Tryptophan orientations in membrane-bound gramicidin and melittin-a comparative linear dichroism study on transmembrane and surface-bound peptides.

In the search for methods to study structure and function of membrane-associated proteins and peptides flow linear dichroism, LD, spectroscopy has emerged as a promising technique. Using shear-aligned lipid vesicles, conformations and binding geometries of membrane-bound bio-macromolecules can be assessed. Here we investigate anchoring properties and specific orientations of tryptophan relative to the peptide backbone and to the membrane normal for the model peptides gramicidin and melittin. We have monitored the conformational change associated with the refolding of non-channel gramicidin into its channel form, and quantitatively determined the average orientations of its tryptophan transition moments, suggesting that these residues adopt a well-defined orientation at the membrane interface. An important conclusion regards the structural variation of gramicidin between these two distinct transmembrane forms. Whilst circular dichroism (CD) spectra, as has been reported before, vary strongly between the two forms suggesting their structures might be quite different, the LD results clearly evidence both the peptide backbone orientation and tryptophan side-chain positioning to be very similar. The latter are oriented in accord with what is expected from their role to anchor peptide termini to the membrane surface. The variations in CD could be due to, the in LD observed, minor shifts in mutual orientation and distance between neighbouring tryptophans sensitively determining their exciton interactions. Our data dispute that the non-channel form of membrane-bound gramicidin would be any of the intertwined forms often observed in crystal as the positioning of tryptophans along the peptide axis would not be compatible with the strong interfacial positioning observed here. The general role of tryptophans as interfacial anchors is further assessed for melittin whose conformation shows considerable angular spread, consistent with a carpet model of its mechanism for induced membrane leakage, and a predominantly surface-aligned membrane orientation governed by amphipathic interactions.