Long‐range 1H‐15N correlation at natural abundance using gradient‐enhanced inverse‐detection

Abstract

Inverse‐detected heteronuclear shift correlation efficiency has been significantly augmented by the incorporation of pulsed field gradients (PFG). Phase‐cycling requirements for t₁‐noise suppression in gradient‐enhanced experiments are, for the most part, obviated, making it feasible to acquire data in one or a few transients/t₁ increment. The benefits which acerue for ¹H‐¹³C correlation (using GHMQC, GHMBC, and variants of GHMQC‐TOCSY) are well documented. Less obvious is the increased facility with which long‐range ¹H‐¹⁵N 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 ¹H‐¹⁵N heteronuclear couplings to the two nitrogen atoms of ajmaline were then probed using a gradient‐enhanced ¹H‐¹⁵N heteronuclear shift correlation experiment derived from HMQC. Long‐range ¹H‐¹⁵N couplings in ajmaline are assigned for the first time.