Cross-correlation between the fluctuations of 13Ca-Ha interactions affects the relaxation behavior of two-spin coherences (zero- and double-quantum coherences) involving the 13Ca nuclei of two successive amino acid residues. The cross-correlation rates are shown to depend on a dihedral angle S defined by two planes subtended by the atoms {Ha(i-1),13Ca(i-1),13Ca(i)} and {13Ca(i-1),13Ca(i),Ha(i)}. This dihedral angle is related to the secondary structure of a protein, and can be used as a constraint in various protein structure calcn. protocols. [on SciFinder (R)]