Distribution of neutral organic compounds betweenn-heptane and methanol orN,N-dimethylformamide

Anisotropic solvent model of the lipid bilayer. 1. Parameterization of long-range electrostatics and first solvation shell effects

A new implicit solvation model was developed for calculating free energies of transfer of molecules from water to any solvent with defined bulk properties. The transfer energy was calculated as a sum of the first solvation shell energy and the long-range electrostatic contribution. The first term was proportional to solvent accessible surface area and solvation parameters (σ(i)) for different atom types. The electrostatic term was computed as a product of group dipole moments and dipolar solvation parameter (η) for neutral molecules or using a modified Born equation for ions. The regression coefficients in linear dependencies of solvation parameters σ(i) and η on dielectric constant, solvatochromic polarizability parameter π*, and hydrogen-bonding donor and acceptor capacities of solvents were optimized using 1269 experimental transfer energies from 19 organic solvents to water. The root-mean-square errors for neutral compounds and ions were 0.82 and 1.61 kcal/mol, respectively. Quantification of energy components demonstrates the dominant roles of hydrophobic effect for nonpolar atoms and of hydrogen-bonding for polar atoms. The estimated first solvation shell energy outweighs the long-range electrostatics for most compounds including ions. The simplicity and computational efficiency of the model allows its application for modeling of macromolecules in anisotropic environments, such as biological membranes.

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.

Distribution model for Folch partition

Partition coefficients for 86 varied compounds were determined for the chloroform-methanol-water (8:4:3 v/v) biphasic partition system and used to derive a model for the distribution of neutral compounds between the water-rich and chloroform-rich layers. The partition coefficients, log K(p), were correlated through the solvation parameter model giving log K(p) = 1.336 - 0.014E + 0.413S + 1.583A + 1.344B - 1.378V with a multiple correlation coefficient of 0.973, standard error of the estimate 0.151, and Fischer statistic 286. In the model the solute descriptors are excess molar refraction, E, dipolarity/polarizability, S, overall hydrogen-bond acidity, A, overall hydrogen-bond basicity, B, and McGowan's characteristic volume, V. The model is expected to be able to estimate further values of the partition coefficient to about 0.13 log units. The chloroform-methanol-water partition system is shown to have different selectivities to 44 common water-organic solvent and totally organic biphasic partition systems.

Distribution of neutral organic compounds between n-heptane and fluorine-containing alcohols

Partition coefficients for a number of varied compounds were determined for n-heptane-2,2,2-trifluoroethanol and n-heptane-1,1,1,3,3,3-hexafluoro-2-propanol and used to derive a general model for the distribution of neutral compounds in the biphasic systems. The partition coefficient, log K(p), was correlated through the solvation parameter model giving log K(p) = 0.160 + 1.190V + 0.856E - 1.538S - 1.325A - 2.965B for the n-heptane-2,2,2-trifluoroethanol system with a multiple correlation coefficient of 0.988, standard error of the estimate 0.136, and Fischer statistic 599 for 77 compounds. For n-heptane-1,1,1,3,3,3-hexafluoro-2-propanol, the model is log K(p) = -0.225 + 1.161V + 0.720E - 1.357S - 0.577A - 2.819B with a multiple correlation coefficient of 0.982, standard error of the estimate 0.148, and Fischer statistic 421 for 84 compounds. In the models, the solute descriptors are excess molar refraction E, dipolarity/polarizability S, overall hydrogen bond acidity and basicity A and B, respectively, and McGowan's characteristic volume V. Either model is expected to be able to estimate further values of the partition coefficient to about 0.13 log units and is applicable to a wide range of compounds. Applications include the choice of partitioning systems for sample clean-up and countercurrent chromatography and for estimating solute descriptors for water insoluble or reactive compounds.