Longitudinal and Transverse 1H−15N Dipolar/15N Chemical Shift Anisotropy Relaxation Interference: Unambiguous Determination of Rotational Diffusion Tensors and Chemical Exchange Effects in Biological Macromolecules

High-resolution proton-detected heteronuclear correlation NMR spectroscopy allows the measurement of 15N spin relaxation rates at multiple sites throughout a biological macromolecule. The rate constants are determined by stochastic internal motions on time scales of picoseconds to nanoseconds, overall molecular rotational diffusion on time scales of nanoseconds, and chemical exchange rates on time scales of microseconds to milliseconds. A new method has been developed for distinguishing the contributions of chemical exchange from the contributions due to anisotropic rotational diffusion by measuring both longitudinal and transverse interference between the 1H−15N dipolar and 15N chemical shift anisotropy interactions. The spectroscopic experiment for measuring the longitudinal cross-correlation rate constant for 1H−15N dipolar/15N chemical shift anisotropy interference is based on the approach for measuring the transverse cross-correlation rate constant (Tjandra, N.; Szabo, A.; Bax, A. J. Am. Chem. Soc. 1...