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54677822 | Thermal Gravimetric Analysis (TGA), Transmission Electron Microscopy (TEM), and X-Ray Analysis. Thermal gravimetric analysis (TGA) of the polymer sample was performed using a SHIMADZU DT-30 thermal analyzer.The weight loss was measured from ambient temperature up to 600 ∘ C at rate of 20 ∘ C/min to determine the rate of degradation of the polymer. The X-ray diffractometer type Philips 1976 Model 1390 was used to investigate the phase structure of the polymer powder under the following condition which kept constant during the analysis processes: Cu: X-ray tube, scan speed = 8/min, current = 30 mA, voltage = 40 kv, and preset time = 10 s. The inner cavity and wall thickness of the prepared polymer were investigated using transmission electron microscopy (TEM) JEOL JEM-1200 EX ^(Japan). | 7 |
54677822 | Conductivity Measurements. The ac conductivity was measured using Philips RCL bridge (digital and computerized) at a frequency range 0.1-100 kHz and over temperature range 30-80 ∘ C. The temperature was controlled by the use of a double wound electric oven. The ac conductivity ac value was calculated using the relation where = 2 and is the applied frequency. | 8 |
54677822 | Effect of HCl Concentration. The effect of HCl concentration on the aqueous oxidative polymerization of OPDA was investigated using constant concentration of K 2 Cr 2 O 7 at 0.3 M and monomer concentration at 0.1 M and using different concentration of HCl at 5 ± 0.2 ∘ C. The yield-time data was represented in Figure 2, from which it is clear that the obtained yield increases in the acid concentration range from 0.04 to 0.2 then decreases gradually up to 0.5 M.This behavior may be due to, at higher concentration of HCl, the degradation of the polymer in the early stages of the reaction, which may be due to the hydrolysis of polyemeraldine chain [13]. | 10 |
54677822 | Effect of O-Phenylenediamine Concentration. The effect of monomer concentration on the polymerization efficiency was investigated in the range of monomer concentration from 0.02 to 0.5 M and the data was represented in Figure 3, from which it is clear that the optimum yield formation is obtained at 0.1 M of the monomer concentration.plotted for different oxidant concentrations and the data are graphically represented in Figure 4(a).The initial and overall reaction rates were determined using the following equation: | 11 |
54677822 | The Kinetic where is the weight of the obtained polymer, is the time in seconds, is the molecular weight of the monomer, and is the reaction volume in liter. The initial and overall reaction rates of the polymerization reaction increase with the increase of oxidant concentration in the range between 0.05 and 0.3 mol/L.The oxidant exponent was determined from the relation between logarithm of the initial rate of polymerization Log ( ) and logarithm of the oxidant concentration.A straight line was obtained which has a slope of 1.011 as represented in Figure 4(b).This means that the polymerization reaction of OPDA is a firstorder reaction with respect to the oxidant. | 12 |
54677822 | Effect of Hydrochloric Acid Concentration. The polymerization of OPDA (0.1 mol) in 25 mL of HCl with different molarities was carried out by addition of 25 mL potassium dichromate (0.3 mol/L) as oxidant at 5 ∘ C for different time intervals.The concentration of the monomer and oxidant were kept constant during the study of HCl effect on the polymerization reaction.The experiments were carried out as described in Section 2.2, and the yield-time curve was plotted for each acid concentration used.The data are graphically represented in Figure 5(a), from which it is clear that both the initial and overall reaction rates of the polymerization reaction increase with the increasing of HCl concentrations in the range between 0.05 and 0.2 mol/L.The HCl exponent determined from the slope of the straight line represented in Figure 5(b) was found to be 0.954, which means that the polymerization order with respect to the HCl concentration is a first-order reaction.concentration used.The data are graphically represented in Figure 6(a).The monomer exponent was determined from the slope of the straight line represented in Figure 6(b) for the relation between log and logarithm of the monomer concentration.The slope of this linear relationship was found to be 1.045.This means that the polymerization reaction with respect to the monomer concentration is a first-order reaction. | 13 |
54677822 | Calculation of the Thermodynamic Activation Parameters. The polymerization of OPDA (0.1 mol/L) in 25 mL of 0.2 mol/L HCl in presence of 25 mL potassium dichromate (0.3 mol/L) as oxidant solution was carried out at 5, 10, and 15 ∘ C for different time periods.The yield-time curves were graphically represented in Figure 7, from which it is clear that both of the initial and overall reaction rates increase with raising the reaction temperature.The apparent activation energy ( a ) of the aqueous polymerization reaction of o-phenylenediamine was calculated using the following Arrhenius equation: where is the rate, is the universal gas constant, is the reaction temperature, and is constant.By plotting log against 1/, which gave a straight line as shown in Figure 8, and from the slope we can calculate the activation energy.The apparent activation energy for this system is 63.658 kJ/mol.The Enthalpy and entropy of activation for the polymerization reaction can be calculated by the calculation of 2 from the following equation: The values of 2 at 5, 10, and 15 ∘ C were 6.131 × 10 −6 , 1.054 × 10 −5 , and 1.594 × 10 −5 , respectively.The enthalpy (Δ * ) and entropy (Δ * ) of activation associated with 2 were calculated using Eyring equation where 2 is the rate constant, is the Avogadro's number, is the universal gas constant, and ℎ is planks constant.By dividing the above equation by and taking its natural logarithm we obtain the following equation: International Journal of Polymer Science Figure 9 shows the relation between 2 / versus 1/, which gives a linear relationship with (−Δ * )/ and intercept line (ln /ℎ + Δ * /) from the slops and intercept; the values of Δ * and Δ * were found to be 61.48kJmol −1 and 29.95 Jmol −1 K −1 , respectively. The intramolecular electron transfer steps for the oxidation reaction are endothermic as indicated by the value of Δ * .The contributions of Δ * and Δ * to the rate constant seem to compensate each other.This fact suggests that the factors controlling Δ * must be closely related to those controlling Δ * .Therefore, the salvation state of the encounter compound could be important in determination of Δ * .Consequently, the relatively small enthalpy of activation can be explained in terms of the formation of more solvated complex. | 14 |
54677822 | The Initial Step.Potassium dichromate in acidified aqueous solution produces chromic acid as shown in This reaction is controlled by the change in pH; the orange red dichromate ions (Cr 2 O 7 ) 2− are in equilibrium with the (HCrO 4 ) − in the range of pH-values between 2 and 6, but at pH below 1 the main species is (H 2 CrO 4 ) and the equilibria can occur as follows: The chromic acid withdraws one electron from each protonated OPDA and probably forms a metastable complex as shown in (11): The complex undergoes dissociation to form monomer cation radical as shown in (12): Generally, the initial step is rapid and may occur in short time, 0-5 min (autocatalytic reaction); no polymeric product is being obtained.After 5 min of the polymerization reaction, the polymeric products are obtained. | 16 |
54677822 | Propagation Step.This step involves the interaction between the formed radical cation and the monomer to form a dimer radical cation.In the case of Cr(VI) oxidation of the organic compounds, Cr(VI) is reduced to Cr(IV) first and then to This reaction is followed by further reaction of the formed dimmer radical cations with monomer molecules to form trimer radical cations and so on.The degree of polymerization depends on different factors such as dichromate concentration, HCl concentration, monomer concentration, and temperature.By adding (7), (11), (12), (13), and ( 14), Termination Step.Termination of the reaction occurs by the addition of ammonium hydroxide solution in an equimolar amount to HCl present in the reaction medium (till pH = 7), which leads to cessation of the redox reaction.The reaction could occur as follows: 1.The complete solubility was found in N-methyl pyrrolidone, then in DMF 2.235 g/L followed by acetone 1.67 g/L, then in methanol 1.074 g/L then followed by iso propanol 0.879 g/L at 20 ∘ C but not soluble in benzene, hexane, and chloroform. | 17 |
54677822 | The Elemental Analysis. The data obtained from elemental analysis using oxygen flask combustion and a dosimat E415 titrator shows that the found carbon content of (POPDA) is lower than the calculated value.This is due to the formation of chromium carbide during step of heating and measuring process while the found values of nitrogen and hydrogen are 20.91 and 4.37, respectively, which are in good agreement with the calculated one for the suggested structure present in Scheme 1.By measuring another sample of the (POPDA) which was prepared by using ammonium persulfate as oxidant, the found value of carbon is higher than sample which is prepared by using potassium dichromate as oxidant.For more information about the chemical composition of (POPDA), the XPS study was conducted as mentioned under point 3.5.2. | 18 |
54677822 | X-Ray Photoelectron Spectroscopy (XPS) Characterization (1) XPS Survey Elemental Composition.X-ray photoelectron spectroscopy (XPS) is used to study the composition of materials, which detect elements starting from Li ( = 3) and higher elements.Hydrogen ( = 1) and helium ( = 2) cannot be detected due to the low probability of electron emission.XPS survey begins from 0 to 1400 (eV) as shown in Figure 10.The XPS survey scan spectrum of the prepared polymer shows the presence of C, N, O, Cl, and Cr.The Cl was present as doping anion in the prepared polymer.Chrome was found due to the polymer prepared using potassium dichromate as oxidant.It is possible for chromium ion (Cr +3 ) to present between polymer chains as a sandwich-bonded C 6 H 6 -C 6 H 6 groups as shown in Scheme 1 and the usual formation procedure is hydrolyzing the reaction mixture with dilute acid which gives the cation (C 6 H 6 ) 2 Cr 3+ [30,31]. The XPS elemental analysis of the prepared polymer is given in Table 2.The data shows that there is a good agreement with the calculated one for the suggested structures present in Scheme 1. (2) XPS Spectra of Poly(OPDA).Four main peaks were obtained for C1s spectra of poly(OPDA) as shown in Figure 11(a).The sharp peak appearing at 283.98 eV is attributed to C-H (C 1 ) bond, while the peak appearing at 284.18 eV is attributed to C-C (C 2 ) bond.The peak appearing at 285.08 eV is assigned to C-N (C 3 ) bond while the peak appearing at 287.58 eV is attributed to C-O or C-N + (C 4 ) bond [32][33][34]. N1s Figure 11(b) shows the XPS N1s spectrum of poly(OPDA) which has three distinct curves.The first two peaks are assigned to imine (-N=) at 398.58 eV and amine (-NH-) at 399.23 eV.Moreover, the peak that appears at 400.58 eV is due to positively charged nitrogen atom (N + ). Two distinct oxygen species contribute to the oxygen 1 s signals in the conducting polymers (Figure 11(c)).The distinct energy peaks at 530.68 and 533.12 eV could be attributed to Cr 2 O 3 and C-OH, respectively. The Cl 2p spectrum of poly(OPDA) is shown in Figure 11(d).In order to estimate the anion Cl at the surface, Cl 2p peaks are fitted with a number of spin-orbit doublets (Cl 2p 1/2 and Cl 2p 3/2 ) with the B.E. for the C12p 3/2 peaks at about 197.36, 198.34, and 200.1 eV.The lowest and the highest B.E. components are attributable to the ionic and covalent chlorine species (Cl − and -Cl), respectively.The chlorine species (Cl * ) with the intermediate appear at B.E. of 198.34 eV.The lower B.E. value of the Cl * species compared to the Cl species suggests the presence of chloride anion in a more positive environment, probably arising from an increase in the number of positively charged nitrogen in the polymer chain associated with the formation of polarons and bipolarons. The Cr spectrum of (POPDA) is shown in Figure 11(e).The main components corresponding to different chemical chromium species were observed in the high-resolution Cr 2 p 3/2 spectrums.The first peak at 576.18 eV ± 0.2 eV was assigned to Cr 2 O 3 which is in agreement with what was found by Chowdhury and Saha [13] and Stefanov et al. [35], also indicated by a distinct O1s peak at 530.68 eV typical for Cr 2 O 3 which may be adsorbed on polymer surface during chromous acid H 2 Cr 2 O 3 oxidation process.There is also a component visible that corresponds to Cr 2 p 1/2 at 586.08 eV, which was attributed to Cr 3+ .This data reveal that chromium ion is present between benzene rings of polymeric chain as shown in Scheme 1. | 19 |
54677822 | The Infrared Spectroscopic Analysis of (OPDA) Monomer and Its Analogs Polymer.The IR spectra of the OPDA and its polymer (POPDA) are represented in Figure 12, while the absorption band values and their assignments are summarized in Table 3.The medium absorption band appearing at 453 cm −1 , which could be attributed to bending deformation of N-H group attached to benzene ring in case of monomer, appears at 518 cm −1 with slight shift in case of polymer.The broad absorption band appearing at 778 cm −1 in case of monomer, which could be attributed to out of plane deformation of CH for 1,2-disubstituted benzene, appears at 748 cm −1 with slight shift in case of polymer.A series of absorption bands appear in the region from 924 to 1152 cm −1 which could be attributed to the out of plane C-H deformation of 1,2-disubistuition benzene ring in case of monomer and 1,2,4 tri-substituted of benzene ring, in case of polymer.The sharp absorption bands appear at 1336 cm −1 in case of the monomer, which could be attributed to symmetric stretching vibration of C-N, appears at 1362 cm −1 with slight shift in case of polymer.The sharp absorption band appearing at 1490 cm −1 in case of the monomer may be attributed to C=C aromatic stretching vibration, which appears at 1500 cm −1 with slight shift in case of polymer.The sharp absorption band appearing at 1630 cm −1 in case of the monomer, which may be attributed to C=C deformation of benzene ring, appears at 1621 cm −1 with slight shift in case of polymer.The shoulder absorption band appears at 3032 cm −1 in case of the monomer which could be attributed to symmetric stretching vibration of C-H in aromatic ring and disappears in case of polymer.The triplet absorption bands appearing at 3190, 3281, and 3370 cm −1 in case of monomer could be attributed to asymmetric stretching vibration for NH group, but in case of polymer a broad absorption band that appears at 3354 cm −1 could be attributed to asymmetric stretching vibration for NH group and OH group present in the polymer structure.and its polymer are represented in Figure 13; the spectra show the following absorption bands: (1) in case of monomer, two absorption bands appear at max = 219 and 240 nm which may be attributed to - * transition (E 2 -band) of the benzene ring and the -band ( 1g − 2u ); (2) in case of polymer, two absorption bands appear at max = 221 and 243 nm which may be attributed to - * transition showing a bathochromic shift.Beside these two bands, broad absorption band appears in the visible region at max = 417 nm which may be due to the high conjugation of the aromatic polymeric chain. 3.5.6.Thermal Gravimetric Analysis (TGA) of Poly O-Phenylenediamine.Thermogravimetric analysis (TGA) for the prepared polymer has been investigated and the TGA-curve is represented in Figure 14.The calculated and found data for the prepared polymers are summarized in Table 4.The thermal degradation steps are summarized as follows: (1) the first stage includes the loss of one water molecule in the temperature range between 29.5 and 122.6 ∘ C; the weight loss of this step was found to be 3.21% which is in a good agreement with the calculated one; (2) in the second stage, in the temperature range between 122.6 and 203.8 ∘ C, the weight loss was found to be 6.65%, which could be attributed to the loss of one HCl molecule.The calculated weight loss is in good agreement with the found one; (3) in the third stage, in the temperature range between 203.8 and 320.3 ∘ C, the weight loss was found to be 11.79%, which is attributed to the loss of four (NH 2 ) groups.The calculated weight loss for this stage is equal to 12.11%; (4) in the fourth stage, in the temperature range between 320.3 and 400.0 ∘ C, the weight loss was found to be 16.95%, which could be attributed to the loss of one molecule of C 6 H 3 -NH.The calculated weight loss of this stage is equal to 17.22%; (5) in the fifth stage, in the temperature range between 400.0 and 523.4 ∘ C, the weight loss was found to be 13.78%, which is attributed to the loss of one molecule of phenyl ring.The calculated weight loss of this stage is equal to 14.19%; (6) in the last stage, above 523.38 ∘ C, the remaining polymer molecule was found to be 47.62% including the metallic residue, but the calculated one was found to be 46.46%.This behavior is in good agreement with the random free energy model proposed by Dyre [36].According to this model, conductance increases as a function of frequency in many solids, including polymers, which can be explained on the basis of any hopping model.The rise in conductivity upon increasing the frequency and temperature is common for disordered conducting polymer.As can be seen, each curve displays a conductivity dispersion, which is strongly dependent on frequency and shows weaker temperature dependant. | 20 |
54677822 | The X-Ray Diffraction Analysis and Transmission The recorded conductivity value at room temperature of POPDA was found 0.0352 S/cm which is higher than conductivity of polyaniline-polyvinyl alcohol blends 10.5 × 10 −5 S cm −1 [37] and ac conductivity of HCl doped polyaniline synthesized by the interfacial polymerization technique 6.2 × 10 −5 S cm −1 [38].Also, the ac conductivity of POPDA is higher than polyaniline loaded with 10% molybdenum trioxide composites 0.025 s/cm [39] but lower than the determined value of ac conductivity polyaniline prepared by K 2 Cr 2 O 7 as oxidant 1.922 S cm −1 .Such difference could be attributed to the different disorder of each composite, substituted function group, and different used dopant. International Journal of Polymer Science 0.15 0.17 In general, for amorphous conducting material, disordered systems, low mobility polymers, and even crystalline materials, the ac conductivity ( ac ) as a function of frequency can obey a power law with frequency [40].The ac conductivity ( ac ) over a wide range of frequencies can be expressed as where is a complex constant and the index () is frequency exponent and is the angular frequency ( = 2). Figure 17 shows the relation between Ln ac and Ln at different temperatures.The value of () at each temperature has been calculated from the slope of ln () versus ln () plot.As shown in Figure 18 the calculated values of () for (POPDA) sample are less than unity.The microscopic conduction mechanisms of disordered systems are governed by two physical processes such as classical hopping or quantum mechanical tunneling of charge carries over the potential barrier separating two energetically favorable centers in a random distribution.The exact nature of charge transport is mainly obtained experimentally from the temperature variation of exponent (s) [41].The temperature exponent(s) dependences for (POPDA) sample reveal that the frequency exponent(s) decreases with the increase of temperature.This behavior is only observed in the correlated barrier hopping model proposed by Elliott [42]. | 21 |
54677822 | Table 1 : Solubility of poly-o-phenylenediamine (POPDA) in different solvents at 20 ∘ C. Transfer of two electrons from two monomer ions radical by H 2 CrO 4 produces para semidine salt along with chromous acid H 2 Cr 2 O 3 (Cr(IV)).The intermediately produced Cr(IV) oxidises para semidine to pernigraniline salt (PS) at suitable low pH and the PS acts as a catalyst for conversion of OPDA to POPDA: 3.6.acConductivity ( ac ).Figure 17 represent the variation in ac conductivity ( ac ) for (POPDA) as a function of frequency and temperature.It is observed that the value of ac conductivity increases with the increase of frequency. | 25 |
3326323 | Article history: Received on: 07/09/2013 Revised on: 08/10/2013 Accepted on: 12/02/2014 Available online: 27/02/2014 Microscopy of the gum revealed various shapes and sizes which disintegrated within a short time (3-5 minutes). They have thick walls, somewhat cracked and striated and also seen as translucent masses. Some were observed to have open fissures. However, the purified ones obtained from the precipitates depicted no open fissures, striations or cracks. Macroscopy revealed that A.occidentale gum has irregular shapes, tasteless, odourless, very coarse texture for the crude to fine coarse for the purified gum. It is yellowish brown colour for the crude to whitish milk for the purified gum. On the other hand, the gum arabic has a bland mucilaginous taste, odourless, varying shapes and sizes with a milky colour. This shows that the plant gum has features similar to available pharmaceutical gums and as such a viable pharmaceutical material and also these features are useful for the preparation of monograph of the plant. Paper and Thin Layer Chromatographic analyses of the carbohydrates in both the gums revealed the presence of sugars such as xylose, arabinose, galactose and glucose. Butanol-Acetic acidWater (BAW) 4:1:5; Butanol-Ethanol-Water (BEW) 4:1:2.2; and Butanol-Acetic acid-Ether-Water (BAEW) 9:6:3:1 were used as solvents systems by ascending technique and sprayed with Aniline phthalate for visualization. | 1 |
3326323 | INTRODUCTION Gums are natural plants hydrocolloids that may be classified as anionic or non-ionic polysaccharides or salts of polysaccharides.They are translucent, amorphous substances that are frequently produced in higher plants as protective agents after injury.Thus, they are the abnormal products of plants metabolism (Kokate et al, 2002).Gums are also considered to be pathological products formed upon injury of the plant or owing to unfavourable conditions such as drought by a breakdown of cell walls (extra cellular formation-gummosis) (Evans, 2002;Kokate et al, 2002).Comparative studies using samples of cashew gum obtained from different geographical sources have been reported to show significant variations in compositions and properties linked with climatic conditions (Lima et al, 2002).Gums are heterogeneous polyuronides which on hydrolysis, they yield sugars such as arabinose, galactose, glucose, mannose, xylose and various uronic acids (Kokate et al, 2002).In most gums, the polyuronides of . .mixed composition are formed by glycosidic linkages and various sugar molecules (Wallis, 1967).The cashew gum was determined by Mothe and Rao (1999) to be acidic to litmus paper which is in agreement with range of cashew gum mucilage Gums consisting of linear polymers are less soluble than those with branched constituents, and linear hydrocolloids yield solutions with greater viscosity.Plants exudates have been the traditional gums for pharmaceuticals purposes and they still find significant application; however preparation of these gums is labour intensive and carries a premium price and their use will probably continue to decline.Marine gums are widely used as utility gums at the present time, and their competitive positions appear stable (Kokate et al, 2002 andTyler et al, 1988). The chromatographic methods of analysis provide information on the homogeneity, molecular size and structure of a carbohydrate and gives useful information especially R F values which are used in the identification of the compound desired.Pharmacopoeias are increasingly employing thin-layer chromatography as a means for assessing quality and purity.The RF value (rate of flow, i.e. distance moved by solute divided by distance moved by solvent front) of a compound, determined under specified conditions, is characteristic and can be used as an aid to identity.The R F values vary from 0.0 to 1.00.Quantitative extracts of crude drugs are prepared and compared chromatographically with the standard reference solutions of the known constituents (Trease and Evans, 1983).The method of separation is also useful in the isolation of carbohydrates and their derivatives.Paula and Rodrigues (1995) also reported the presence of arabinose, glucose, rhamnose, mannose and glucuronic acid appearing as terminal residues in the polysaccharide of A. occidentale gum Gums find diverse applications in pharmacy as tablet binders, emulsifiers, gelatine agents, suspending agents, stabilizers and thickeners.They are also ingredients in dental and other adhesives and in bulk laxatives (Tyler et al, 1988).Zakaria and Rahman (1996) observed that differences in gum sources seem to influence the pH and the viscosity of the gum mucilage. | 2 |
3326323 | MATERIALS AND METHODS Following the identification of the A. occidentale, the gum was collected upon wounding or injuring the bark using a sterile beaker container.The gum was allowed to air dry under the shade and some brownish to red particles suspected to be cork or fragments of bark were mechanically removed.The hardened cakes were size reduced to fine powder by the use of pestle and mortar.About 500g of the gum powder was dissolved in about 1 litre of hot water and the resultant solution was strained to free it from insoluble matter (organic matter) by filtering, through a clean piece of linen cloth.The gum from the filterate was then extracted or precipitated using the method of Karawya et al (1971) for gum purification and extraction.The gum was extracted severally with 95% alcohol and finally washed and dried in the oven at a temperature of 40⁰С for at least 3hours and kept in an air tight container for further use. Detailed macroscopical and microscopical studies of the cashew gum with respect to gum Arabic were carried out.Following the acid hydrolysis, the chromatographic analysis of the gum was done using paper and Thin layer chromatography by calculating the R F values and compared with the reference sugars.Butanol-Acetic acid-Water (BAW) 4:1:5; Butanol-Ethanol-Water (BEW) 4:1:2.2;and Butanol-Acetic acid-Ether-Water (BAEW) 9:6:3:1 were used as solvents systems by ascending technique and sprayed with Aniline phthalate for visualization. | 3 |
3326323 | RESULTS AND DISCUSSIONS From the physical examination of the gum, it can be deduced that it has varying shapes and sizes, odourless, tasteless.The crude gum was seen to be yellowish brown in colour while the purified or precipitated gum is whitish to milk in colour.Under the microscope, the crude gum appeared as large masses which dissolve when cleared with a chloral hydrate as a clearing agent.The precipitated gum on the other hand showed irregular with varying shapes and sizes.They have thick walls, somewhat cracked and striated and also seen as translucent masses.Some were observed to have open fissures (figure 1).However, the purified ones obtained from the precipitates depicted no open fissures, striations or cracks (figure 2).The microscopy of the gum Arabic also shows similar features like that of the precipitated cashew gum as in figure 2. The fissures/cracks have reduced due to partial purification.The observed features disintegrated within 3-5 minutes when mounted with dilute glycerol.Negligible, limited fibres were also seen.The macroscopical and microscopical feature of gum Arabic is much alike to that of cashew gum.The rapid disintegration of the A. occidentale gum may probably suggest its low stability property.Based on the R F values of the various reference sugars and compared with the test gum sample, it can be inferred that the later have xylose, glucose, arabinose and galactose sugars as revealed on both the paper and thin layer chromatography.The TLC in BEW revealed that the R F value of the gum sample (0.58) closely corresponded to that of glucose (0.55).Similarly, the TLC in BAEW solvent system indicated the R F value of the gum (0.36) closely related to galactose (0.37).These are similar to earlier | 4 |
29282189 | O caráter lipofílico de doze cumarinas foi investigado por cromatografia de camada fina de fase reversa (RP TLC) em sílica RP-18. Três diferentes sistemas de solvente binário compostos por água e o modificador orgânico (metanol, tetrahidrofurano ou acetonitrilo) foram utilizados para determinar o parâmetro de retenção (RM ) e o coeficiente de partição de octanol-água (log POW) como medida de lipofilicidade dos compostos testados. O parâmetro de lipofilicidade (log POW) foi determinado experimentalmente usando-se oito padrões de soluto com valores de log POW conhecidos, que foram analisados sob as mesmas condições cromatográficas de substâncias alvos. Parâmetros de lipofilicidade junto com descritores moleculares 2D foram submetidos à análise estatística multivariável (análise de componentes principais (PCA) e regressão por mínimos quadrados parciais (SLS)) para determinar os fatores mais importantes para retenção, ou seja, lipofilicidade dos compostos investigados. Os modelos quantitativos de relação entre as propriedades de estrutura e retenção revelam a importância de descritores referentes ao tamanho e ao formato da molécula assim como suas propriedades polares. | 1 |
29282189 | Introduction The methods of relating molecular structure of solutes (expressed via descriptors) to their chromatographic (retention) behavior are commonly denoted as quantitative structure-retention relationships (QSRRs).Similarly, the aim of quantitative structure property relationship (QSPR) research is to find a functional dependence between molecule structure and its physicochemical properties. Lipophilicity is a very important molecular parameter used in the QSR(P)R studies and plays an important role in drug discovery.Knowing the lipophilicity of potential drugs helps understanding their absorption, distribution, metabolism, excretion and toxicity (ADMET). 1 Lipophilicity is expressed by the logarithm of the partition coefficient (log P), which represents the tendency of a molecule to distribute between water and a water-immiscible solvent.Liquid chromatographic (LC) techniques can be considered as a traditional approach to fast estimation of lipophilicity. Recently, a comparative study on several approaches for the determination of lipophilicity by means of thin-layer chromatography (TLC) was presented by Komsta et al. 2 In the case of TLC, the QSRR studies are usually based on the use of the R M value defined by Bate-Smith and Westall equation, 3 (1) where R F is the retardation factor.Generally, the R M values determined by means of reversed-phase thin-layer chromatography (RP TLC) are linearly dependent on the concentration of the organic modifier (j) in the mobile phase (2) where m and R M 0 are, respectively, the slope and the intercept of equation 2. The extrapolation of the R M value to pure water based on the Soczewinski-Wachtmeister model 4 allows the estimation of lipophilicity. 5 The OECD (Organization for Economic Cooperation and Development) Guidelines for the Testing of Chemicals (Test 117) 6 describes the method for the determination of the partition coefficient (log P OW ) using reversed-phase high performance liquid chromatography (HPLC).The appropriate reference substances with log P OW values which encompass the log P OW of the test substances (i.e., at least one reference substance has P OW above that of the test substance and another P OW below that of the test substance) need to be selected and chromatographed under the same conditions as test substance in isocratic mode.A calibration graph obtained by correlation of the measured retention data of reference substances with their partition coefficients is used for the determination of the log P OW value of test substances.In many articles, HPLC method is substituted by thin-layer chromatography, 7 keeping the same principles as in Test 117 with RP-18 silica stationary phase and the composition of the mobile phase that provide the best selectivity (in accordance with isocratic HPLC mode). In the past decade, our research was focused on QSRR of various organic compounds that are believed to exhibit biological activity.][10][11] In a previous publication, the results on the chromatographic behavior of 4-hydroxycoumarin rodenticides (coumatetralyl, bromadiolone and brodifacoum) and biocidal material impurities in various normal-and reversed-phase chromatographic systems were reported. 12The results proved the RP TLC to be suitable for the estimation of the relative lipophilicity of coumarine derivatives. Coumarins are naturally occurring benzopyrone derivatives identified in plants and are characterized by extensive chemodiversity and various pharmacological activities.The majority of coumarins have been isolated from green plants.The genus Seseli (part of Apiaceae family) is a well-known source of linear or angular pyranocoumarins, an interesting subclass of coumarins possessing antiproliferative, 13 antiviral 14 and antibacterial activities. 15Numerous species of the genus have been used in folk medicine since ancient times. Continuing research in this field, we selected Seseli montanum subsp.tommasinii as a source of some natural coumarins.From the aerial parts of the plant, five known coumarins were isolated.They were studied together with another two natural (isolated from the roots of Seseli annuum and Achillea tanacetifolia) and five synthetic coumarins.A study here presented deals with several topics: (i) retention behavior of coumarins in the reversed-phase chromatographic systems using different organic modifiers, (ii) comparison of different modifiers in lipophilicity assessment, (iii) comparison of two experimentally obtained lipophilicity parameters (R M 0 and log P OW ) in terms of better lipophilicity evaluation and (iv) selection of a subset of descriptors that are the most relevant for retention of coumarins.Principal component analysis (PCA) and partial least squares (PLS) were selected as ones of the most widely used chemometrical methods to build QSRR models. | 2 |
29282189 | Isolation procedure The chemical structures of the investigated coumarins 1-12 are presented in Figure 1. The plant material was collected at Gorica Hill (area of Podgorica City, Montenegro, Serbia) in Autumn 2009.A voucher specimen (P167/09) was deposited at Herbarium of the Faculty of Natural Sciences and Mathematics, University of Montenegro (Podgorica City). All relevant 1 H and 13 C NMR data and 1 H NMR spectra of compounds 1-6 are given in Supplementary Information (SI). | 3 |
29282189 | Reversed-phase thin-layer chromatography The TLC experiments were performed on a commercially available RP-18 TLC plates, (Art.5559, E. Merck, Germany).The plates were spotted with 1 μL aliquots of 2 mg mL -1 solutes of each compound (dissolved in CH 2 Cl 2 ), and developed by the ascending technique, without preconditioning.The detection of the zones was performed under UV light (λ = 254 nm).The R F values were determined as an average of the three chromatograms.Three solvent systems were used as mobile phase: methanol-water, acetonitrile-water and tetrahydrofuran-water binary mixtures, with a varying content of organic modifier (from 100 to 60 vol.% in the case of methanol and acetonitrile and from 100 to 40 vol.% of tetrahydrofuran (increment 10 vol.%)).All the components of the mobile phases were of the analytical grade of purity.All experiments were performed at ambient temperature (22 ± 2 °C). | 4 |
29282189 | Calculations For the geometry optimization, the structures were subjected to the Hyperchem Program (version 7.0, Hypercube).The optimization of three-dimensional structure was calculated by semi-empirical quantum chemical calculations with AM1 Hamiltonian.A set of molecular descriptors was selected to reflect geometrical, electronic and physicochemical properties of the investigated compounds.Hyperchem calculates electronic properties, optimized geometries, total energy and QSAR properties.A set of additional physicochemical parameters was generated from the optimized structures by Molecular Modeling Program Plus program (MMP Plus).Virtual Computational Chemistry Laboratory at website http://www.vcclab.org was used for the calculation of lipophilicity of the compounds by various methods based on different theoretical procedures. | 5 |
29282189 | Multivariate statistical analysis and modeling PCA and PLS were performed using demo version of PLS Toolbox statistical package (Eigenvectors, Inc., version 5.7) for the MATLAB version 7.4.0.287 (R2007a) (MathWorks, Inc., Natick, MA, USA).The data were mean-centered and scaled to unit variance before any statistical operations in order to prevent highly abundant components dominating in the final result over the components present in much smaller quantities. PCA was carried out as an exploratory data analysis by using single value decomposition (SVD) algorithm and 0.95 confidence level for Q and T 2 Hotelling limits for outliers.A limited number of PC reduces the dimensionality of the retention data space, simplifying further analysis and grouping the substances according to their intrinsic ability for specific interactions.2][23] Validation of the models was performed by leave one out cross-validation procedure.The quality of the models was monitored with the following parameters: R 2 cal (cum) (the cumulative sum of squares of the Ys explained by all extracted components), R 2 CV (cum) (the cumulative fraction of the total variation of the Ys that can be predicted by all extracted components), showing as higher as possible values, and root mean square errors of calibration (RMSEC) and root mean square errors of cross-validation (RMSECV) showing as lower as possible values, with the lowest difference in between them.Low value of RMSEC is desirable but if the high values of RMSECV are present at the same time, this can be an indication of the poor predictability of the calibration model. 24,25 nsidering the other multivariate linear regression techniques as multiple linear regression (MLR) and principal component regression (PCR), PLS was chosen as a target analysis due to a number of advantages.Namely, the number of predicted variables is greater than the number of the compounds and it is better to reduce their number to just a few latent variables (using PLS or PCR) than select a few predictor variables, by MLR.In addition, a lot of variables are correlated and have constant values, so MLR would not be appropriate method.An important feature of PLS is that it takes into account errors in both independent and response variables, while PCR assumes that the estimation of molecular descriptors are error free. 26s it is previously mentioned, the best selectivity was obtained with methanol-water mobile phase and these results were used for the evaluation of the possible relationship between the lipophilicity characteristics and the physicochemical parameters of the molecules.The lipophilicity parameter R M 0 (chromatographic system RP-18/methanol-water) and log P OW were the response variables in the QSRR study.These values were regressed against the molecular structural descriptors as independent variables. | 6 |
29282189 | Lipophilicity of the analytes The retention parameters (R F and R M ) of coumarins were determined at several compositions of the three different binary solvent systems composed of organic modifier and water: methanol-water, acetonitrile-water and tetrahydrofuran-water.For each compound, the R M value was extrapolated to the zero volume of the organic modifier by using equation 2, thus obtaining the lipophilicity parameter (R M 0 ).The slope (m) and intercept (R M 0 ) values, and the statistical data (correlation coefficient (r) and standard deviation (s)) for each binary system are listed in Table 1. The R M values were linearly dependent on the concentration of organic modifier in the mobile phase, with r ≥ 0.99.Also, the majority of substances show the highest R M 0 values in methanol, which has the lowest elution strength among all the organic modifiers applied on RP-18 silica. Taking into account the observed retention, it can be concluded that tricyclic compounds (1-4 and 6) exhibited stronger retention compared to byciclic coumarines (5, 7-12).Also, increased retention of 1, 2 and 6 coumarins can be ascribed to the presence of 2-butenoil and 3-methylbut-2-enyloxy group.Similar chromatographic behavior was observed for compounds 6 and 7, with identical side-chain substituent, indicating that the presence of the bulky 3-methylbut-2-enyloxy group defines their chromatographic behavior.Among all investigated coumarins, bicyclic compounds with hydroxy (9 and 10) and methoxy groups (11 and 12) demonstrated decrease of retention. The determination of log P OW by TLC is based on the linear relationship between the chromatographic retention R M and the octanol-water partition coefficient determined by shake-flask method for a set of standard compounds.For that purpose, the investigated coumarins were simultaneously chromatographed with the standard solutes, and the retention parameters were determined (R M values are presented in brackets: 4-methoxyphenon (−0.45), 2,6-dimethylphenol (−0.13), 1,3,5-trihydroxybenzene (−1.19), anthracen (0.69), 4-hydroxybenzaldehyde (−0.57), 1-naphthol (−0.10), benzophenon (0.21), and phenol (−0.52)).As the best selectivity was obtained with methanol-water (75:25%, v/v), this mobile phase was chosen for the determination of log P OW .To characterize lipophilicity of coumarins, linear calibration between R M values of eight standards and their literature log P OW values was used R M = −1.176+ 0.423 log P OW (3) r = 0.992, N = 8, SD = 0.078, P < 0.0001 R M values of the studied compounds were substituted into equation 3 to calculate log P OW values, listed in Table 2.The same table contains calculated log P values of selected coumarins. The determination of linear dependences between lipophilicity parameters obtained in chromatographic investigations and calculated log P values is an indispensable step for QSRR.These correlations provide evidence that the chromatography based measurements of lipophilicity are valid.A number of methods based on different approaches for calculating log P from chemical structures are available.Extrapolated R M 0 values for chromatographic system RP-18/methanol-water and experimentally established log P OW values were compared with calculated log P (log P calc ), and statistical parameters of these dependences are given in Table 3.Although linear dependence exists in most cases with satisfactory correlation coefficient values over 0.93, observing the slope and the intercept of the relevant equations, it could be concluded that the deviations from the ideal correlation (slope ca. 1 and intercept ca.0) are more pronounced in the case of experimentally obtained log P OW values, i.e., R M 0 is better lipophilicity estimate.Determined lipophilicity of the investigated compounds is in accordance with their chromatographic behavior.Additional pyran and furan ring attached to 2-benzopyran-1-on aromatic core provide increased lipophilicity versus corresponding derivatives possessing no extra ring.Incorporating polar hydroxy and methoxy groups have a more pronounced negative impact on lipophilicity.Lipophilicity is also raised with increasing substitution on the basic benzopiranon, i.e., derivatives with 2-butenoil and 3-methylbut-2-enyloxy group are more hydrophobic than compounds that possess methyl, methoxy, hydroxy, acethyl and epoxide substituents. | 7 |
29282189 | Principal component analysis (PCA) PCA carried out on the set of calculated molecular descriptors and retention data can reveal some similarities among studied compounds governed by both their intrinsic structural properties and specific interactions that occur in different chromatographic systems.Loading plots highlight the mostly influential variables responsible for such a clustering and provide a picture on the similarity between R M 0 values and the other molecular descriptors. PCA applied on a set of molecular descriptors resulted in a three-component model explaining 91.79% of the data variation (first principal component comprises 71.94% of variances).The score plot of the three principal components (Figure 2) indicates that all data were lying inside the Hotelling T 2 ellipse, suggesting that there are no outliers among the analytes. Considering the score plot, PCA reveals different classification.Samples are clustered into two main separate groups: coumarins 7-12 and 5 with different substituents attached to 2-benzopyran-1-on are positioned in one group; while coumarins with one more pyran or furan ring connected to benzopyranon core are in the second group (compounds 1-4 and 6).First principal component distinguished samples according to the number of the rings present in the molecule (bicyclic and tricyclic compounds).Second principal component separates those with hydroxyl group in the molecule (3, 4, 9 and 10) from the other investigated.The mutual projections of loading vectors are shown in Figure 3.The highest positive impact to the PC1 is recorded by the parameters which describe the size and the shape of the molecule.PC2 separates compounds mainly according to their polar properties, i.e., physicochemical descriptors such as the count of hydrogen-bond donor, hydrophiliclipophilic balance, solubility parameter, dipole moment, etc.On the loading plot, the three R M 0 variables are in the group with those relating the size and the shape of a molecule such as refractivity, polarizability, surface area, molecular volume, molecular weight, molecular depth and molecular width.These facts could indicate the most influential factors for observed chromatographic behavior of the coumarins. Quantitative structure-retention relationship (QSRR) PLS modeling was performed in order to qualify relationships between the factors governing the lipophilicity.The number of latent variables was selected on the basis of the minimum RMSECV, and the minimum difference between RMSEC and cross-validation.In both models a minimum value of RMSECV was obtained with two latent variables.The obtained models are summarized in Table 4. The application of PLS methods revealed that the statistical results of these two models are comparable, and that they are statistically significant.The main descriptors in both PLS models are those relating the size and the shape of a molecule such as refractivity, polarizability, surface area, molecular volume, weight, parachor, volume and mass.Observing the X loading plot of the models, it was supposed that a simpler PLS model can be obtained after removing some variables. The contribution of descriptors that are most influential on the chromatographic behavior was done using variable importance in projection (VIP) scores.The variables with VIP scores higher than 1 were considered as the most relevant for explaining the response variable Y, while the other are of extremely low or almost no contribution.After removing the variables that only contribute to noise (variables with low values of coefficients and low VIP values), a simpler and better PLS models were obtained.The descriptors included in the final models are presented in Table 4 in order from the highest to the lowest value of their regression coefficient, with notification of the sign of their contribution on the response variable.Taking into account the parameters that represent the quality of the model, it can be concluded that both PLS models are statistically significant.The descriptors included in the final models are of similar nature and significance. The results obtained indicate that the most relevant descriptors influencing lipophilicity parameters are: surface area, molecular length, density, solubility parameter, Hansen polarity, Hansen dispersion and hydrophilic-lipophilic balance.From the sign of the regression coefficients, it can be observed that the descriptors describing polarity of the investigated compounds, i.e., their ability for hydrophilic interactions makes negative contribution to the R M 0 values.Solubility parameter, Hansen polarity and dispersion provide a numerical estimate of the degree of intermolecular attractions between molecules (i.e., existence of the dispersion, polar and hydrogen bonding forces), and indicate that the stronger the intermolecular interactions between molecules and the mobile phase are, the analytes are less retained on the stationary phase and the lower R M 0 and log P OW values are obtained.Surface area and molecular length influence the lipophilicity parameters on the opposite way.They have positive coefficients in models and give the higher value of R M 0 and log P OW when they are higher.The surface area of substance is a sum of all areas that cover the surface of the molecule.The higher value of this descriptor indicates the larger molecule which is stronger retained on the stationary phase causing the higher value of R M 0 , i.e., log P OW molecular length determines the size of the molecule and influences on the lipophilicity parameter on the same way as previous descriptor.Hydrophilic-lipophilic balance of a solute is a measure of a degree to what extent its hydrophilic or lipophilic properties are expressed.Its negative regression coefficients reveal the lower the values of these balances are, the greater the values of R M 0 and log P OW are observed, suggesting that more hydrophobic solutes, exhibiting stronger nonspecific dispersive interaction between their own nonpolar moieties, and those of the stationary phase are more retained under applied chromatographic conditions. | 8 |
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