COSY representation of two-dimensional homonuclear double-quantum spectra

Abstract

Many different 2D experiments are available for the analysis of J-coupling networks in liquid phase NMR spectra, such as correlated spectroscopy (COSY) (I, 2), doublequantum-filtered COSY (3), relayed coherence transfer (4, 5), and doublequantum spectroscopy (2, 6-8). Complicated spectra consisting of a superposition of many isolated networks, such as *H NMR spectra of proteins, are best analyzed by comparing the results of several different kinds of experiments. The 2D doublequantum (DQ) experiment is especially versatile for establishing J connectivities between resonances with a small chemical-shift difference, i.e., multiplets which would show cross peaks close to the diagonal in COSY experiments. Furthermore, information with respect to remotely coupled multiplets can be extracted from those spectra (6, 7). However, a major drawback of the DQ experiment is its twodimensional representation, which makes comparisons of complicated DQ spectra with COSY type spectra a tedious task. Recently, a modification of the basic DQ experiment which results in a COSY representation of the direct connectivity patterns has been suggested (9). In that experiment data acquisition commences at a time t, after the detection pulse, causing a mixing of the t, and t2 acquisition times analogous to what occurs in spin-echo correlated spectroscopy (SECSY) (10). The pulse sequence requires an anti-echo suppression phase cycle which causes phase twisted lineshapes. Therefore, the results of this elegant experiment can be displayed only in the absolute-value mode, which is undesirable for crowded spectra of larger biomacromolecules. Pure absorption phase DQ spectra can be obtained with a symmetrical excitation/ observation pulse sandwich terminated by a purge pulse (8). The experiment allows for the editing of direct and remote connectivities due to phase differences of the respective cross peaks, and when combined with preparation time averaging, for uniform doublequantum coherence excitation (8). The results of this superior experiment can easily be cast in the COSY representation, without affecting the experimental setup, in the following way. Prior to the tI wI Fourier transformation the interferograms are zero filled to obtain in the final frequency-domain spectrum a number of points in w, twice as large as in ~2. After the transformation, the points i of the wI slices n are left-shifted to the positions j according to (j, n) = (i n, n).