PROTEIN-DEPENDENT REDUCTION OF THE PYRENE EXCIMER FORMATION IN MEMBRANES

The presence of proteins in lipid bilayers always decreases the excime r formation rate of pyrene and pyrene lipid analogues in a way that is related to the protein-to-lipid ratio. Energy transfer measurements f rom intrinsic tryptophans to pyrene have shown (Engelke et al., 1994), that in microsomal membranes,the excimer formation rate of pyrene and pyrene fatty acids is heterogeneous within the membrane plane, becaus e a lipid layer of reduced fluidity surrounds the microsomal proteins. This study investigates whether of not liposomes prepared from egg yo lk phosphatidylcholine with incorporated gramicidin A give results com parable to those from microsomal membranes. The results indicate that the influence of proteins on the lipid bilayer cannot be described by one unique mechanism: Small proteins such as gramicidin A obviously re duce the excimer formation rate by occupying neighboring positions of the fluorescent probe and thus decrease the pyrene collision frequency homogeneously in the whole membrane plane, while larger proteins are surrounded by a lipid boundary layer of lower fluidity than the bulk l ipid. The analysis of the time-resolved tryptophan fluorescence of gra micidin A incorporated liposomes reveals, that the tryptophan quenchin g by pyrene is stronger for tryptophans located closely below the phos pholipid headgroup region because of the pyrene enrichment in this are a of the lipid bilayer.