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

Using tryptophan 13C-enriched at the C4 (Cε3) of the indole, the orientation of the Cε3 chemical shift tensor relative to the Cε3–H dipolar axis was determined from the 13C chemical shift/13C–1H dipolar 2D NMR powder pattern. The principal values obtained were 208, 137 and 15 ppm with σ33 perpendicular to the indole plane, and σ11 (least shielded direction) 5° off the Cε3–H bond toward Cξ3. The side off the Cε3–H bond was determined by comparing the reduced chemical shift anisotropies obtained by solid-state NMR and from molecular dynamics calculations of [4-13C] tryptophans in gramicidin A aligned in phospholipid membranes.