COSY spectra of paramagnetic macromolecules: Observability, scalar effects, cross‐correlation effects, relaxation‐allowed coherence transfer

Abstract

Abstract1H‐1H homonuclear correlated spectroscopy (COSY) cross peaks are detected at high fields in the case of paramagnetic macromolecular systems, even between signals with linewidths of 103 hertz. Because they cannot be ascribed to J‐coupling effects, we began to look for novel or undiscovered physical phenomena. Cross correlation occurs between proton‐proton dipolar coupling and Curie spin‐nuclear spin relaxation. The effect on a dipolar coupled AX system is that each signal is made up of two overlapped components with different linewidths. The cross correlation produces antiphase coherence that generates COSY cross peaks when a 90° ‐t1 −90°‐ t2 experiment is performed, even though the phase of the cross peak with respect to a normal J‐coupling scheme is different. Here, a theoretical derivation of the effect is given. Because the observation of this cross‐correlation effect is absolutely general in paramagnetic metalloproteins, the pulse sequences that quench the effect are shown. It also is stressed that it would be desirable to quench concomitant J‐coupling effects and to detect cross peaks that result only from cross‐correlation effects; a sequence to achieve this is shown.