Efficient determination of angles subtended by Cα-Hα and N-HN vectors in proteins via dipole-dipole cross-correlation

The angle Theta(C alpha H alpha,NHN) subtended by the internuclear vectors C-13(alpha)-H-alpha and N-15-H-N in doubly-labeled proteins can be determined by observing the effect of cross-correlation between the dipolar interactions on zero- and double-quantum coherences involving C-13(alpha) and N-15. Two complementary 2D experiments with the appearance of N-15-H-N correlation spectra yield signal intensities that depend on the rate of interconversion through cross-correlated relaxation of in-phase and doubly antiphase zero- and double-quantum coherences. The ratio of the signal intensities in the two experiments bears a simple relationship to the cross-correlation rate, and hence to the angle Theta(C alpha H alpha,NHN). Assuming planarity of the peptide bond, the dihedral angle Psi (between C-alpha and C') can be determined from the knowledge of Theta(C alpha H alpha,NHN). The experiments are very time-effective and provide good sensitivity and excellent spectral resolution.