Quantitative Measurement of Cross-Correlations between 15N and 13CO Chemical Shift Anisotropy Relaxation Mechanisms by Multiple Quantum NMR

This paper describes an experiment that allows the quantitative measurement of the CSA−CSA cross-correlation between backbone 15 N and 13 CO nuclei in uniformly enriched proteins. The CSA−CSA cross-correlation is obtained from the cross-peak intensity ratios of the double- and the zero-quantum components observed with a modified triple-quantum 2D CT-HNCO experiment. In addition, the 1 H N − 15 N/ 1 H N − 13 CO dipole−dipole cross-correlation was measured without relying on resolved scalar couplings using a complementary quantum 2D CT-HNCO experiment. The cross-correlation rates measured for the protein binase (12.3 kDa) were obtained with high precision but show a surprisingly large range of values. Calculations show that this range is at least partially caused by dynamical processes. The potential use of this information to characterize internal anisotropic motion is discussed.