Hydrogen bonding

Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

Response to "comment on 'structural determinants of drug partitioning in surrogates of phosphatidylcholine bilayer strata'"

We used the solvatochromic correlation to explain the influence of characteristics of studied compounds on the partition coefficients (P) measured using n-hexadecane (C16) and the novel headgroup surrogate (diacetyl phosphatidylcholine, DAcPC), and compared them with those in other systems, including the C16/water (W) system. The comment analyzes why our correlation for the C16/W system has the standard deviation (SD) higher than that published previously. The main reason is that in our, much smaller, data set the measured P values are complemented by the P values predicted by a reliable, unrelated method. We believe that this approach is acceptable for the aforementioned comparison. We did not use just experimental values, as suggested in the comment, because the solvatochromic correlation, although exhibiting 35% reduction in the SD, was accompanied by a sign change of one of the regression coefficients. The recommended use of special solvatochromic solute characteristics for a few compounds and replacement of a predicted PC16/W value by the experimental value resulted in improved correlations. The observed differences between our correlation and those published in the comment and in a previous article do not affect our main conclusions regarding the solvation of solutes in the surrogates (DAcPC and C16) of intrabilayer strata.

Linear Solvation Energy Relationships in the Determination of Specificity and Selectivity of Stationary Phases

The retention of fifty structurally different compounds has been studied using linear solvation energy relationships. Investigations were performed with the use of six various stationary phases with two mobile phases (50/50 % v/v methanol/water and 50/50 % v/v acetonitrile/water). Packing materials were home-made and functionalized with octadecyl, alkylamide, cholesterol, alkyl-phosphate and phenyl molecules. This is the first attempt to compare all of these stationary phases synthesized on the same silica gel batch. Therefore, all of them may be compared in more complex and believable way, than it was performed earlier in former investigations. The phase properties (based on Abraham model) were used to the classification of stationary phases according to their interaction properties. The hydrophilic system properties s, a, b indicate stronger interactions between solute and mobile phase for most of the columns. Both e and v cause greater retention as a consequence of preferable interactions with stationary phase by electron pairs and cavity formation as well as hydrophobic bonds. However, alkyl-phosphate phase has different retention properties, as it was expressed by positive sign of s coefficient. It may be concluded that most important parameters influencing the retention of compounds are volume and hydrogen bond acceptor basicity. The LSER coefficients showed also the dependency on the type of organic modifier used as a mobile phase component.

Molecular probes of solvation phenomena

The properties of the molecules present in any chemical or biological system are dependent on interactions with the environment, and a quantitative understanding of solvation phenomena remains a major challenge. Molecular recognition probes provide a new approach to quantitatively measure the properties of solvents. Traditionally, solvent polarity scales have been based on spectroscopic probes that provide insight into the nature of solvent-solute interactions. This review compares the solvent polarity parameters obtained from the wavelengths of UV/Visible absorption maxima with solute H-bond parameters obtained from the free energies of solution equilibria. The similarity of the solvent and solute H-bond scales leads to a general H-bond scale that uses the same parameters to describe both solvent and solute. The general H-bond scale provides a framework for understanding the relationship between local intermolecular interactions and the properties of the bulk medium. Intermolecular interactions are sensitive to solvation equilibria, so molecular recognition probes provide fundamentally different information from spectroscopic probes that are sensitive to the populations of different solvation states of the solute. Studies of mixed solvents demonstrate the potential of molecular recognition probes for providing new insights into solvation phenomena.

Separation characteristics of wall-coated open-tubular columns for gas chromatography

The application of the solvation parameter model for the classification of wall-coated open-tubular columns for gas chromatography is reviewed. A system constants database for 50 wall-coated open-tubular columns at five equally spaced temperatures between 60 and 140 degrees C is constructed and statistical and chemometric techniques used to identify stationary phases with equivalent selectivity, the effect of monomer chemistry on selectivity, and the selection of stationary phases for method development. The system constants database contains examples of virtually all commercially available common stationary phases.

Determination of descriptors for organosilicon compounds by gas chromatography and non-aqueous liquid–liquid partitioning

Measurements of retention factors by gas chromatography on up to 10 complementary stationary phases at up to 5 temperatures for each stationary phase and liquid-liquid partition coefficients in three biphasic organic solvent systems (n-hexane-acetonitrile, n-heptane-N,N-dimethylformamide and n-heptane-2,2,2-trifluoroethanol) were used to estimate solute descriptors for 54 organosilicon compounds for use in the solvation parameter model. Many of the E descriptor values (electron lone pair interactions) are negative for simple siloxanes and silanes indicating that these compound bind electron lone pairs more tightly than n-alkanes. Silanes and siloxanes with alkyl groups have near zero dipolarity/polarizability (S descriptor). The S descriptor is only modest for simple phenylsilanes, silazanes, silanols, orthosilicates, and alkoxides. All organosilicon compounds with silicon-oxygen bonds are reasonably strong hydrogen-bond bases (B descriptor) but only the silanol group is a reasonably strong hydrogen-bond acid (A descriptor). Silanes (SiH) and silazanes (SiNHSi) are weak hydrogen-bond acids. Cavity formation and dispersion interactions (V or L descriptor) are often the main component of solvation models for siloxanes and silanes that have simple alkyl and aromatic substituents. A number of physicochemical properties (vapor pressure, aqueous solubility, biphasic partition coefficients, sorption coefficients, etc.) for linear and cyclic dimethylsiloxanes can be reliably predicted from their descriptors in established models for organic compounds.

Revised solute descriptors for characterizing retention properties of open-tubular columns in gas chromatography and their application to a carborane–siloxane copolymer stationary phase

An iteration procedure is used to calculate revised solute descriptors for 103 varied compounds suitable for characterizing the retention properties of stationary phases for gas chromatography using the solvation parameter model. The iteration procedure utilizes a database of retention factors obtained on up to 39 open-tubular columns and up to five temperatures in the range 60-140 degrees C for the 103 solutes. The average of the standard deviation [Sigma(logk(exp)-logk(calc))(2)/(n(c)-1)](0.5) where logk(exp) is the experimental retention factor, logk(calc) the model predicted retention factor, and n(c) the total number of retention factors) on all columns is 0.018 for the revised solute descriptors compared with 0.045 for the original values. When used to characterize the retention properties of six open-tubular columns selected to represent different selectivity groups the revised solute descriptors afford improved values for the multiple correlation coefficient and standard deviations of the system constants, and about a three-fold improvement in the standard error of the estimate compared with the original solute descriptors. The revised solute descriptors were used to model retention on the carborane-siloxane copolymer stationary phase Stx-500. This phase has low cohesion, is weakly electron lone pair repulsive, weakly dipolar/polarizable, and weakly hydrogen-bond basic. It has no hydrogen-bond acidity. Its separation properties are similar to those of the poly(diphenyldimethylsiloxane) stationary phases containing 5% diphenylsiloxane monomer, but it is not selectivity equivalent to these phases, being more dipolar/polarizable and a weaker hydrogen-bond base.

Determination of gas–liquid partition coefficients by gas chromatography

This review covers theoretical principles and experimental procedures for the determination of gas-liquid partition coefficients, KL, by gas chromatography. In order to precisely define the relationship between KL, retention time and experimental parameters, the retention theory, both for ideal and for imperfect gas phase, is expounded. The most important sources of systematic error, as peak asymmetry, mixed retention mechanisms, column hold-up time and stationary phase mass determination, are discussed. Although the review is focussed on packed columns, comparison to capillary columns is discussed in those aspects in which these last show advantages.

Determination of sets of solute descriptors from chromatographic measurements

The use of gas-liquid chromatographic (GLC) retention data to obtain sets of solute descriptors is outlined, with reference to the schemes of Laffort and of Weckwerth. The method of Snyder and Dolan to obtain a set of solute descriptors from reverse phase high performance chromatographic (RP-HPLC) measurements is described. The work of Abraham on the construction of solvation parameters, or descriptors, from water-solvent partitions, GLC retention data and RP-HPLC data is considered in some detail. A comparison is made between the schemes of Laffort, Weckwerth and Abraham, and it is shown that the latter two yield exactly the same fits for a test data set of gas-methanol partition coefficients, although the distribution of chemical information amongst the terms in the multiple linear regressions is not quite the same. A comparison between the above 'experimental' descriptors and theoretical descriptors is made, and it is shown that the experimental Abraham and the theoretical Klamt descriptors encode almost the same chemical information. It is concluded that for processes that entail transfer of a solute from one phase to another, only a small number of solute descriptors, no more than five or six, is needed to provide a reasonably accurate analysis of the process.

Using temperature gradient gas chromatography to determine or predict vapor pressures and linear solvation energy relationship parameters of highly boiling organic compounds

An isothermal chromatographic method allowing determination of sigmabetaH2 and sigmaalphaH2 descriptors of the linear solvation energy relationship (LSER) was tested and results obtained are presented. This method is based on the use of four stationary phases of various polarity. On the other hand, it was demonstrated that the temperature gradient chromatography may be successfully used to determine LSER descriptors. Results of piH2, sigmabetaH2 and log L16 determination are reported. This approach opens new possibilities of precise and rapid determination of LSER descriptors of high boiling compounds using a small number of phases. It was demonstrated that the log L16 descriptor may be used to estimate vapor pressures of high boiling organic compounds with a better accuracy than those usually obtained with chromatographic methods.

Column selectivity from the perspective of the solvation parameter model

The solvation parameter model is a useful tool for delineating the contribution of defined intermolecular interactions to retention of neutral molecules in separation systems based on a solute equilibrium between a gas, liquid or fluid mobile phase and a liquid or solid stationary phase. The free energy for this process is decomposed into contributions for cavity formation and the set up of intermolecular interactions identified as dispersion, electron lone pair, dipole-type and hydrogen bonding. The relative contribution of these interactions is indicated by a series of system constants determined by the difference of the defined interaction in the two phases. The interpretation of these system constants as a function of experimental factors that affect retention in the chromatographic system provides the connection between relative retention (selectivity) and the control variables for the separation system. To aid in the understanding of these processes we perform an analysis of system constants for gas chromatography, liquid chromatography, supercritical fluid chromatography and micellar electrokinetic chromatography as a function of different experimental variables as a step towards gaining a theoretical understanding of selectivity optimization for method development.

Characterization of phosphorus-containing gas chromatographic stationary phases by linear solvation energy relationships

Linear solvation energy relationships allow the prediction of a variety of solubility interactions based on a set of descriptors found in the following equation: [equation: see text]. SP refers to an intrinsic thermodynamic property that can be found experimentally for a series of solutes. Phases containing phosphate, phosphite and phosphine functional groups were studied in this work. Coefficients obtained during this work, as well as those available for previously characterized phases, were correlated with molecular structural descriptors. When effects of non-phosphorus functional groups are estimated and subtracted out, hydrogen bond acceptor capability, a1, shows a positive trend when correlated with percent functional group. Correlation of the dipolarity/polarizability coefficient, s, with calculated atomic polarizability shows stationary phases group according to like functional groups. A similar correlation with dipole moment gives a trend of increasing dipole as s1 increases. Further quantitative structure-solubility relationship work is planned to better describe the contributions of inner shell and valence electrons to the chemical and physical properties of these compounds.

Prediction of internal standards in reversed-phase liquid chromatography. 1. Initial study on predicting internal standards for use with neutral samples based on linear solvation energy relationships

This paper describes the results of an initial study on the application of linear solvation energy relationships (LSERs) to the prediction of internal standard compounds in reversed-phase liquid chromatographic (RPLC) method development. Six neutral samples are separated on an Inertsil ODS(3) column by either acetonitrile-water or methanol-water mobile phases under either isocratic or linear gradient conditions. After the separation conditions are optimized, the desired positions for internal standard candidates are selected based on the "open windows" of the chromatograms. The compounds with the desired retention range are then predicted based on LSERs from a database consisting of more than 700 compounds with defined physicochemical properties. The prediction requires the use of LSER coefficients under the separation conditions for each sample. They are determined a priori by performing multivariable linear regression on the retention of 20 reference solutes against their physicochemical properties. It can be concluded from the study that LSER is an excellent approach to the selection of internal standard compounds for RPLC under either isocratic or gradient elution. The average prediction error is usually within 10%, but no more than 20%. Finally, LSER approach is fast and systematic, and will save a significant amount of time and resources during RPLC method development.

Experimental determination and prediction of the gas-liquid n-hexadecane partition coefficients

Experimental methods based on gas-phase chromatography were tested with a view to determine the gas-liquid n-hexadecane partition coefficients, log L16 of non-volatile compounds at 298.2 K. It was demonstrated that reliable values of log L16 of compounds more volatile than n-docosane can be obtained using either capillary, or packed columns. The main limitation of both methods is the column stability at high temperatures. Here we propose a new method based on the temperature gradient mode, to obtain log L16 of high-boiling compounds. A group contribution model is also presented in view to predicting log L16 values of non-volatile compounds.

Laterally attached liquid crystalline polymers as stationary phases in reversed-phase high-performance liquid chromatography. V. Study of retention mechanism using linear solvation energy relationships

A linear solvation energy relationship model was used to characterize the retention behavior of a stationary phase based upon a nematic side-on liquid crystalline polymer (SOLCP) in reversed-phase liquid chromatography. The set of solutes was constituted of a high variety of compounds whose molecular sizes were considerably smaller than the mesogenic unit size. The results showed good statistical fits for these retention data in 65:35, 75:25 and 85:15 (v/v) methanol-water mobile phases. Both the cavity term and excess molar refraction are the most important favorable retention-governing parameters, whereas the solute hydrogen bond acceptor basicity is the most unfavorable retention parameter. Hydrophobicity and pi-pi interactions decrease strongly when the percentage of methanol increases, leading to an important retention decrease despite the fact that the hydrogen bond interaction weakens as the organic solvent is added. The shape recognition ability of this side-on liquid crystalline stationary phase on polycyclic aromatic hydrocarbon solutes is partly explained by the solutes' high polarizability due to the presence of pi-electrons. However, the solute polarizability is not sufficient and a stationary phase's "structure effect" must to be taken into account for the shape discrimination observed. The strong interaction between liquid crystal molecules caused likely a adsorption retention mechanism rather than a partition mechanism.

Evaluation of the retention dependence on the physicochemical properties of solutes in reversed-phase liquid chromatographic linear gradient elution based on linear solvation energy relationships

This paper describes the results of the evaluation of retention dependence on the physicochemical properties of solutes in linear gradient elution by reversed-phase liquid chromatography (RPLC) based on linear solvation energy relationships (LSERs). Retention time data on Inertsil ODS(3) column by linear gradient elution were collected for both acetonitrile-water and methanol-water binary mobile phases under various gradient steepness. Based on the LSERs, the retention times were linearly correlated with the physicochemical properties (size, dipolarity, and hydrogen bond donor-acceptor acidity and basicity) of solutes. As predicted by LSERs, very acceptable linear relationships are observed for both mobile phases. While the magnitudes of the coefficients are modified by the gradient steepness, their signs are consistent with those obtained by isocratic elution. As obtained for isocratic elution, the dominant factors to retention in linear gradient elution of RPLC are the solutes' size and hydrogen bond acceptor basicity. The conclusions of the study allow us to predict retention in chromatographic method development by gradient elution.

Surfactant aggregates (solloids) adsorbed on silica as stationary chromatographic phases: structures and properties

The structure and physical properties of solloids (surfactant aggregates adsorbed on surfaces) adsorbed on particles are of general interest. The relationship between solloid structure and properties of hexadecyltrimethylammonium bromide (HTAB), cetylpyridinium chloride (CPC) and cetylpyridinium salicylate (CPS) adsorbed on silica particles was studied by electron paramagnetic resonance (EPR) spectroscopy using the spin-probes peroxylaminedisulfonate (PADS) and 4-[N,N-dimethyl-N-(n-hexadecyl)ammonium]-2,2,6,6-tetramethylpiperidin yl-N-oxy bromide (HTAB*). Using HTAB* incorporated in HTAB, CPC and CPC solloids and comparing the results to those in micelles, it was determined that for silica around pH 4 the solloids are very similar in properties to the micelles. This is consistent with a linear solvation-energy relationship (LSER) analysis of solute equilibration data which indicates that at pH 5 HTAB solloids have similar properties to HTAB micelles. The PADS spin-probe appears to be more sensitive to changes in the properties of the double layer, and substantial differences were observed between HTAB, CPC and CPS and as a function of HTAB concentration for HTAB solloids on silica.

Estimation of partitioning parameters of nonionic surfactants using calculated descriptors of molecular size, polarity, and hydrogen bonding

A study was carried out to verify the capacity of the Abraham's solute descriptors (R2, an excess molar refraction; pi 2H, the dipolarity/polarizability; sigma alpha 2H and sigma beta 2H, the summation hydrogen-bond acidity and basicity; and Vx, the characteristic volume of McGowan) to predict the alkane-water partition coefficient (log Palk) of nonionic surfactants. Log P values were taken from the literature and examined for linear solvation energy-related (LSER) equations with calculated physicochemical descriptors. A stepwise multiple linear regression (MLR) analysis allowed us to derive LSER models which, unlike those previously published for a standard set of solutes, revealed that R2 and sigma beta 2H are the major contributors to log Palk. A minor contribution of Vx was also detected, whereas surprisingly sigma alpha 2H required a positive coefficient. The less relevant size effect seems to indicate that for nonionic surfactants the energy needed for creating a water cavity is largely compensated for by the favorable interactions of ethoxy groups in the polar chain with water, whereas the log P-increasing effect of hydrogen-bond donor groups could be explained by considering the occurrence of folded, more lipophilic conformers, favored by the formation of intramolecular hydrogen bonds in the apolar phase. This hypothesis was substantiated by the calculation of the molecular lipophilicity potential on the water-accessible surface of extended and folded conformers of two representative surfactants, selected by high-temperature molecular dynamics. Besides MLR, a principal component analysis on a larger set of descriptors, comprising three solubility parameters (delta D, based on dispersion forces; delta H, including the contribution of hydrogen bonding; delta O, overall solubility parameter) afforded the so-called principal properties, which were able to characterize nonionic surfactants and to satisfactorily assess their lipophilicity-related properties.

Partition coefficients of indoles and betacarbolines

Partition coefficients for substituted indoles and betacarbolines were determined in octan-1-ol/water and cyclohexane/water. A comparative study of the results in both systems allows us to discuss the effects played by the different molecular structures, substituents, and aromaticity on the distribution properties of these compounds. In particular, the hydrogen-bond donor (HBD) and hydrogen-bond acceptor (HBA) properties of these solutes were characterized and compared with those of structurally related compounds. The Abraham solute descriptors were estimated and partition coefficients (log P) calculated and compared with the experimental values. The results show that the HBD properties are similar for indoles and betacarbolines, and the HBA capacity, as expected, is significantly enhanced by the contribution of the extra pyridinic or piperidinic ring in betacarbolines. The effects of the substituent groups are presented in relation to their contribution to the distribution properties of the compounds studied.

Unified model for the corneal permeability of related and diverse compounds with respect to their physicochemical properties

Corneal permeability data taken from the literature were analyzed for possible quantitative relationships with physicochemical properties. Although a parabolic relationship was obtained with good correlation between lipophilicity, as expressed by the 1-octanol-water partition coefficients, log Poctanol (or the distribution coefficients, log D for ionizable compounds), and the permeability in individual analyses of compound classes such as beta-adrenoceptor blockers and steroids, the correlation was reduced when taken together. However, delta log P (i.e., log Poctanol-log Palkane) correlated inversely with the combined permeability data for beta-blockers and steroids and played a key role as a unifying variable. To a lesser extent, lipophilicity itself also contributes positively to corneal permeation. Even with the addition of miscellaneous compounds such as methanol and ibuprofen, the delta log P and lipophilicity terms were still significant. However, small molecules were likely to be underestimated, which is consistent with penetration via another pathway besides that governed by delta log P and lipophilicity.

Solvent selection for whole cell biotransformations in organic media

Although they were used historically as antimicrobial agents, there is a modern requirement to devise organic solvent systems for exploitation in the biotransformation by intact cells of substrates that are poorly soluble in water. Water-immiscible solvents are normally less cytotoxic than are water-miscible ones. While a unitary mechanism is excluded, damage to the membrane remains the likeliest major mechanism of cytotoxicity, and may be conveniently assessed using an electronic biomass probe. Studies designed to account for the mechanisms of action of general anesthetics and of uncouplers parallel those designed to account for the cytotoxicity of organic solvents. Although there are hundreds of potential physical descriptors of solvent properties, many are broadly similar to each other, such that the intrinsic dimensionality of solvent space is relatively small (< 10). This opens up the possibility of providing a rational biophysical basis for the optimization of the solvents used for biotransformations. The widely used descriptor of solvent behavior, log P (the octanol:water partition coefficient), is a composite of more fundamental molecular descriptors; this explains why there are rarely good correlations between cytotoxicity and log P when a wide variety of solvents is studied. Although the intrinsic dimensionality of solvent space is relatively small, pure solvents still populate it rather sparsely. Thus, mixtures of solvents can and do provide the opportunity of obtaining a solvent optimal for a biotransformation of interest.

Hydrogen bonding. 30. Solubility of gases and vapors in biological liquids and tissues

The general solvation equation log L = c + rR2 + pi H2 + a alpha H2 + b beta H2 + l log L16 has been used to analyze the solubility of solute gases and vapors, as log L values, in water, blood, and a variety of other biological fluids and tissues. The explanatory variables are R2, the solute excess molar refraction; pi H2, the solute dipolarity/polarizability; alpha H2 and beta H2, the solut hydrogen-bond acidity and basicity; and log L16, where L16 is the solute Ostwald solubility coefficient of hexadecane. The obtained coefficients then serve to characterize the biological phase as follows: r + s is the phase dipolarity/polarizability, a is the phase hydrogen-bond basicity, b is the phase hydrogen-bond acidity, ald l is the phase lipophilicity. In addition to characterization of phases, the equation can be used to determine quantitatively solute/phase interactions and predict further log L values. A similar equation in which McGowan's characteristic volume, Vx, replaces the log L16 descriptor can be used to analyze partitions between phases. For example, water/phase and blood/phase partition coefficients are analyzed, and the analysis leads again to coefficients that characterize phases and to the prediction of partition coefficients.

Hydrogen bonding. 33. Factors that influence the distribution of solutes between blood and brain

It is shown that neither the set of directly determined blood-brain concentration ratios (BB) of Young and Mitchell nor the set of indirectly obtained values of Abraham and Weathersby are suitable for the construction of a general equation for the interpretation and prediction of log BB values. However, combination of both sets leads to the general equation log BB = -0.038 + 0.198R2 - 0.687 pi H2 - 0.715 alpha H2 - 0.698 beta H2 + 0.995Vx (n = 57, rho = 0.9522, sd = 0.197, F = 99.2), where the solute descriptors are R2, an excess molar refraction; pi H2, the dipolarity/polarizability, alpha H2 and beta H2, the effective or summation hydrogen-bond acidity and basicity; and Vx, the characteristic volume of McGowan. Thus solute dipolarity/polarizability, hydrogen-bond acidity, and hydrogen-bond basicity favor blood, and solute size, as Vx, favors brain. Methods are given for the estimation of solute descriptors through fragment schemes, so that log BB values themselves may be obtained simply from knowledge of solute molecular structure.

Hydrogen bonding. 32. An analysis of water-octanol and water-alkane partitioning and the delta log P parameter of seiler

A general linear solvation energy equation has been used to analyze published partition coefficients in the systems water-octanol (613 solutes), water-hexadecane (370 solutes), water-alkane (200 solutes), and water-cyclohexane (170 solutes). The descriptors used in the equation are R2, an excess molar refraction; phi H2, the solute dipolarity/polarizability; sigma alpha H2 and sigma beta H2, the effective solute hydrogen-bond acidity and basicity; and Vx, the characteristic volume of McGowan. It is shown that the water-octanol partition coefficient is dominated by solute hydrogen-bond basicity, which favors water, and by solute size, which favors octanol, but solute excess molar refraction and dipolarity/polarizability are also significant. For the water-alkane partition coefficients, the same factors are at work, together with solute hydrogen-bond acidity as a major influence that favors water. An analysis of 288 delta log P values shows that solute hydrogen-bond acidity is the major factor but that solute hydrogen-bond basicity and, to a lesser extent, solute dipolarity/polarizability and size are also significant factors that influence the delta log P parameter.