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  • Author or Editor: Tatjana Djaković Sekulić x
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The separation properties of five different unconventional TLC stationary phases (rice starch, microcrystalline cellulose, aminoplast, talc, and paraffin oil-impregnated silica gel) with three-component mobile phases (water-ethyl methyl ketone-dioxane) have been compared. Aliphatic linear alcohols with one to twenty carbon atoms, as the esters of 3,5-dinitrobenzoic acid, were used as test compounds. The stationary phases were characterized by use of separation factor, α , resolution, R S , and Δ R F values. Irregular changes of the retention factors with increasing number of methylene groups of the compounds were sometimes observed. Linear correlations were obtained between R M and concentration of dioxane in the mobile phase.

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Chromatographic retention data have been studied for the 3,5-dinitrobenzoic acid esters of a homologous series of aliphatic C 1 –C 20 linear alchols on five unconventional TLC stationary phases — rice starch, microcrystalline cellulose, aminoplast, talc, and paraffin oil-impregnated silica gel. The stationary phases (supports) were characterized by means of retention scores obtained by principal-components analysis (PCA). PCA shows that although the main retention principles are similar, the unconventional supports differ in secondary retention mechanisms. The stationary phases were also compared on the basis of their capacity to reflect analyte lipophilicity and the correlation of this with their experimental partition coefficients, log P .

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Estimation of retention data by use of correlation equations and physicochemical properties is a useful tool in liquid chromatography. In this work, relationships between physicochemical structural properties and the HPTLC retention of an homologous series of s -triazines on C 18 , cyano, and amino layers has been investigated. Comparisons based on partial least-squares (PLS) regression have been used to model R M 0 values by means of 30 different descriptors. Significant results were obtained in the prediction of retention indices.

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The behavior of 5,5-disubstituted hydantoin derivatives has been studied using four chromatographic systems, two normal-phase (silica gel stationary phase with ethyl acetate-toluene and acetonitrile-toluene mobile phases) and two reversed-phase (RP-18 systems with methanol-water and acetonitrile-water mobile phases). The effect of the structures of the derivatives on their retention in both normal and reversed-phase modes was investigated by use of QSRR and molecular descriptors. By means of multiple linear regressions results were interpreted in terms of understandable and physically meaningful variables. Among six models proposed, the two best were selected and tested for prediction of retention — one for normal-phase chromatography (NPC) and the other for reversed-phase chromatography (RPC). The most informative properties in the best NPC model, with similar effects on retention, are log P and valence connectivity indices. The informative properties in the best RPC model were log P and molecular weight, but the statistics was much worse than for NPC. Cross-validation indicated the best models are reliable QSRR models.

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The chromatographic behavior of para -substituted amides of α -phenylacetic, benzoic, propanoic, and 2,2-dimethylpropanoic acid has been studied in reversed-phase thin-layer chromatography on silica gel impregnated with paraffin oil. Mobile phases used were acetone-water, methanol-water, and dioxane-water. R M 0 values were calculated by linear extrapolation of retention to 0% organic modifier for each mobile phase. The relationships between R M 0 values obtained with different organic modifiers in the mobile phases are discussed. C 0 values were obtained from the slopes and intercepts of linear relationships between retention and volume fraction of modifier. Correlation between partition coefficient, log P , and R M 0 and between log P and C 0 was examined. Results show that C 0 is a better measure of the lipophilicity of the amides than R M 0 .

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Chromatographic retention data R F and R M 0 of some benzimidazole and benztriazole derivatives have been estimated by reversed-phase thin-layer chromatography on paraffin oil-impregnated silica gel plates with methanol-water mixtures as mobile phases. A quantitative structure-retention relationship (QSRR) correlation study was performed on a matrix containing retention data derived from chromatographic regression, retention scores provided by principal-components analysis (PCA), and different computed molecular descriptors. By means of multiple regression analysis statistically significant equations relating lipophilicity (estimated as R M 0 values) to different descriptors were derived for the sixteen compounds.

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The chromatographic behavior of newly synthesized para -substituted propanoic acid amides has been studied in normal- and reversed-phase thin-layer chromatography. The retention mechanism on different TLC supports was investigated. The retention constants determined for the amides are discussed in terms of the physicochemical properties of the solute and the stationary and mobile phases. R M,w , derived by extrapolation from retention in reversed-phase chromatography, was correlated with hydrophobic parameters π and log P , calculated by the Rekker fragmental method and by use of the commercial program ACD/log P .

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The chromatographic behavior and mechanism of retention of some N -phenylsuccinimide derivatives has been studied. Retention constants were determined for normal- and reversed-phase TLC on silica gel and on silica gel impregnated with paraffin oil. Two lipophilic constants C 0 (calculated from normal-phase chromatography data) and R M,w (extrapolated RPC retention to 100% water as mobile phase) were derived, and partition coefficients between n -octanol and water, log P , were calculated by use of two different procedures — use of ACD/logP software and use of the interactive analysis log P predictor web site. Correlation between chromatographic lipophilic constants ( C 0 or R M,w ) and log P values calculated by use of the different procedures has been studied. Results show that both R M,w and C 0 can be used as a measure of the lipophilicity of the compounds investigated.

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Modeling properties of newly synthesized compounds is important for both understanding their activity and predicting their interactions. This paper describes the evaluation of the lipophilicity of the newly synthesized cycloalkylspiro-5-hydantoins by experimental and calculation methods. The chromatographic lipophilicity (R M 0) of the analyzed compounds was determined by reversed-phase liquid chromatographic system consisting of RP-18 stationary phase and methanol—water mobile phase. Correlation coefficients between R M 0 values and the predicted log P were above 0.93, indicating a strong linear relationship between the variables. Multivariate data analysis applied in this study enabled the comparison of the lipophilicity and the structure of the investigated compounds using solely information obtained from thin-layer chromatography. Hierarchical clustering analysis (HCA) reveals a high similarity between R M 0 and miLogP whereas squared log Ps such as ALOGP2 and MLOGP2 were distinct from R M 0. HCA classifies the investigated compounds into three clusters according to lipophilicity. Principal component analysis (PCA) yields two principal components which explained >99.19% total variance. The changes in the lipophilicity of the substituents attached in the molecule are reflected in the value of PCs. An increase in the lipophilicity of spiro substituent results in a decrease of values of both PC1 and PC2. The increase in the lipophilicity of R substituent decreases the value of PC1 and increases the value of PC2. The positive value of PC1 is typical for the compounds with the electron withdrawing groups such as CN, NO 2 , or OCH 3 .

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Quantitative structure-retention relationships (QSRR) have been used to study the chromatographic behavior of some s -triazines. Retention factors, R M 0 , on C 18 layers corresponding to zero percent organic modifier in the aqueous mobile phase were determined for five mobile phase mixtures: methanol-water, acetone-water, acetonitrile-water, 2-propanol-water, and tetrahydrofuran-water and relationships between R M 0 values obtained with different organic mobile phase modifiers were examined.A variety of partition coefficients ( A log P , IA log P , C log P , X log P , log P Kowin , and ACD log P ) were calculated by use of different software products. The correlation between partition coefficients and chromatographically obtained lipophilicity was analyzed. On the basis of correlations between R M 0 and log P , C 18 with methanol-water as mobile phase was selected as the best RP HPTLC system for determination of the octanol/water partition coefficient and thus the lipophilicity of the molecules.

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