Determination of chemical shielding tensor of an indole carbon and application to tryptophan orientation of a membrane peptide

Using tryptophan C-13-enriched at the C-4 (C epsilon(3)) of the indole, the orientation of the C epsilon(3) chemical shift tensor relative to the C ep silon(3)-H dipolar axis was determined from the C-13 chemical shift/C-13-H- 1 dipolar 2D NMR powder pattern. The principal values obtained were 208, 13 7 and 15 ppm with sigma(33) perpendicular to the indole plane, and sigma(11 ) (least shielded direction) 5 degrees off the C epsilon(3)-H bond toward C xi(3). The side off the C epsilon(3)-H bond was determined by comparing th e reduced chemical shift anisotropies obtained by solid-state NMR and from molecular dynamics calculations of [4-C-13] tryptophans in gramicidin A ali gned in phospholipid membranes. (C) 1999 Elsevier Science B.V. All rights r eserved.