Effects of random copolymers of lysine on the thermotropic behaviour of dipalmitoylphosphatidylglycerol vesicles. A fluorescence anisotropy study.

The effects of some random copolymers of lysine on the thermotropic behaviour of phospholipid vesicles, mainly dipalmitoylphosphatidylglycerol (DPPG), were studied by monitoring the steady-state fluorescence anisotropy with 1,6-diphenyl-1,3,5-hexatriene (DPH) as a membrane probe. A characteristic effect of the polylysine-tyrosine 4:1 copolymer was observed: DPPG vesicles are strongly stabilized, the gel-to-fluid transition going from 40 degrees C in the absence to 52 degrees C with an excess of copolymer (corresponding to a lipid/amino-acid ratio R = 0.5). For R greater than 0.5, the gel-to-fluid transition shows a three-step profile. This triphasic transition, which appears with a much better resolution than with polylysine, demonstrates the formation of stable distinct domains of reasonable size, coexisting from 41 to 51 degrees C. Such stability, over a temperature range of 10 degrees C, is quite unusual and unexpected for a phase separation of mixed lipid phases. The peculiarity of this copolymer was proved by investigating the interactions of DPPG vesicles with polylysine-tryptophan 4:1, polyornithine-lysine 4:1 and polylysine-tyrosine 1:1. We hypothesize that the observed effects are correlated with the ability of these copolymers to adopt/not adopt an alpha-helical or beta-sheet conformation upon interacting with anionic vesicles.