SUPPRESSION OF THE EFFECTS OF CROSS-CORRELATION BETWEEN DIPOLAR AND ANISOTROPIC CHEMICAL-SHIFT RELAXATION MECHANISMS IN THE MEASUREMENT OF SPIN SPIN RELAXATION RATES

Cross correlation between dipolar and anisotropic chemical shift relaxation mechanisms complicates measurements of heteronuclear spin-spin relaxation rate constants by the Carr-Purcell-Meiboom-Gill (CPMG) technique if the magnitudes of the chemical shift anisotropy and the dipolar interaction are comparable. Experimental schemes are described that attenuate the effects of cross correlation and permit accurate measurements of spin-spin relaxation rate constants for heteronuclei with significant chemical shift anisotropies. The theoretical analysis is confirmed by measurements of N-15 and C-13 spin-spin relaxation in a peptide. Application of 180-degrees pulses to the protons directly attached to the heteronuclei in synchrony with the even echoes of the heteronuclear spins yields more accurate results than continuous irradiation of the protons with a composite pulse decoupling sequence or application of a single 180-degrees pulse to the protons in the middle of the CPMG pulse train.