INFLUENCE OF CROSS-CORRELATION BETWEEN DIPOLAR AND ANISOTROPIC CHEMICAL-SHIFT RELAXATION MECHANISMS UPON LONGITUDINAL RELAXATION RATES OF N-15 IN MACROMOLECULES

The intramolecular dipole-dipole and chemical shift anisotropy relaxation mechanisms for the N-15 nucleus of the peptide bond have comparable magnitudes. The cross-correlation (or interference) between these two relaxation mechanisms is analysed using the protein human epidermal growth factor (hEGF). It is demonstrated that the longitudinal N-15 relaxation recovery is not defined by a unique time constant. A two-dimensional experiment to monitor specifically the cross-correlation between the two relaxation mechanisms is discussed. It is also shown that a simple procedure, the inclusion of H-1 broadband decoupling, removes the effects of the cross-correlation upon the N-15 longitudinal relaxation and leads to exponential N-15 recovery.