Nuclear magnetic relaxation of methyl protons in a paramagnetic protein: cross-correlation effects

Multi-exponential relaxation and concomitant relaxation allowed multiple qu antum coherence among methyl protons in paramagnetic proteins arising from cross-correlation between Curie spin relaxation (CSR) and dipolar relaxatio n (DD) are investigated. Numerical simulations and semi-quantitative multi- quantum experiments in the cyanide-inhibited paramagnetic protein horseradi sh peroxidase underscore the presence of CSR X DD cross-terms. The magnitud e of the resulting two-spin order depends on the distance and relative orie ntation of the methyl axis to the paramagnetic centre which can be exploite d to derive structural constraints. (C) 2000 Elsevier Science B.V. All righ ts reserved.