Spin-locked multiple quantum coherence for signal enhancement in heteronuclear multidimensional NMR experiments

Summary For methine sites the relaxation rate of ¹³C-¹H two-spin coherence is generally slower than the relaxation rate of the individual ¹³C and ¹H 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 and C chemical shifts. To avoid dephasing of the two-spin coherence caused by ¹H-¹H J-couplings, the ¹H 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-time 2D HSQC experiment. The approach is applicable to many multidimensional NMR experiments, as demonstrated for a 3D ¹³C-separated ROESY CT-HMQC spectrum.