Comparison of the interaction of methionine and norleucine‐containing peptides with phospholipid bilayers

Norleucine is a structural analog of methionine with a methylene group replacing the thio ether. Despite the close structural similarity of these two amino acids, norleucine-containing peptides have markedly different behaviour with phospholipids compared with methionine-containing peptides. For example, HCO-L-Ahx-L-Leu-L-Phe-OMe behaves as a hydrophobic peptide when mixed with dimyristoylphosphatidylcholine. This peptide lowers the enthalpy of the lipid phase transition. The effect is independent of the rate of heating. With the homologous peptide, HCO-L-Met-L-Leu-L-Phe-OMe, the results are markedly dependent on scan rate with a higher enthalpy observed at faster scan rates. Only at a scan rate of 0.2 K min-1 do the two peptides approach similar behaviour. The higher enthalpy observed for samples with the methionine peptide at higher scan rates can be explained assuming that the peptide aggregates at low temperature. As the phase transition temperature is approached, the more hydrophilic methionine peptide partitions more slowly into the membrane than the norleucine peptide. Partitioning of the peptides between aqueous and lipid phases was measured at 37 degrees by centrifuging down the lipid-bound fraction. At a peptide concentration of 15 microM and a lipid concentration of 1.4 mM, 89% of the HCO-L-Ahx-L-Leu-L-PheOMe and 97% of the HCO-L-Met-L-Leu-L-PheOMe remained in the supernate; indicating a greater tendency of the norleucine-containing peptide to partition into the lipid phase. The peptides Ac-L-Phe-L-Met-L-Arg-L-Phe-NH2 and Ac-L-Phe-L-Ahx-L-Arg-L-Phe-NH2 are readily soluble in water.(ABSTRACT TRUNCATED AT 250 WORDS)