Relative orientation of (CH alpha)-H-alpha-bond vectors of successive residues in proteins through cross-correlated relaxation in NMR

Cross-correlation between the fluctuations of C-13(alpha)-H-alpha interacti ons affects the relaxation behavior of two-spin coherences (zero- and doubl e-quantum coherences) involving the C-13(alpha) nuclei of two successive am ino acid residues. The cross-correlation rates are shown to depend on a dih edral angle Sigma defined by two planes subtended by the atoms {H-alpha(i-1 ),C-13(alpha)(i-1),C-13(alpha)(i)} and {C-13(alpha)(i-1),C-13(alpha)(i),H-a lpha(i)}, This dihedral angle is related to the secondary structure of a pr otein, and can be used as a constraint in various protein structure calcula tion protocols.