The structure and orientation of class-A amphipathic peptides on a phospholipid bilayer surface

The amphipathic alpha-helix is a recognised structural motif that is shared by membrane-associating proteins and peptides of diverse function. The aim of this paper is to determine the orientation of an alpha-helical amphipat hic peptide on the bilayer surface. We use five amphipathic 18-residue pept ide analogues of a class A amphipathic peptide that is known to associate w ith a bilayer surface. Tyrosine and tryptophan are used as spectroscopic pr obes to sense local environments in the peptide in solution and when bound to the surface of unilamellar phosphatidylcholine vesicles. In a series of peptides, tryptophan is moved progressively along the sequence from the non polar face (positions 3, 7, 4) to the polar face of the peptide (positions 2, 12). The local environment of the tryptophan residue at each position is determined using fluorescence spectroscopy employing quantum yield, and th e wavelength of the emission maximum as indicators of micropolarity. The ex posure of the tryptophan residues at each site is assessed by acrylamide qu enching. On association with vesicles, the tryptophan residues at positions 3, 7 and 14 are in nonpolar water-shielded environments, and the tryptopha n at position 12 is in an exposed polar environment. The tryptophan at posi tion 2, which is located near the bilayer-water interface, exhibits interme diate behaviour. Analysis of the second-derivative absorption spectrum conf irmed that the tyrosine residue at position 7 is in a nonpolar water-shield ed environment in the peptide-lipid complex. We conclude that these class A amphipathic peptides lie parallel to the lipid surface and penetrate no de eper than the ester linkages of the phospholipids.