Cross-correlated relaxation for the measurement of angles between tensorial interactions

Theory, experimental aspects, and use in structure calculation of cross-cor related relaxation rates measured on zero- and double-quantum coherences in liquid state NMR are presented. The relative size of the interaction depen ds on the projection angle between the two tensorial interactions. The tens orial interaction can be either a dipolar interaction or a chemical shift a nisotropy relaxation mechanism (CSA). Effects of additional sources of rela xation on the cross-correlated relaxation rates are analyzed. Also, an easy -to-use formalism is given to manipulate different cross-correlated relaxat ion interactions. The application addresses measurement of the backbone ang le psi in a protein by measuring dipole(N-15-H-1)-dipole(C-13(alpha)-H-1(al pha)) and CSA(N-15)- dipole(C-13(alpha)-H-1(alpha)) cross-correlated relaxa tion rates. It is shown that ambiguities due to the 3 cos(2)theta-1 depende nce of one cross-correlated relaxation rate can be overcome by measuring ad ditional cross-correlated relaxation rates. The use of cross-correlated rel axation rates is demonstrated in structure calculations. (C) 2000 Academic Press.