Double-quantum NOESY. Coupled coherent and incoherent transfer of magnetization

Abstract

The application of 2D Ff NMR to the structural analysis of biological macromolecules has revolutionized the field. On the one hand networks of scalar coupled spins in complicated molecules such as proteins and nucleic acids can be analyzed by making use of coherent magnetization transfer in two-dimensional correlated spectroscopy (COSY) (l-4), on the other hand the proximity of specific nuclei in such molecules can be investigated by detecting incoherent magnetization transfer by means of nuclear Overhauser enhancement spectroscopy (NOESY) (5-7). During the last five years the number of techniques using coherent magnetization transfer has grown substantially: examples are relayed coherence spectroscopy and doublequantum spectroscopy (8-13). The latter techniques were shown to be a prerequisite in the unraveling of networks of scalar coupled spins which, due to ambiguities arising from extensive overlap, could not be resolved within COSY or SECSY (14-16). The process of incoherent transfer of magnetization, taking place via dipolar cross relaxation, is fundamentally different from coherent magnetization transfer which occurs via the J coupling between spins. In methods developed so far, the two phenomena have been employed strictly independently from one another; this is true even in the so-called COCONOSY experiment (I 7). However, there is no reason why a 2D FT NMR experiment should be based solely on either coherent or incoherent magnetization transfer (18). In this paper an experiment is described which couples coherent and incoherent transfer. It will be shown that such an experiment can be used as an aid in resolving ambiguities that may arise in NOESY spectra. To illustrate the basic principles of this new experiment, which we call doublequantum NOESY (DQ NOESY), we shall compare it with the conventional NOESY experiment (see Table 1). Both experiments can be divided in six periods and both experiments start by creating coherences which are frequency labeled during the evolution period I,. In a conventional NOESY experiment a 7r/2 pulse is used to create in-phase single-quantum coherence (i.e., transverse magnetization (5)). In DQ NOESY the preparation sequence of double-quantum spectroscopy (II, 19, 20), namely r/2-r-r-r--1r/2 is used, which gives rise to double-quantum coherences that are frequency labeled during the evolution period t, . After the evolution period,