LONG-RANGE H-1-N-15 CORRELATION AT NATURAL-ABUNDANCE USING GRADIENT-ENHANCED INVERSE-DETECTION

Inverse-detected heteronuclear shift correlation efficiency has been significantly augmented by the incorporation of pulsed field gradients (PFG). Phase-cycling requirements for t(1)-noise suppression in gradient-enhanced experiments are, for the most part, obviated, making it feasible to acquire data in one or a few transients/t(1) increment. The benefits which accrue for H-1-C-13 correlation (using GHMQC, GHMBC, and variants of GHMQC-TOCSY) are well documented. Less obvious is the increased facility with which long-range H-1-N-15 correlation spectra can be acquired. An IDR-(Inverted Direct Response)-GHMQC-TOCSY was used to establish unequivocal proton resonance assignments for the alkaloid ajmaline. Long-range H-1-N-15 heteronuclear couplings to the two nitrogen atoms of ajmaline were then probed using a gradient-enhanced H-1-N-15 heteronuclear shift correlation experiment derived from HMQC. Long-range H-1-N-15 couplings in ajmaline are assigned for the first time.