The lipophilicity of parabens estimated on reverse phases chemically bonded and oil‐impregnated plates and calculated using different computation methods

The chromatographic behaviour of the parabens has been investigated on RP‐18F_254S, RP‐18WF_254S, CNF_254S, Diol F_254s and silica gel 60F₂₅₄ plates impregnated with different oils (paraffin, olive, sunflower and corn) using methanol–water mixtures in different volume proportions as mobile phases, the regression determination coefficients being excellent (higher than 0.98 for the majority of compounds). Moreover, highly significant correlations were obtained between different experimental indices of lipophilicity (R_M0, b and scores corresponding to the first principal component (PC1)) and computed log P values. All types of stationary phases investigated appear to be highly suited for estimating the lipophilicity of the parabens.     

Micellar liquid chromatography for lipophilicity determination of new biologically active 1,3‐purinodiones

A series of newly synthesized 1,3‐purinodiones with potential anticonvulsant activity, exhibiting affinity to adenosine A₁ and/or A_2A receptors, were subjected to micellar LC (MLC) with SDS as micelle‐forming agent and n‐propanol as organic modifier. Two C18 silica‐based columns were employed in MLC: a particle one and a monolithic. In parallel, those derivatives were also analyzed in RP‐LC on four silica‐based columns and on an immobilized artificial membrane column. The correlations between the relevant logarithms of the retention factors of analytes obtained in MLC, immobilized artificial membrane and RP‐LC systems on the one hand, and the calculated log P (clog P) and log D values (clog D) on the other, were examined. The level of the correlations of retention data from MLC and RP‐LC systems with clog P and clog D obtained is similar but it could be stressed that MLC allows increasing the speed of analysis and using only one mobile phase. Moreover, there is no need of applying an extrapolation procedure in lipophilicity determination. Therefore, the MLC systems, providing chromatographic data in a fast and efficient manner, were demonstrated as promising alternatives to the classical RP‐LC systems to estimate the lipophilicity of drugs and drug candidates.     

Use of a database of plots of pesticide retention (R F) against mobile-phase composition.Part I. Correlation of pesticide retention data in normal- and reversed-phase systems and their use to separate a mixture of ten pesticides by 2D TLC

Summary Ten pesticides have been completely separated by two-dimensional (2D) development on TLC plates coated with coupled layers of octadecyl silica (reversed-phase, RP) and plain silica (normal-phase, NP). The binary mobile phases, aqueous-organic for RP chromatography and nonaqueous for NP chromatography, were chosen from plots ofR _F against mobile-phase composition and graphicalR _F(RP)-R _F(NP) correlations. The different selectivity of the RP and NP systems enabled dispersion of spots over the plate area and good separation.     

A Comparative Study of the Lipophilicity of Metformin and Phenformin

The results presented in this paper confirm the beneficial role of an easy-to-use and low-cost thin-layer chromatography (TLC) technique for describing the retention behavior and the experimental lipophilicity parameter of two biguanide derivatives, metformin and phenformin, in both normal-phase (NP) and reversed-phase (RP) TLC systems. The retention parameters (R_F, R_M) obtained under different chromatographic conditions, i.e., various stationary and mobile phases in the NP-TLC and RP-TLC systems, were used to determine the lipophilicity parameter (R_MW) of metformin and phenformin. This study confirms the poor lipophilicity of both metformin and phenformin. It can be stated that the optimization of chromatographic conditions, i.e., the kind of stationary phase and the composition of mobile phase, was needed to obtain the reliable value of the chromatographic lipophilicity parameter (R_MW) in this study. The fewer differences in the R_MW values of both biguanide derivatives were ensured by the RP-TLC system composed of RP2, RP18, and RP18W plates and the mixture composed of methanol, propan-1-ol, and acetonitrile as an organic modifier compared to the NP-TLC analysis. The new calculation procedures for logP of drugs based on topological indices ⁰χ^ν, ⁰χ, ¹χ^ν, M, and M^ν may be a certain alternative to other algorithms as well as the TLC procedure performed under optimized chromatographic conditions. The knowledge of different lipophilicity parameters of the studied biguanides can be useful in the future design of novel and more therapeutically effective metformin and phenformin formulations for antidiabetic and possible anticancer treatment. Moreover, the topological indices presented in this work may be further used in the QSAR study of the examined biguanides.     

Optimization of chromatographic systems for the high-performance thin-layer chromatography of flavonoids

Summary To characterize the retention and selectivity of separations of 23 flavonoids (aglycones and glycosides) relationships betweenR _F and modifier concentration were determined for silica and diol adsorbents (with mixtures of ethyl acetate and methanol as mobile phases), for cyanopropyl silica (with mixtures of ethyl acetate and dichloromethane as mobile phases), for aminopropyl silica (with mixtures of ethyl acetate, methanol and water as mobile phases) and for octadecyl silica (with mixtures of methanol and water as mobile phases). Owing to large polarity differences between aglycones and glycosides, these groups of compounds cannot be separated other than by use of reversed-phase systems, for which the selectivity is lower. It follows from correlation plots ofR _F1 againstR _F2 that for some pairs of adsorbents (e. g. silica and diol) selectivity differences are small; for others the points in the plot are widely dispersed, indicating selectivity differences. The chemometric database obtained can be used to choose optimum chromatographic systems for the separation of given sets of flavonoids and for planning gradient elution programs for separation of flavonoid aglycones and glycosides in a single TLC experiment.     

Zirconia—A stationary phase capable of the separation of polar markers of myocardial metabolism in hydrophilic interaction chromatography

Creatine, phosphocreatine, and adenine nucleotides are highly polar markers of myocardial metabolism that are poorly retained on RP silica sorbents. Zirconia represents an alternative material to silica with high promise to be used in hydrophilic interaction chromatography (HILIC). This study describes a first systematic investigation of the ability of ZrO₂ to separate creatine, phosphocreatine, adenosine 5′‐monophosphate, adenosine 5′‐diphosphate, and adenosine 5′‐triphosphate and compares the results with those obtained on TiO₂. All analytes showed a HILIC‐like retention pattern when mobile phases of different strengths were tested. Stronger retention and better column performance were achieved in organic‐rich mobile phases as compared to aqueous conditions, where poor retention and insufficient column performance were observed. The effect of mobile phase pH and ionic strength was evaluated as well. The analysis of myocardial tissue demonstrated that all compounds were separated in a relevant biological material and thus proved ZrO₂ as a promising phase for HILIC of biological samples that deserves further investigation.     

Evaluation of N-Substituted 2,4-Dihydroxyphenylthioamide Fungicide Lipophilicity Using the Chromatographic Techniques HPLC and HPTLC

2,4-Dihydroxyphenylthioamide derivatives modified on the N-aryl ring have substantial fungicidal activity. To determine their quantitative structure–activity relationships their lipophilicity was determined by use of the chromatographic methods column liquid chromatography and thin-layer chromatography. Methanol–water systems were used as mobile phases and the linear dependences of retention (R_M and log k) on volume fraction of organic modifier, φ, were determined. This enabled precise determination of lipophilicity (R_Mw and log k_w) by extrapolation. Correlations were found between quantities characterizing the lipophilicity of the compounds. Deviations enabled discovery of compound structural features which increase or reduce lipophilicity. When these data were correlated with biological activity against the phytopathogenic fungi Alternaria alternata and Botrytis cinerea parabolic dependences were obtained.     

Estimating the lipophilicity of a number of 2‐amino‐1‐cyclohexanol derivatives exhibiting anticonvulsant activity

The lipophilicity of a number of N‐acyl derivatives of trans‐ or cis‐: racemic, (1R,2R)‐ or (1S,2S)‐aminocyclohexanol (1–13) exhibiting anticonvulsant activity was investigated. Their lipophilicity (R_{m 0}) was determined using reversed‐phase thin‐layer chromatography (RP‐TLC) with mixtures of methanol and water as mobile phases. The partition coefficients of compounds 1–13 (log P) were also calculated using two computer programs (Pallas and Chem DU) and compared with R_{m 0}. Copyright © 2009 John Wiley & Sons, Ltd.     

Insights into the Retention Mechanisms on Perfluorohexylpropylsiloxane-Bonded (Fluophase-RP) and Octadecylsiloxane-Bonded (Betasil C18) Stationary Phases Based on the Same Silica Substrate in RP-LC

The solvation parameter model is used to elucidate the retention mechanism on a perfluorohexylpropylsiloxane-bonded (Fluophase RP) and octadecylsiloxane-bonded (Betasil C18) stationary phases based on the same silica substrate with acetonitrile–water and methanol–water mobile phase compositions. Dewetting affects the retention properties of Fluophase RP at mobile phase compositions containing less than 20% (v/v) acetonitrile or 40% (v/v) methanol. It results in a loss of retention due to an unfavorable change in the phase ratio as well as changes in specific intermolecular interactions. Steric repulsion reduces retention of bulky solutes on fully solvated Betasil C18 with methanol–water (but not acetonitrile–water) mobile phase compositions but is not important for Fluophase RP. The retention of weak bases is affected by ion-exchange interactions on Fluophase RP with acetonitrile–water, and to a lesser extent, methanol-water mobile phases but these are weak at best for Betasil C18. The system constants of the solvation parameter model and retention factor scatter plots are used to compare selectivity differences for Fluophase RP, Betasil C18 and a perfluorophenylpropylsiloxane-bonded silica stationary phase Discovery HS F5 for conditions where incomplete solvation, steric repulsion and ion-exchange do not significantly contribute to the retention mechanism. Lower retention on Fluophase RP results from weaker dispersion and/or higher cohesion moderated to different extents by polar interactions since solvated Fluophase RP is a stronger hydrogen-bond acid and more dipolar/polarizable than Betasil C18. Retention factors for acetonitrile–water mobile phases are highly correlated for Fluophase RP and Betasil C18 except for compounds with a large excess molar refraction and weak hydrogen-bonding capability. Selectivity differences are more significant for methanol–water mobile phases. Retention factors on Fluophase RP are strongly correlated with those on Discovery HSF5 for acetonitrile–water mobile phases while methanol–water mobile phases retention on Fluophase RP is a poor predictor of the retention order on Discovery HS F5.     

Drug–Membrane Interaction on Immobilized Liposome Chromatography Compared to Immobilized Artificial Membrane (IAM), Liposome/Water,and Octan‐1‐ol/Water Systems

The objective of this study was to investigate drug–membrane interaction by immobilized liposome chromatography (ILC; expressed as lipophilicity index log K_s) and the comparison with lipophilicity indices obtained by liposome/H₂O, octan‐1‐ol/H₂O, and immobilized artificial membrane (IAM) systems. A set of structurally diverse monofunctional compounds and drugs (nonsteroidal anti‐inflammatory drugs and β‐blockers) were selected in this study. This set of solutes consists of basic or acidic functionalities which are positively or negatively charged at physiological pH 7.4. No correlation was found between log K_s from ILC and lipophilicity indices from any of the other membrane model systems for the whole set of compounds. For structurally related compounds, significant correlations could be established between log K_s from ILC and lipophilicity indices from IAM chromatography and octan‐1‐ol/H₂O. However, ILC and liposome/H₂O systems only yield parallel partitioning information for structurally related large molecules. For hydrophilic compounds, the balance between electrostatic and hydrophobic interactions dominating drug partitioning is different in these two systems.     

Survey and Trends in the Preparation of Chemically Bonded Silica Phases for Liquid Chromatographic Analysis

In order to increase chromatographic selectivity and to extend the analytical capability of reversed phase liquid chromatography (RP HPLC) many investigators have concentrated on the preparation of silica based column packings with chemically bonded phases (CBP). These phases have also been successfully used in sample preparation techniques, mainly in solid phase extraction (SPE). Although alkyl bonded phases (e.g., C₂, C₈, and C₁₈) are the most widely used packings in RP HPLC and SPE, various specific applications require CBPs with polar functional groups (e.g., -NH₂, -NO₂, -CN, and/or -OH). The solution of problems with separation of complicated chiral compounds was attempted by applying stationary phases with chiral selectors (e.g., cyclodextrins, Pirkle phases, crown ethers, etc.). On the other hand, packings with pseudo-membrane or liquid crystal properties have been utilized for the separation of various substances of natural origin. Porous silica is commonly used as a support in the preparation of CBPs. Its physico-chemical characteristics, such as: type and structure of siliceous matrix, porosity, type and concentration of silanol groups, as well as surface purity, strongly influence the density and structure of chemically bonded phases. Recognition of these properties is helpful in optimizing separation processes based on RP HPLC elution and/or extraction of substances with polar character.     

Octanol/water partitioning simulation by RP‐HPLC for structurally diverse acidic drugs: Comparison of three columns in the presence and absence of n‐octanol as the mobile phase additive

The advantageous effect of n‐octanol as a mobile phase additive for lipophilicity assessment of structurally diverse acidic drugs both in the neutral and ionized form was explored. Two RP C₁₈ columns, ABZ⁺ and Aquasil, were used for the determination of logk_w indices, and the results were compared with those previously reported on a base‐deactivated silica column. At pH 2.5, the use of n‐octanol‐saturated buffer as the mobile phase aqueous component led to high‐quality 1:1 correlation between logk_w and logP for the ABZ⁺ column, while inferior statistics were obtained for Aquasil. At physiological pH, the correlations were significantly improved if strongly ionized acidic drugs were treated separately from weakly ionized ones. In the latter case, 1:1 correlations between logD_7.4 and logk_w^oct indices were obtained in the presence of 0.25% n‐octanol. Concerning strongly ionized compounds, adequate correlations were established under the same conditions; however, slopes were significantly lower than unity, while large negative intercepts were obtained. According to the absolute difference (diff = logD_7.4–logk_w) pattern, base‐deactivated silica showed a better performance than ABZ⁺, however, the latter seems more efficient for the lipophilicity assessment of highly lipophilic acidic compounds. Aquasil may be the column of choice if logD_7.4<3 with the limitation, however, that very hydrophilic compounds cannot be measured.     

Stationary and mobile phases in hydrophilic interaction chromatography: a review

Hydrophilic interaction chromatography (HILIC) is valuable alternative to reversed-phase liquid chromatography separations of polar, weakly acidic or basic samples. In principle, this separation mode can be characterized as normal-phase chromatography on polar columns in aqueous-organic mobile phases rich in organic solvents (usually acetonitrile). Highly organic HILIC mobile phases usually enhance ionization in the electrospray ion source of a mass spectrometer, in comparison to mobile phases with higher concentrations of water generally used in reversed-phase (RP) LC separations of polar or ionic compounds, which is another reason for increasing popularity of this technique. Various columns can be used in the HILIC mode for separations of peptides, proteins, oligosaccharides, drugs, metabolites and various natural compounds: bare silica gel, silica-based amino-, amido-, cyano-, carbamate-, diol-, polyol-, zwitterionic sulfobetaine, or poly(2-sulphoethyl aspartamide) and other polar stationary phases chemically bonded on silica gel support, but also ion exchangers or zwitterionic materials showing combined HILIC-ion interaction retention mechanism. Some stationary phases are designed to enhance the mixed-mode retention character. Many polar columns show some contributions of reversed phase (hydrophobic) separation mechanism, depending on the composition of the mobile phase, which can be tuned to suit specific separation problems. Because the separation selectivity in the HILIC mode is complementary to that in reversed-phase and other modes, combinations of the HILIC, RP and other systems are attractive for two-dimensional applications. This review deals with recent advances in the development of HILIC phase separation systems with special attention to the properties of stationary phases. The effects of the mobile phase, of sample structure and of temperature on separation are addressed, too.     

Quantitative Determination of Four Water-Soluble Compounds in Herbal Drugs from Lamiaceae Using Different Chromatographic Techniques

HPTLC-densitometric and HPLC–UV techniques were used for qualitative and quantitative determination of luteolin-7-O-glucuronide, lithospermic acid, rosmarinic acid and caffeic acid in several herbal drugs from the Lamiaceae family: Thymi herba, Serpylli herba, Majoranae herba and Menthae piperitae folium. Unmodified silica gel (HPTLC Si60) and silica gel chemically modified with aminopropyl groups (HPTLC NH₂) were used during the investigation process. Among HPTLC methods the best resolution and selectivity was achieved with mobile phases: diisopropyl ether–acetone–formic acid–water (50:30:10:10, v/v/v/v) and acetone–formic acid (85:15, v/v), respectively. Plates were densitometrically evaluated. Contents of analyzed compounds in the studied aqueous extracts prepared from herbal drugs were established using both techniques. The results from the HPTLC-densitometric analysis have been compared with those from HPLC–UV on a C18 column with acetonitrile–water–formic acid as a mobile phase. The chromatographic methods were validated for linearity, LOD, LOQ, repeatability, intermediate precision and recovery. An analysis of variance showed that the HPTLC-densitometric and HPLC–UV methods are equivalent and sufficiently precise for the estimation of polyphenolic compounds mentioned above, in investigated herbal drugs. All of the suggested methods (HPTLC NH₂, HPTLC Si60 and HPLC RP18) give results with good agreement.     

Effect of analyte lipophilicity on the resolution of α‐ and β‐amino acids on liquid chromatographic ligand exchange chiral stationary phases

Separation of the two enantiomers of racemic α‐ and β‐amino acids on two ligand exchange chiral stationary phases (CSPs) prepared previously by covalently bonding sodium N‐((S)‐1‐hydroxymethy‐3‐methylbutyl)‐N‐undecylaminoacetate or sodium N‐((R)‐2‐hydroxy‐1‐phenylethyl)‐N‐undecylaminoacetate on silica gel was studied with variation of the organic modifier (methanol) concentration in the aqueous mobile phase. In particular, the variation of retention factors with changing organic modifier concentration in the aqueous mobile phase was found to be strongly dependent on both the analyte lipophilicity and the stationary phase lipophilicity. In general, the retention factors of relatively lipophilic analytes on relatively lipophilic CSPs tend to increase with increasing organic modifier concentration in the aqueous mobile phases while those of less lipophilic or hydrophilic analytes tend to increase. However, only highly lipophilic analytes show decreasing retention factors with increasing organic modifier concentration in the aqueous mobile phase on less lipophilic CSPs. The contrasting retention behaviors on the two CSPs were rationalized by the balance of the two competing interactions, viz. hydrophilic interaction of analytes with polar aqueous mobile phase and the lipophilic interaction of analytes with the stationary phase.     

Chromatographic investigations of adsorption of some aromatic compounds on aluminium oxide and silica gel from solutions

Summary Using adsorption TLC R_M values were measured for a number of aromatic compounds on aluminium oxide and silica gels having different specific surface areas, using the following binary mobile phases: benzene+toluene, benzene+carbon tetrachloride and chloroform+carbon tetrachloride. The results are graphically presented and compared with theoretically calculated values. Generally, a good agreement was found between the calculated and measured R_M values. The agreement is valid both when the R_M values were calculated using experimentally (indirectly) determined partition coefficients or when the coefficients are obtained with help of Ocik's equation [cf. Roczn. Chem.34, 745 (1960) and Chromatographia4, 516 (1971)]. One can assume that in the case of systems in which strong intermolecular interactions are absent, the statically determined partition coefficients of the compounds may be used for the calculation of their R_M values.     

Preparation and separation of mixed-ligand Fe(II) complexes containing 1,10-phenanthroline-5,6-dione as ligand

The synthesis, separation, and characterization of mixed-ligand iron(II) complexes containing 1,10-phenanthroline (phen), 1,10-phenanthroline-5,6-dione (pdon), and NCS^? are reported. The mixed-ligand complexes [Fe(phen)(pdon)₂]²⁺ and [Fe(phen)₂(pdon)]²⁺ were prepared from iron(II) sulfate hepta hydrate and both ligands. The mixture of both complexes formed regardless the ratio of the ligands or the reaction time; therefore, the complexes were separated successfully on the reversed phase (RP) Develosil RP-Aqueous [C30] 5?µm, 150?×?4.6?mm column by two different methods. The first method was the ion paired RP chromatography performed under gradient elution with acetonitrile–water containing 0.001?mol?L^?1 KPF₆ aqueous as mobile phases. The second method was the RP chromatography performed under gradient elution with methanol and water as mobile phases. The gradient elution with water–methanol as eluents was preferred for the semi preparative separations allowing one to use the complexes without further purification upon separation, different than the first method and its variations so far. Three complexes (5, 6, and 7) were characterized by electrospray ionization mass spectrometry, NMR, UV-Vis, and IR.     

A Study of the Relative Importance of Lipophilic, π–π and Dipole–Dipole Interactions on Cyanopropyl, Phenyl and Alkyl LC Phases Bonded onto the Same Base Silica

Cyano (CN), butyl (C₄), phenyl and octadecyl (C₁₈) phases prepared from the same base silica gel were chromatographically characterized in order to assess the relative importance of lipophilic, π–π and dipole–dipole interactions in governing retention on these differing phases. Dipole interactions of analytes (possessing dipole moments and low lipophilicity) with CN phases were primarily responsible for the elution order. However, as the analytes’ lipophilicity increased, the lipophilic interaction predominated over the dipole interaction. In comparison, retention on the phenyl phase appeared to be complex, being controlled by a mixture of lipophilic, π–π and dipole–dipole interactions. Retention on the C₄ and C₁₈ phases was dictated by the analyte’s lipophilicity and its accessibility into the phase.