Torsion angle determination in solid C-13-labeled amino acids and peptides by separated-local-field double-quantum NMR

A novel two-dimensional double-quantum NMR technique for determining the torsion angle psi in doubly-C-13-labeled amino acid residues of solid peptides is presented. The intensity pattern in the two-dimensional NMR spectrum reflects the orientation of the C-alpha-H bond with respect to the carbonyl moiety, by correlating the C-alpha-H dipolar coupling with the CO chemical-shift anisotropy. This approach eliminates problems caused by C-13-N-14 dipolar couplings and the relatively small chemical-shift anisotropy of the C-alpha carbon in two-dimensional double-quantum spectra based only on chemical-shift anisotropies. The double-quantum selection achieves isolated-spin background suppression and increases the spectral resolution by partially removing the inhomogeneous spectral broadening. The experiment is demonstrated on C-13(alpha)-(CO)-C-13-labeled leucine. With 50 mg, a useful spectrum was obtained in 3 h. The potential of the technique for distinguishing different secondary structures in peptides is demonstrated by spectral simulations.