SENSITIVITY OPTIMIZED HCN AND HCNCH EXPERIMENTS FOR C-13 N-15 LABELEDOLIGONUCLEOTIDES/

Triple resonance HCN and HCNCH experiments used in studies of C-13/N-1 5 labeled oligonucleotides include extended evolution periods (typical ly up to 100 ms) to allow coherence transfer through a complex heteron uclear spin network. Unfortunately, most of the magnetization is lost during the evolution due to fast spin-spin relaxation dominated by one -bond H-1-C-13 dipolar interaction. As demonstrated recently, the sens itivity of the experiments can be dramatically improved by keeping the spin system in a state of proton-carbon multiple-quantum coherence wh ich is not affected by the strong dipolar coupling. However, the multi ple-quantum coherence is very sensitive to homonuclear as well as long -range heteronuclear interactions. Unwanted magnetization transfer due to these interactions can reduce the sensitivity back to the level of a single-quantum experiment and, for some spin moieties, even elimina te the signal completely. In the present paper we show that a modified HCN scheme that refocuses the interfering coherences improves sensiti vity routinely by a factor of 1.5 to 4 over a nonselective experiment. In addition, novel multiple-quantum 2D and 3D HCNCH experiments with substantially enhanced sensitivity are presented.