Estimation of average depth of penetration of melanotropins in dimyristoylphosphatidylglycerol vesicles.

The interaction of alpha-melanocyte stimulating hormone (alpha-MSH) and its analogs [Nle4,D-Phe7]-alpha-MSH (MSH-I) and [Nle4,Asp5,D-Phe7,Lys10]-alpha-MSH(4-10) (MSH-II) with vesicles of dimyristoylphosphatidylglycerol (DMPG) was studied by steady-state fluorescence spectroscopy. The association constants for the interaction were obtained from binding isotherms. Electrostatic effects on the interaction were taken into account through calculation of Gouy-Chapman potentials. The quenching of fluorescence of the peptides by acrylamide and nitroxide labeled lipids demonstrated that insertion of the peptides into the lipid phase of the vesicles causes the changes in the hormone's fluorescence in the presence of DMPG. The parallax method was employed for the estimation of an average depth of penetration of the peptides in the DMPG vesicles. It was found that the Trp residue in alpha-MSH and in MSH-II is positioned around the carbons 6 and 8 of the aliphatic chain. The analog MSH-I goes deeper into the bilayer compared to the others peptides, and the Trp residue locates between carbons 10 and 11 of the acyl chain. The average depth of penetration shows correlation with the number of lipid molecules that interact with one molecule of peptide. There is no direct correlation between the association constants for the lipid-peptide interactions and the depth of penetration of the hormone.