PULSE SEQUENCES FOR REMOVAL OF THE EFFECTS OF CROSS-CORRELATION BETWEEN DIPOLAR AND CHEMICAL-SHIFT ANISOTROPY RELAXATION MECHANISM ON THE MEASUREMENT OF HETERONUCLEAR T1 AND T2 VALUES IN PROTEINS

The effects of cross correlation between dipolar and chemical-shift anisotropy relaxation interactions on the measurement of heteroatom T1 and T2 relaxation times in proteins is considered. It is shown that such effects can produce errors of approximately 25% in the measurement of 15N transverse relaxation times at a field strength of 11.8 T. Cross correlation has a less significant effect on the measurement of 15N spin-lattice relaxation rates and for proteins the errors in T1 decrease as a function of increasing molecular weight. Nevertheless, for T1 measurements at 11.8 T errors of approximately 15 and 5% are calculated for proteins with correlation times, τc, of 5 and 9 ns, respectively. Pulse sequences which eliminate dipolar and chemical-shift anisotropy cross-correlation effects are described. These sequences are used to make more accurate measurements of 15N T1 and T2 values of staphylococcal nuclease and to determine errors in these parameters that result when cross correlations are present. © 1992.