Tryptophan rotamer distributions in amphipathic peptides at a lipid surface

The fluorescence decay of tryptophan is a sensitive indicator of its local environment within a peptide or protein. We describe the use of frequency d omain fluorescence spectroscopy to determine the conformational and environ mental changes associated with the interaction of single tryptophan amphipa thic peptides with a phospholipid surface. The five 18-residue peptides stu died are based on a class A amphipathic peptide known to associate with lip id bilayers. The peptides contain a single tryptophan located at positions 2, 3, 7, 12, or 14 in the sequence. in aqueous solution, the peptides are u nstructured and a triple-exponential function is required to fit the decay data. Association of the peptides with small unilamellar vesicles composed of egg phosphatidylcholine reduces the complexity of the fluorescence decay s to a double exponential function, with a reduced dependence of the preexp onential amplitude on peptide sequence. The data are interpreted in terms o f a rotamer model in which the modality and relative proportions of the lif etime components are related to the population distribution of tryptophan c hi(1) rotamers about the C-alpha-C (beta) bond. Peptide secondary structure and the disposition of the tryptophan residue relative to the lipid and aq ueous phases in the peptide-lipid complex affect the local environment of t ryptophan and influence the distribution of side-chain rotamers, The result s show that measurement of the temporal decay of tryptophan emission provid es a useful adjunct to other biophysical techniques for investigating pepti de-lipid and protein-membrane interactions.