Alexey Odinokov

Benzene and other nonpolar organic solvents can accumulate in the lipid bilayer of cellular membranes. Their effect on the membrane structure and fluidity determines their toxic properties and antibiotic action of the organic solvents on the bacteria. We performed molecular dynamics simulations of the interaction of benzene with the dimyristoylphosphatidylcholine (DMPC) bilayer. An increase in the membrane surface area and fluidity was clearly detected. Changes in the acyl chain ordering, tilt angle, and overall bilayer thickness were, however, much less marked. The dependence of all computed quantities on the benzene content showed two regimes separated by the solubility limit of benzene in water. When the amount of benzene exceeded this point, a layer of almost pure benzene started to grow between the membrane leaflets. This process corresponds to the nucleation of a new phase and provides a molecular mechanism for the mechanical rupture of the bilayer under the action of nonpolar compounds.

This work explains the unordinary solvent effect which was observed in the photochemical decay kinetics for the cyanine dye thiacarbocyanine iodide (Cy(+)I(-)) in binary solvent mixtures toluene/dimethylsulfoxide. The interpretation is formulated in terms of the probability density F(R) describing the distribution of interionic distances R in the ion pair Cy(+)I(-) and depending on the solvent composition. The proper normalization of this distribution is expressed via the degree of association α for the ion pair in a given solvent mixture. The α values are, in turn, extracted by means of the mass action law from the ionic association constants computed in a separate publication. The detailed kinetic scheme includes the empirical parametrization of the R-dependent kinetic constants for different decay channels. The multiparameter fitting procedure represents, with the reasonable parameter values, the dependence of the observed quantum yields on the solvent composition.

The computations of the association constants K(ass) were performed at the microscopic level for the ion pair Cy(+)I(-) composed of the complex cyanine dye cation Cy(+) coupled to the negative iodine counterion. The wide array of K(ass) values is arranged by a variation of the composition of the binary solvent mixtures toluene/dimethylsulfoxide with the accompanying change of the solvent polarity. The potentials of mean force (PMFs) are calculated for a set of interionic separations R in the Cy(+)I(-) by a methodology which combines the quantum-chemical techniques for the treatment of the electronic structure of the Cy(+)I(-) system with the recent dielectric continuum approach which accounts for the solvation effects. For a given solute/solvent system the probability function P(R), which describes the distribution of interionic separations, is constructed in terms of the PMFs and implemented for the evaluation of the K(ass).

The values of steady-state solvatochromic Stokes shifts (SS) in absorption/emission electronic spectra of organic chromophores are studied theoretically in the framework of the Hush-Marcus model. Charge distributions for chromophore solutes in their S0 and S1 states are found by means of conventional quantum-chemical methods combined with the continuum PCM approach for treating solvation effects. The solvent reorganization energies, which are expected to correlate with the solvent-induced part of 1/2 SS, are found in a molecular dynamics (MD) simulation which invokes a novel method for separation of the inertial piece of the electrostatic response (Vener, et al. J. Phys. Chem. B 2006, 110, 14950). Computations, performed in two solvents (acetonitrile and benzene), consider three organic dyes: coumarin 153 as a benchmark system and two other chromophores, for which experimental spectra are also reported. The results are found to be in reasonable agreement with the experiment. A consistent treatment of nonlinear effect in the solvent response, promoted by the polarizability of solutes and contributing to the solvent reorganization energies (Ingrosso, et al. J. Phys. Chem. B 2005, 109, 3553), improves the results of computations.

A continuum model for solvation effects in binary solvent mixtures is formulated in terms of the density functional theory. The presence of two variables, namely, the dimensionless solvent composition y and the dimensionless total solvent density z, is an essential feature of binary systems. Their coupling, hidden in the structure of the local dielectric permittivity function, is postulated at the phenomenological level. Local equilibrium conditions are derived by a variation in the free energy functional expressed in terms of the composition and density variables. They appear as a pair of coupled equations defining y and z as spatial distributions. We consider the simplest spherically symmetric case of the Born-type ion immersed in the benzene/dimethylsulfoxide (DMSO) solvent mixture. The profiles of y(R) and z(R) along the radius R, which measures the distance from the ion center, are found in molecular dynamics (MD) simulations. It is shown that for a given solute ion z(R) does not depend significantly on the composition variable y. A simplified solution is then obtained by inserting z(R), found in the MD simulation for the pure DMSO, in the single equation which defines y(R). In this way composition dependences of the main solvation effects are investigated. The local density augmentation appears as a peak of z(R) at the ion boundary. It is responsible for the fine solvation effects missing when the ordinary solvation theories, in which z=1, are applied. These phenomena, studied for negative ions, reproduce consistently the simulation results. For positive ions the simulation shows that z>1 (z=5-6 at the maximum of the z peak), which means that an extremely dense solvation shell is formed. In such a situation the continuum description fails to be valid within a consistent parametrization.