Vol. 117, No. 2, 1983 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS December 16, 1983 Pages 479-485 IMPROVED SPECTRAL RESOLUTION IN COSY 1H NMR SPECTRA OF PROTEINS VIA DOUBLE QUANTIJM FILTERING +* M. Rance , O.M. s!.?wensen+, G. Bodenhausen+, G. Waqner*, + R.R. Ernst and K. Wiithrich* * Institut fiir Molekularbioloqie und Biophysik, Eidgenassische Technische Hochschule, CH-8093 Ziirich, Switzerland t Laboratorium fiir Physikalische Chemie, Eidqenossische Technische Hochschule, CH-8092 Ziirich, Switzerland Received October 4, 1983 SUMMARY: A double quantum filter is inserted into a two-dimensional correlated (COSY) 1~ NMR experiment to obtain phase-sensitive spectra in which both cross peak and diagonal peak multiplets have anti-phase fine structure, and in which the cross peaks and the major contribution to the diagonal peaks have absorption lineshapes in both dimensions. The elimination of the dispersive character of the diaqonal peaks in phase-sensitive, double quantum-filtered COSY spectra allows identification of cross peaks lyinq immediately adjacent to the diaqonal, which represents a siqnificant improvement over the conventional COSY experiment. Two-dimensional correlated spectroscopy (COSY) (l-3) has for some time been employed as an efficient method for the delineation of the spin-spin coupling networks of the individual amino acid residues in proteins (4-6). While the early investigations used absolute value presentations of the data, more recent work showed that phase-sensitive COSY spectra recorded with high diqital resolution provide further important advantaqes (7). For example, with the resolved fine structure of the individual cross peaks unambiquous resonance assiqnments can be obtained even in crowded spectral reqions, and quantitative measurements of the spin-spin couplinq constants can be made. A limitation in the use of phase-sensitive presentations of conventional COSY experiments arises because cross peaks and diagonal peaks have different lineshapes (1). which can cause enhancement of the diagonal peaks over the cross peaks in the spectra of macromolecules. As a consequence, important spectral regions near the diagonal are usually not amenable to spectral analysis. The present paper demonstrates the usefulness of applyinq a double quantum filter (DQF) (8,9) in the COSY experiment of a protein to obtain phase-sensitive spectra in which cross peaks and diagonal peaks have the same lineshapes. The entire spectrum 0006-291X/83 $1.50 479 Copyright 0 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.