Tatjana Djaković-Sekulić

HPTLC chromatography of androstene derivates: Application of normal phase thin-layer chromatographic retention data in QSAR studies

The chromatographic behavior of seven 16-oximino derivatives of 3beta-hydropxy-5-androstene have been investigated using the normal-phase (NP) HPTLC chromatographic mode of the type silica-non-polar diluent (benzene)-polar modifier (acetonitrile, ethyl acetate, or dioxane). The linear relationship between the retention constants (R(M)) and the logarithm of the organic modifier content in the mobile phase allowed for the calculation of R(M)0 values. The influence of substituent in the molecule on extrapolated retention data is discussed. To better understand the retention mechanism in the separation of androstene compounds, the functional group contributions (tauX) were compared with Hansch substituent constants (pi). An attempt to quantitate the lipophilicity of the investigated compounds using normal phase thin-layer chromatographic R(M)0 value was made. Also, the relative lipophilicity values determined previously by RPC as well as activity were compared with NPC data.

Evaluation of the predictive power of calculation procedure for molecular hydrophobicity of some estradiol derivates.

Several calculation procedures for log P values based on the fragmental and atomic contributions are compared with experimental reversed-phase liquid chromatography (RPLC) retention of estradiol derivates. The RPLC experiments were performed on HPTLC and HPLC commercially available stationary phases. Binary solvent mixtures of methanol-water and acetonitrile-water were used as mobile phases. The correlation between log P and various chromatographically obtained hydrophobicity parameters (R(M)0, log k(w) and phi0) are quantified. The R(M)0, i.e., log k(w) were obtained by linear extrapolation of retention to 0% organic modifier. Phi0 values were obtained from the slopes and intercepts of such linear relationship. The mutual relationship between phi(0,MeOH) and phi(0,ACN) values of the compounds were discussed. The obtained statistical results can be summarized in the following order of reliabilities for different log P calculation methods: Broto>ACD/logP>Crippen>Rekker>Viswanadhan.

Study of quantitative structure—retention relationships for s -triazine derivatives in different RP HPTLC systems

Quantitative structure-retention relationships (QSRR) have been used to study the chromatographic behavior of some s-triazines. Retention factors, RM0, on C18 layers corresponding to zero percent organic modifier in the aqueous mobile phase were determined for five mobile phase mixtures: methanol-water, acetone-water, ace-tonitrile-water, 2-propanol-water, and tetrahydrofuran-water and relationships between RM0 values obtained with different organic mobile phase modifiers were examined. A variety of partition coefficients (Alog P, IAlog P, Clog P, Xlog P, log PKowin, ACDlog 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 RM0 and log P, C18 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.

A comparative study of the lipophilicity of benzimidazole and benztriazole derivatives by RPTLC

Chromatographic retention data RF and RM0 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 RM0 values) to different descriptors were derived for the sixteen compounds.

Normal- and Reversed-Phase Chromatography of para-Substituted Propanoic Acid Amides

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. RM,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.

Study of the Characteristics and Separating Power of Unconventional TLC Supports. II. Principal-Components Analysis

Chromatographic retention data have been studied for the 3,5-dinitrobenzoic acid esters of a homologous series of aliphatic C1-C20 linear alcohols on five unconventional TLC stationary phases rice starch, microcrystalline cellulose, aminoplast, talc, and paraffin oilimpregnated 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.

Influence of the mobile-phase modifier on the retention of some para -substituted amides; correlation with compound lipophilicity

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. RM0 values were calculated by linear extrapolation of retention to 0% organic modifier for each mobile phase. The relationships between RM0 values obtained with different organic modifiers in the mobile phases are discussed. C0 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 RM0 and between log P and C0 was examined. Results show that C0 is a better measure of the lipophilicity of the amides than RM0.

Study of the characteristics and separation properties of unconventional TLC supports. Part I

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, RS, and ΔRF values. Irregular changes of the retention factors with increasing number of methylene groups of the compounds were sometimes observed. Linear correlations were obtained between RM and concentration of dioxane in the mobile phase.

Effect of the stationary phase and the mobile-phase modifier on the retention of some thiazoles. Correlation with the lipophilicity of the compounds

The chromatographic behavior of 12 newly synthesized thiazoles has been studied by thin-layer chromatography. The relationship between the RF values of the thiazoles and mobile-phase composition has been determined for normal-phase (NP) systems of the type silica—non-polar diluent (hexane)—polar modifier (ethyl acetate, acetone, or dioxane), for reversed-phase (RP) systems of the type paraffin oil-impregnated silica—concentrated ammonia—polar modifier (methanol, acetone, or dioxane), and for rice starch—concentrated ammonia—polar modifier (methanol, acetone, or dioxane). The retention observed for different mobile-phase modifiers and different stationary phases was compared. The correlation between the retention of the compounds investigated in normal- and reversed-phase liquid chromatography and their octanol—water partition coefficients expressed as log P (which ranged between 2.33 and 5.94), was examined.

Partial Least-Squares Study of the Effects of Organic Modifier and Physicochemical Properties on the Retention of Some Thiazoles

Reversed-phase (RP) HPTLC with aqueous ammonia-organic modifier (acetonitrile, dioxane, acetone) mobile phases has been used to study the effect on retention of the chromatographic system and the physicochemical properties of twelve 2,4-dioxotetrahydro-1,3-thiazoles. A multivariate approach to the retention behavior of these 1,3-thiazoles with three quite different organic solvents was used to explain the interactions between the analytes and the mobile phases. Principal-component analysis (PCA) and partial least-squares (PLS) regression were used to determine the molecular properties with the greatest effect on retention for each modifier. Good correlation was obtained between experimental and calculated retention data.