Multiple-quantum-filtered homonuclear RELAY NMR

Abstract

The 2D NMR RELAY experiment (1, 2) has proven to be extremely useful for identifying spin systems in proteins (3-7), nucleic acids (8) and oligosaccharides (9). Although RELAY is being replaced in many instances by isotropic mixing experiments, there are still reasons for obtaining RELAY spectra: (a) RELAY spectra can be selectively optimized for some amino acids (2, 6); and (b) phase-sensitive RELAY spectra contain antiphase cross peaks, which are useful in amino acid spin-system identification (10). Phase-sensitive RELAY spectra suffer, however, in that the diagonal and cross peaks are 90” out-of-phase, and that additional mixed-phase contributions to the diagonal and COSY peaks (2, 4) are present. (Cross peaks arising from directly coupled spins are referred to as COSY peaks; cross peaks between indirectly coupled spins are referred to as RELAY peaks.) Multiple-quantum filtering is invaluable for collecting phase-sensitive COSY data; it is somewhat surprising that similar filtering techniques have not been applied to the RELAY experiment, although the theoretical suggestion has been made by Sorensen (II). In this communication we discuss and demonstrate the results of inserting two multiple-quantum filters into the RELAY pulse sequence. The resulting MQF-RELAY pulse sequence is