Gradient-enhanced multiple-quantum filter (ge-MQF). A simple way to obtain single-scan phase-sensitive HMQC spectra

Abstract

Gradient-enhanced NMR techniques (ge-NMR) recently have gained much attention because of improvements in gradient technology. These methods are especially useful for ‘H-detected heteronuclear experiments (Z-4) and exchange spectroscopy (5). A number of experiments have been published which allow recording of highquality single-scan two-dimensional (2, 6, 7) and three-dimensional (a-20) spectra with inherent frequency-independent solvent suppression. Although ge-NMR has a number of important advantages over NMR experiments employing phase cycling, it also suffers from a number of drawbacks. Probably most important is the fact that coherence selection by gradient pulses results in phasemodulated signals in the indirectly detected dimensions (4, 6, 11) . As a consequence, spectra must be displayed in absolute-value mode. In order to get rid of the broad dispersive tails, time-domain filter functions which enhance resolution at the cost of signal-to-noise are usually applied. Generally, it is worth the time and disk space to record the additional data needed for amplitude-modulated signals, so that phasesensitive spectra can be obtained. A number of ways to obtain phase-sensitive spectra have been proposed. Twodimensional J-modulated spectra have been processed using maximum-entropy methods to yield in-phase spectra (12) and such procedures may possibly also be applicable to the time-domain data of a ge-NMR experiment. However, the procedures employing MEM or MEM-like techniques are computationally very demanding and this may presently prohibit actual use for 3D or 4D datasets. An alternative solution (SWAT) has been proposed by Hurd et al. (I3), in which two coherence pathways are alternatingly rephased and sampled during acquisition. The 2D time-domain signal is reconstructed in the processing stage. Some drawbacks, however, are the need for very short gradient pulses and the increased bandwidth of the electronic filters, which will result in extra noise.