Peptide models of helical hydrophobic transmembrane segments of membrane proteins. 2. Differential scanning calorimetric and FTIR spectroscopic studies of the interaction of Ac-K2-(LA)12-K2-amide with phosphatidylcholine bilayers.

The interactions of the hydrophobic helical transmembrane peptide Ac-K2-(LA)12-K2-amide [(LA)12] with a series of n-saturated diacylphosphatidylcholines (N:0 PC) were studied by high-sensitivity differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The incorporation of (LA)12 into these lipid bilayers results in a broadening of the chain-melting phase transitions of the lipids and progressive decreases in the characteristic temperatures and enthalpies of their gel/liquid-crystalline phase transitions. At low peptide/lipid ratios, the DSC thermograms exhibited by mixtures of (LA)12 with the short chain PCs (13:0 and 14:0) and with very long chain PCs (21:0 and 22:0) appear to be a summation of sharp and broad components, the former diminishing in intensity with increases in peptide concentration. This behavior can be approximated by that of a macroscopic mixture of peptide-poor and peptide-rich lipid domains, the relative proportions of which change with changes in peptide concentration. For peptide mixtures with the medium-chain PCs, the hydrocarbon chain-melting phase transition endotherms are not clearly resolvable into similar sharp and broad components. Instead, at all finite peptide concentrations the DSC heating thermograms appear as broad and highly asymmetric endotherms, the transition temperatures of which decrease significantly with increases in peptide concentration. For mixtures of (LA)12 with each of the lipids studied, the total hydrocarbon chain-melting transition enthalpy decreases with increasing peptide concentration but does not vanish at high peptide/lipid ratios. The FTIR spectra of (LA)12 in these PC bilayers indicate that the peptide retains a predominantly alpha-helical conformation in both the gel and liquid-crystalline phases of the short to medium chain PCs studied (N < 18). However, when incorporated into bilayers composed of the longer chain PCs (N > or = 18), (LA)12 undergoes a reversible conformational change at the gel/liquid-crystalline phase transition of the mixture. In the liquid-crystalline phase, the amide I regions of the FTIR spectra of these mixtures are indicative of a predominantly alpha-helical peptide conformation. However, upon freezing of the lipid hydrocarbon chains, populations and/or domains of (LA)12 giving rise to a sharp conformationally unassigned band near 1665 cm-1 are formed.(ABSTRACT TRUNCATED AT 400 WORDS)