Site-specific tryptophan dynamics in class A amphipathic helical peptides at a phospholipid bilayer interface

The amphipathic helix plays a key role in many membrane-associating peptide s and proteins. The dynamics of helices on membrane surfaces might be of im portance to their function. The fluorescence anisotropy decay of tryptophan is a sensitive indicator of local, segmental, and global dynamics within a peptide or protein. We describe the use of frequency domain dynamic depola rization measurements to determine the site-specific tryptophan dynamics of single tryptophan amphipathic peptides bound to a phospholipid surface. Th e five 18-residue peptides studied are based on a class A amphipathic pepti de that is known to associate at the interface of phospholipid bilayers. Th e peptides contain a single tryptophan located at positions 2, 3, 7, 12, or 14 in the sequence. Association of the peptides with egg phosphatidylcholi ne vesicles results in complex behavior of both the tryptophan intensity de cay and the anisotropy decay. The anisotropy decays were biphasic and were fitted to an associated model where each lifetime component in the intensit y decay is associated with a particular rotational correlation time from th e anisotropy decay. In contrast, an unassociated model where all components of the intensity decay share common rotational modes was unable to provide an adequate fit to the data. Two correlation times were resolved from the associated analysis: one whose contribution to the anisotropy decay was dep endent on the exposure of the tryptophan to the aqueous phase, and the othe r whose contribution reflected the position of the tryptophan in the sequen ce. The results are compared with existing x-ray structural data and molecu lar dynamics simulations of membrane-incorporated peptides.