The in situ determination of the 15N chemical-shift tensor orientation in a polypeptide

Abstract

An analytical approach for the determination of the orientation of the variable σ 33 component of the amide 15N chemical-shift tensor element with respect to the molecular frame is described. Advantage is taken of the known orientation of the unique axis of the heteronuclear dipolar interaction between 15N and 13C in doubly labeled samples through observations of the dipolar-coupled 15N chemical-shift powder patterns. Certain singularities in the powder spectrum have a unique dependence on a single polar angle and hence an analytical solution can be found from the frequency differences of the singularities. This approach is demonstrated for two peptide backbone sites in the pentadecapeptide, gramicidin A, in a lipid environment.