FTIR SPECTROSCOPIC STUDIES OF THE CONFORMATION AND AMIDE HYDROGEN-EXCHANGE OF A PEPTIDE MODEL OF THE HYDROPHOBIC TRANSMEMBRANE ALPHA-HELICES OF MEMBRANE-PROTEINS

The conformation and amide hydrogen exchangeability of the hydrophobic peptide Lys2-Gly-Leu24-Lys2-Ala-amide were studied by Fourier transform infrared spectroscopy. In these studies information on the secondary structure of the peptide was obtained from an examination of the contours of both the amide I and amide II absorption bands. The conformationally sensitive amide I and amide II regions of the infrared spectra suggest that the peptide is predominantly alpha-helical and that it contains some non-alpha-helical structures which are probably in an extended conformation. Studies of the exchangeability of the amide protons of the peptide indicate that there are two populations of amide protons which differ markedly with respect to their exchangeability with the bulk solvent phase, whether the peptide is dissolved in methanol or dispersed in hydrated lipid bilayers. One population of amide protons is very readily exchangeable, and our data suggest that it arises primarily but not exclusively from the extended regions of the peptide. The other population exchanges very slowly with thc bulk solvent and appears to originate entirely from the alpha-helical domain of the peptide. This latter population is virtually unexchangeable when the peptide is dispersed in hydrated phosphatidylcholine bilayers but can be largely exchanged when the peptide is solubilized with methanol. We suggest that this slowly exchanging population of amide protons arises from the central part of the hydrophobic polyleucine core which forms a very stable alpha-helix that would be deeply buried in the hydrophobic domain of hydrated lipid bilayers. Our results also suggest that the readily exchangeable population of amide protons probably arises from the amino acid residues near the ends of the peptide. These residues probably comprise the extended domains of the peptide and small contiguous portions of its alpha-helical polyleucine core. Our studies indicate that the alpha-helical structure of the peptide remains stable under all our experimental conditions. We also show that, by utilizing initial H/D exchange, the conformation of both the transbilayer domain and the hydrophilic end regions of this peptide, and presumably also those of transmembrane proteins, can be differentially probed by infrared spectroscopy.