Supramolecular events modulate flunitrazepam partitioning into natural and model membranes

Abstract

In the present paper we investigate the effects of chemical and physical variables which modulate supramolecular membrane organization on the partitioning of FNTZ into natural and model membranes. In dpPC the value of P dpPC-B increased almost discontinuously at a temperature close to the gel-liquid crystalline phase transition temperature of dpPC (41.5°C). In natural membranes, P m b remained constant upto 37°C and increased significatively at higher temperatures. However, the ascending slope was less sharp than in liposomes, reflecting a lower cooperativity of the change due to the higher complexity of the natural membrane compared with that of the pure lipid. As a function of pH, P values increased slightly in dpPC, increased markedly in dpPE, decreased in dpPS and, in dpPA, P decreased up to pH 8 and, at higher pH values, it tended to increase again. The variation in P as a function of pH resulted from the balance between two opposite tendencies: (1) changes in the viscosity of phospholipid bilayers as a function of pH due to modification of the surface net charge leading to changes the packing of the lipid molecules, and (2) the reduction of interface hydration induced by the decreasing pH which enhances its hydrophobicity. The former membrane property is favored up to pH 7. Partitioning of FNTZ is favored in less hydrated and less ordered interfaces. At pH < 3, FNTZ stablishes with the lipid-interface attractive (dpPS and dpPA) or repulsive (dpPE) electrostatic interactions, which can be screened by the ions arising from salt dissociation over a certain concentration level. The increase in the proportion of cholesterol in dpPC-cholesterol mixed liposomes induces a decrease in the value of P. This result may be explained by the assumption that possible vacancies which might incorporate FNTZ are already filled with cholesterol, thus inhibiting further FNTZ incorporation. The membrane-buffer partition coefficient of FNTZ is strongly influenced by the physical state of the bilayer membrane. Particular interactions may lead to a strong local accumulation of FNTZ within the membrane due to a coupling to lateral density fluctuations.