ARE TRUE SCALAR PROTON PROTON CONNECTIVITIES EVER MEASURED IN COSY SPECTRA OF PARAMAGNETIC MACROMOLECULES

The problem of the detectability of H-1 COSY cross peaks between hyperfine broadened signals in paramagnetic macromolecules in order to ascertain whether they are observable and, if so, to understand their nature, is addressed. It is shown that cross peaks often arise from relaxation-allowed coherence transfer rather than from true scalar couplings. The effect arises from cross correlation between interproton dipolar coupling and Curie relaxation. The latter, which is often the dominant source of H-1 line broadening in paramagnetic macromolecules, effectively acts as a chemical shift anisotropy relaxation mechanism, which is known to cross correlate with proton-proton dipolar relaxation. Literature data are critically evaluated in the light of the above finding.