Application of multiple-quantum filters in zero-quantum spectroscopy

Abstract

Homonuclear zero-quantum (ZQ) 2D NMR offers several advantages over other correlation spectroscopies in the analysis of scalar coupling networks, e.g., insensitivity to magnetic field inhomogeneity (I), a generally smaller spectral range in w1 and a concomitant increase in o1 resolution for an FID data set of fixed size (2), and the ability to discern the relative signs of certain pairs of scalar coupling constants from the wI multiplet structure (2). ZQ spectroscopy has found limited application, however, due in part to the inability to separate zeroquantum coherence (ZQC) and longitudinal magnetization during ti by phase cycling (3, 4). The presence of longitudinal magnetization during tl gives rise to axial peaks in the 2D ZQ spectrum. In macromolecules, the existence of strong, in-phase axial peaks may obscure the presence of weak, antiphase ZQ resonances, especially those resonances presenting small ZQ frequencies. It is therefore desirable to develop an efficient and reliable method whereby axial peaks may be purged from 2D ZQ spectra. Bolton has suggested that the suppression of axial peaks in zero-quantum spectra may be accomplished using the pulse sequence of Fig. 1A (5). The suppression is achieved by taking a linear combination of two experiments obtained with different tip angles for the conversion pulse, e.g., /3 = 7r/4 and p = ST/~. The level of suppression, however, will be compromised by the presence of sources which contribute to tip angle errors, such as resonance offset, RF inhomogeneity, and pulse h-reproducibility (6-8). Recognizing that (&,+r is identical to (?r/2),+,(?r/2),+, with all preceding pulses concomitantly phase shifted by 8 (8, 9), one can recast this pulse sequence in an attempt to obtain axial peak suppression which is insensitive to tip angle errors, in analogy to conventional multiple-quantum-filtered correlation sequences (10) and POMMIE (I I). An analysis of this recast pulse sequence, however, indicates that the level of axial peak suppression for any spin obtained by combining experiments with phases P = ~1 4 and p = 57r/4 is inversely proportional to cos ~yi, where (Y~ represents the spindependent, effective tip angle for the first pulse in the 2-pulse sandwich immediately following tr , The pulse sequence suggested by Mtiller to achieve an improved excitation of ZQC in homonuclear systems (12) also suppresses axial peaks to a large extent. When