SPIN-LOCKED MULTIPLE-QUANTUM COHERENCE FOR SIGNAL ENHANCEMENT IN HETERONUCLEAR MULTIDIMENSIONAL NMR EXPERIMENTS

For methine sites the relaxation rate of C-13-H-1 two-spin coherence is generally slower than the relaxation rate of the individual C-13 and H-1 single spin coherences. The slower decay of two-spin coherence can be used to increase the sensitivity and resolution in heteronuclear experiments, particularly those that require correlation of H-alpha and C alpha chemical shifts. To avoid dephasing of the two-spin coherence caused by H-1-H-1 J-couplings, the H-1 spin is locked by the application of a weak rf field, resulting in a spin-locked multiple quantum coherence. For a sample of calcium-free calmodulin, use of the multiple quantum approach yields significant signal enhancement over the conventional constant-rime 2D HSQC experiment. The approach is applicable to many multidimensional NMR experiments, as demonstrated for a 3D C-13-separated ROESY CT-HMQC spectrum.