Correlation of aliphatic proton chemical shifts in proteins using two-dimensional double-quantum correlated spectroscopy

Abstract

Double-quantum coherences have been used in several ways to aid the interpretation of ‘H NMR spectra of proteins (1-3). For example, double-quantum filtration has been employed in COSY experiments to reduce the intensity of diagonal peaks (2). The detection of remote connectivities in two-dimensional double-quantum spectra has been used to overcome the problems of overlap in the identification of coupled spin systems (3). Recently we discussed the application of two-dimensional dosblequantum spectroscopy to the identification of spin systems of the aromatic amino acids in the ‘H NMR spectra of proteins (I). In this communication we sh,ow that doublequantum coherences are also useful for the identification of resonances arising from methyl groups. In the study of the aromatic amino acids double-quantum coherence was found to be created with good sensitivity when preparation periods of 7 = 15 ms ( l/W) and 7 = 30 ms (1/4J) were used in the INADEQUATE pulse sequence (I, 4). The coupling between the methyl protons of alanine, threonine, leucine, valine, and isoleucine and the adjacent methylene or methyne protons is approximately 7 Hz, similar in magnitude to that between ring protons of aromatic amino acids (approximately 8 Hz) (5). Therefore, with a preparation period, T, of either 15 or 30 ms double-quantum coherence should be created with quite good sensitivity for these aliphatic amino acids. In all the experiments reported here a detection pulse of 3?r/4 was used to suppress correlations from remote spins and to suppress peaks which could arise from multiple-quantum coherence creation within methyl groups (6, 7). Both of these types of peaks were previously shown to occur in doublequ.antum spectra of alanine and threonine when a a/2 detection pulse was used (6). The double-quantum spectra of alanine, threonine, and valine obtained with a T value of 30 ms and of isoleucine obtained with 7 values of 15 and 30 ms are shown in Figs. 1 and 2. The doublequantum spectrum of alanine, threonine, and valine obtained with a r value of 15 ms is very similar to the r = 30 ms spectrum and provides no additional information. The double-quantum spectrum of leucine is qualitatively similar to that of valine and is not reproduced here. Several interesting features emerge from an examination of the double-quantum spectra. All the peaks observed correspond to direct connectivities. The most intense peaks in spectra obtained with a 7 value of 30 ms arise from the doublet methyl