Response-intensity modulation in long-range heteronuclear two-dimensional NMR chemical-shift correlation

Abstract

Two-dimensional NMR spectroscopy has had a very significant impact on the way in which structure elucidation and spectral assignment studies are performed. Investigators have begun to exploit long-range heteronuclear coupling constants by utilizing delays in the conventional 2D NMR heteronuclear chemical-shift correlation experiment optimized for long-range magnetization transfer through two, three, and/or four bonds (‘Jcu, 3JcH, and “J,,) (1-16). Alternatively, several new experiments specifically for the observation of long-range heteronuclear spin-coupling information (14, 15, 2 7-22) have been reported. Despite the now numerous applications of longrange heteronuclear chemical-shift correlation contained in the literature, factors affecting the magnetization transfer step in these experiments have not been carefully investigated. Most long-range experiments are optimized for a 10 Hz magnetization transfer based upon the work of Reynolds and co-workers (2) or that of Wemly and Lauterwein (9), although other optimizations have also been used with success (I, 4, 10). As a consequence of the absence of responses in some of our own work despite optimization of the experiment at or near the magnitude of the coupling in question we now wish to communicate the results of our preliminary investigation into factors affecting response intensity in long-range heteronuclear chemical-shift correlation. Schenker and von Philipsborn (23) have reported a systematic investigation of the optimization of the INEPT and DEPT experiments in weakly coupled spin-f systems. They have shown that when magnetization is transferred via small heteronuclear couplings, that the transfer may be modulated by other homoand/or heteronuclear spin couplings which are experienced by those nuclei involved in the polarization transfer. Population transfer involved in the long-range heteronuclear chemical-shift correlation experiments may also be modulated in similar fashion, as recently suggested by Freeman (18) and Bax (21).