Peptide models of helical hydrophobic transmembrane segments of membrane proteins. 1. Studies of the conformation, intrabilayer orientation, and amide hydrogen exchangeability of Ac-K2-(LA)12-K2-amide.

The secondary structure, amide hydrogen exchangeability, and intramembrane orientation of the hydrophobic peptide Ac-K2-(LA)12-K2-amide [(LA)12] were studied by a combination of circular dichroism (CD), Fourier transform infrared (FTIR), and proton nuclear magnetic resonance (1H NMR) spectroscopic techniques. All three techniques indicate that (LA)12 adopts predominantly helical conformations in various organic solvents, detergent micelles, and phospholipid bilayers. Also, attenuated total reflectance FTIR studies of oriented phospholipid bilayers demonstrate that (LA)12 is arranged with the long helical axis perpendicular to the bilayer plane. FTIR and 1H NMR studies of the exchangeability of the amide protons of (LA)12 indicate that in all media there are at least two populations of amide protons which exchange with the bulk solvent at markedly different rates. Moreover, the 1H NMR spectroscopic studies indicate that, in organic solvents and micellar dispersions, amide proton exchange rates decrease progressively from the N- or C-terminus of the peptide toward the central region. Our results are thus consistent with (LA)12 retaining a predominantly helical structure with so-called frayed ends in all media. The amide proton exchange studies also indicate that when (LA)12 is dispersed in lipid bilayers, the slowly exchanging population of amide protons is larger than that observed in organic solvents or in micellar dispersions and that most of that proton population is virtually unexchangeable. Such observations are consistent with the sequestration of the central regions of the peptide in the hydrophobic domains of the lipid bilayer. The CD and FTIR data indicate that although (LA)12 seems to retain conformations with a high alpha-helical content in all media examined, its conformation is sensitive to the composition of the surrounding medium, in contrast to the polyleucine-based analogues which have been studied previously. In particular, the FTIR spectroscopic data indicate that (LA)12 may exhibit an amide I absorption band between 1633 and 1639 cm-1 under some circumstances. The relative intensity of this band changes with the composition of the surrounding medium and its appearance has previously been correlated with the formation of 3(10)-helical structures [Miick et al. (1992) Nature 359, 653-655]. Thus (LA)12 may be interconverting between different helical conformations in response to changes in the physical properties of the medium in which the peptide is dispersed. Our results suggest that (LA)12 should serve as a good peptide model of hydrophobic, transmembrane helices which are conformationally sensitive to the properties of the lipid bilayer in which they reside.