PARTITIONING OF GRAMICIDIN A' BETWEEN COEXISTING FLUID AND GEL PHOSPHOLIPID PHASES

The partitioning behavior of gramicidin A' was investigated in four bi nary phospholipid mixtures with coexisting fluid and gel phases. The r atio of the equilibrium peptide concentration in the fluid phase to th at in the gel phase (i.e., the partition coefficient, K-p) was determi ned by analysis of the quenching of gramicidin A' tryptophanyl fluores cence by a spin-labeled phosphatidylcholine. The partition coefficient was used as a measure of the relative solubility of gramicidin A' in the four types of gel phases analyzed. The composition of the gel phas e was entirely Ca(dioleoylphosphatidylserine), (Ca(di18:1-PS)(2)), or was rich in either distearoylphosphatidylcholine (di18:0-PC), dipalmit oylphosphatidylcholine (di16:0-PC), or dimyristoylphosphatidylcholine (di14:0-PC). Except in the last case, the gel phase was depleted of gr amicidin A': K-p similar to 30 when the gel phase was Ca(di18:1-PS)(2) or di18:0-PC-rich, K-p similar to 10 when the gel phase was di16:0-PC -rich, and K-p similar to 1 when the gel phase was di14:0-PC-rich. The hydrophobic mismatch between the length of gramicidin A' and the leng th of the phospholipid acyl chains in the bulk gel phase is greatest w ith di18:1-PS and di18:0-PC, intermediate with di16:0-PC, and least wi th di14:0-PC. The K-p measurements presented here are consistent with increasing solubility of gramicidin A' in the gel phase with decreasin g hydrophobic mismatch.