Interaction of hydrophobic organic compounds with mercury adsorbed dioleoylphosphatidylcholine monolayers

The interaction of hydrophobic organic compounds with biological membranes is an important factor in their biological activity. A fundamental insight into the mechanisms can be obtained by examining the influence of these substances on model membrane systems. The effect of four groups of hydrophobic aromatic compounds on a mercury-adsorbed dioleoylphosphatidylcholine (DOPC) monolayer is described in this paper. The compounds studied were: (1) polynuclear aromatic hydrocarbons (PAH), (2) polychlorinated biphenyls (PCB), (3) neurotoxic pesticides, and (4) phenothiazines. The monolayer properties were measured using phase-sensitive a.c. voltammetry and cyclic voltammetry. The response of the monolayer to these compounds is recorded as a change in the form of the capacity-potential curve especially with respect to two capacity peaks which correspond to two well-defined phase transitions. Planar aromatic molecules cause a negative shift of the capacity peaks, however, as the molecule becomes more globular the response becomes less and the peaks become suppressed. It is shown that planar aromatic molecules exert their effect by penetrating the hydrocarbon region of the monolayer and that there is a molecular size cut-off for higher membered PAH molecules. In addition, the monolayer is not so sensitive to the bulky PCB which have a more disruptive effect on the phase transitions and thus on the mechanisms of self-assembly of the monolayer. A direct correlation is shown between the biological membrane activity of the phenothiazines and their effect on the monolayer at submicromolar levels. A molecular selectivity of phospholipid monolayers to these compounds is indicated which has implications for their effect on biological membranes.