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 amphipathic peptide on the bilayer surface. We use five amphipathic 18-residue peptide analogues of a class A amphipathic peptide that is known to associate with a bilayer surface. Tyrosine and tryptophan are used as spectroscopic probes 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 nonpolar 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 the wavelength of the emission maximum as indicators of micropolarity. The exposure of the tryptophan residues at each site is assessed by acrylamide quenching. On association with vesicles, the tryptophan residues at positions 3, 7 and 14 are in nonpolar water-shielded environments, and the tryptophan at position 12 is in an exposed polar environment. The tryptophan at position 2, which is located near the bilayer-water interface, exhibits intermediate behaviour. Analysis of the second-derivative absorption spectrum confirmed that the tyrosine residue at position 7 is in a nonpolar water-shielded environment in the peptide-lipid complex. We conclude that these class A amphipathic peptides lie parallel to the lipid surface and penetrate no deeper than the ester linkages of the phospholipids.