CONFORMATION OF THE ACYLATION SITE OF PALMITOYLGRAMICIDIN IN LIPID BILAYERS OF DIMYRISTOYLPHOSPHATIDYLCHOLINE

Gramicidin A (gA) call be palmitoylated by means of an ester linkage t o the OH group of the terminal ethanolamine that sits at the membrane- water interface in the functional gA channel. We have investigated pal mitoyl-gA as a model transmembrane acylprotein. Ethanolamine-d(4) (NH2 CD2CD2OH) was incorporated into gA by total synthesis, and a portion o f the labeled gA was palmitoylated. Solid-state H-2-NMR spectra of acy l- and nonacyl-gA in hydrated dimyristoylphosphatidylcholine (DMPC) bi layers were compared. The spectra for both oriented and nonoriented sa mples at 4 and at 40 degrees C indicate that the ethanolamine of gA is highly mobile prior to acylation, but essentially immobile after palm itoylation. The H-2 quadrupolar splittings allow the conformation of t he ethanolamine group in acyl-gA to be determined. By combining our da ta with the previously determined quadrupolar splittings for deuterium labels on the palmitoyl chain [Vogt, T. C. B., Killian, J. A, & de Kr uijff, B. (1994) Biochemistry 33, 2063-2070], we also propose a model for the acyl chain. The ethanolamine group rotates over Leu(10) and to ward the outside of the gA channel's cylinder upon acylation, so that the attached acyl chain passes between the side chains of Trp(9) and L eu(10). To accommodate the acyl chain, the six-membered portion of the indole ring of Trp(9) is displaced by about 0.9 Angstrom, by means of 1-2 degrees rotations in chi(1) and chi(2).