Environments and conformations of tryptophan side chains of gramicidin A in phospholipid bilayers studied by Raman spectroscopy.

Raman spectroscopy has been used to investigate the hydrophobic interaction of the indole ring with the environments, the water accessibility to the N1H site, and the conformation about the C beta-C3 bond for the four tryptophan side chains of gramicidin A incorporated into phospholipid bilayers. Most of the tryptophan side chains of the head-to-head helical dimer transmembrane channel are strongly interacting with the lipid hydrocarbon chains, and the hydrophobic interactions for the rest increase with increasing hydrocarbon chain length of the lipid. One tryptophan side chain (probably Trp-15) is accessible to water molecules, another (Trp-9) is deeply buried in the bilayer and inaccessible, and the accessibilities of the remaining two (Trp-11 and Trp-13) depend on the bilayer thickness. The torsional angle about the C beta-C3 bond is found to be +/- 90 degrees for all the tryptophans irrespective of the membrane thickness. Binding of the sodium cation to the channel does not change the torsional angles but decreases the water accessibilities of two tryptophans (Trp-11 and Trp-13) considerably. In conjunction with a slight spectral change in the amide III region, it is suggested that the sodium binding causes a partial change in the main-chain conformation around Trp-11 and Trp-13, which results in the movements of these side chains toward the bilayer center. Two models consistent with the present Raman data are proposed for the tryptophan orientation in the dominant channel structure.