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

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Abstract
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 Hα and Cα 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.