Zero-quantum spectroscopy with TOCSY transfer

Abstract

Two-dimensional zero-quantum (ZQ) NMR spectra represent a very special kind of multiple-quantum (MQ) spectra ( I ) : zero-quantum coherences (ZQC) evolve during the evolution period, giving rise to the appearance of frequency differences in the F, dimension where the resonances are split by the so-called ZQ splittings (2). The unusual appearance of ZQ spectra has probably limited their use. However, in the last few years renewed interest has evolved in ZQ spectroscopy (3-7) as these spectra obviously possess several advantages over single-quantum (SQ) or doublequantum (DQ) spectra: (i) ZQC are independent of magnetic field inhomogeneity and of the transmitter frequency (8); (ii) they deliver additional information (e.g., relative sign of J couplings (9)) and contain fewer lines than SQ spectra (9); (iii) the frequency differences in ZQ spectra, in comparison to DQ spectra, often allow a reduction of the spectral range in the F, dimension, which has obvious advantages in 3D ZQ NMR spectra. Furthermore, the possibility of removing assignment ambiguities is of great importance: peaks that overlap in a SQ or DQ spectrum might be resolved in a ZQ spectrum because of the totally different resonance positions in those spectra. Hence, Hall and Norwood recently recommended the combined use of ZQ and DQ spectra to get complementary information (4). Although a uniform excitation of ZQC is difficult to achieve due to the dependency of the excitation efficiency on J couplings and chemical shifts ( IO), a similar problem exists for DQ spectra, but DQ spectra are often used. Variation of the excitation delay has been found to help in severe cases ( 10). The purpose of this paper is to introduce the implementation of the MLEV-17 sequence ( I1 ) within ZQ pulse sequences. Miiller and Pardi ( 22) described a relayed ZQ spectrum for detecting remote connectivities, but no attempt was made to achieve a more distant magnetization transfer by applying a TOCSY step. Each ZQ frequency in F, should ideally represent the whole spin system and make a multiple check of the resonance assignment possible. Therefore a ZQ spectrum with TOCSY transfer, which we call ZQ-TOCSY, could be a useful alternative method to normal TOCSY ( 11, 13) due to the different resonance line positions. Peaks which overlap in the normal TOCSY spectrum may be unraveled in a ZQ-TOCSY spectrum and vice versa. We obtained a series of ZQ spectra (pulse sequences are given in Fig. 1) for the cyclic hexapeptide cycle ( -D-Leu ‘-Tyr 2-Leu 3-Gin 4-Ser 5-Leu6) , in which two ex-