A three-dimensional heteronuclear multiple-quantum coherence homonuclear hartmann-hahn NOESY experiment

Abstract

Since the introduction of two-dimensional NMR a further extension into three (or more) frequency dimensions, by the insertion of one or two additional evolution times, seemed only a logical extension of the method. In the area of the high-resolution NMR, three-dimensional experiments seemed impracticable for the same reasons that were at the heart of the slow introduction of 2D NMR: very large data matrices were generated and long measuring times were needed. Increased sensitivity of NMR spectrometers, ever increasing computer power, and speed of mass storage devices have alleviated these problems. Three-dimensional NMR can nowadays be done within a reasonable amount of time and without generating excessively large data matrices as judged by today’s standards. This is borne out by a number of recently published 3D NMR experiments (1-9). The majority of these experiments entails the coupling of two 2D proton-proton correlation experiments, e.g., NOESYCOSY (3) and NOESY-HOHAHA (5). To limit the measuring time and the size of the data matrices in most cases the proton frequency domain was restricted by using selective excitation pulses, although also two 3D NOESY-HOHAHA experiments have been done in which the complete proton frequency domain was utilized (6, 7). In addition reports have appeared of 3D heteronuclear NMR experiments, where a 2D heteronucleus-proton correlation experiment is combined with a 2D protonproton correlation experiment (8, 9). For uniformly 15N- or ‘3C-enriched samples, the sensitivity of such 3D heteronuclear NMR experiments is similar to that of 3D proton NMR experiments, since the heteronucleus is detected via the attached proton and thus with proton sensitivity ( 10-12). Therefore, in terms of measuring time and size of data matrices the same conditions apply to such 3D heteronuclear NMR experiments as to 3D proton NMR. The great promise of 3D NMR experiments lies in the increased spectral resolution with respect to 2D NMR experiments, which is of prime importance in the elucidation of the molecular structure by means of NMR of proteins as well as of nucleic acids and carbohydrates. In this Communication we present a new 3D NMR experiment in which a ‘Hdetected heteronuclear multiple-quantum coherence experiment (HMQC) ( 10-12)