Water accessibility to the tryptophan indole N-H sites of gramicidin A transmembrane channel: detection of positional shifts of tryptophans 11 and 13 along the channel axis upon cation binding.

Gramicidin A analogues, in which one of four Trp residues was selectively substituted by carbon-deuterated Trp, were incorporated into phospholipid liposomes and their Raman spectra were recorded in the absence and presence of Na+. Detailed analyses of the Raman spectra have revealed the conformation, strength of hydrophobic interaction, and water accessibility of each individual Trp residue of the gramicidin transmembrane channel. The absolute value of the torsion angle chi2,1 about the CalphaCbeta-C3C2 linkage is found to be 94 degrees +/- 6 degrees in both the cation-free and Na+-bound states. The Trp side chains are generally involved in strong hydrophobic interactions with the lipid acyl chains of the membrane and/or with another Trp residue. The water accessibility to the indole N1H site is in the order Trp-15 > Trp-13 >> Trp-11 > Trp-9 in the cation-free state. In the Na+-bound state, however, the water accessibility significantly decreases for Trp-13 and increases for Trp-11 without change for Trp-15 and -9. The site-specific changes of water accessibility are explained by a combination of positional shifts of Trp-13 and -11 toward the channel center and the channel mouth, respectively. Model building has shown that such positional shifts of Trp indole rings can be linked with deflections of amide C&dbd;O bonds toward the channel pore, suggesting a cation-induced conformational transition of the channel backbone structure.