Temperature-dependent interactions between poly-L-lysine and phosphatidylcholine vesicles

Abstract

Studies on the effect of temperature on the optical density of suspensions of dipalmitoyl phosphatidylcholine vesicles, in the presence of polylysine, indicate that the polypeptide can undergo two different kinds of interactions with the vesicles. At temperatures below the phase transition there seems to be a small surface–polar interaction, which maximizes at about 36 °C (i.e., about 5 °C below the phase transition). Upon transition of the lipid membranes to the liquid crystalline phase, there is a much larger rapid interaction, which seems to be hydrophobic. Whereas the first interaction is not temperature reversible, the second interaction rapidly reverses when the temperature is decreased below the phase transition of dipalmitoyl phosphatidylcholine. Incorporation of as little as 5% unsaturated lecithin, either dioleoyl or dilinoleoyl, into the dipalmitoyl vesicles, stabilizes the liquid phase interaction, so that it does not readily reverse when the temperature is decreased below that of the phase transition. Whereas dioleoyl only partially and temporarily interferes with the temperature reversal, dilinoleoyl almost completely stabilizes the interaction. Associated studies using dilatometry and electron microscopy show that the liquid phase interaction between polylysine and phosphatidylcholine vesicles results in the formation of a complex which is of high molecular and electron density.