2D Heteronuclear NMR Measurements of Spin-Lattice Relaxation Times in the Rotating Frame of X Nuclei in Heteronuclear HX Spin Systems

Theoretical and experimental aspects of T,, are discussed for a heteronuclear HX twospin system (T$) where only the X nucleus is spin-locked. An expression for TE in terms of spectral density functions and the effective magnetic field parameters is developed. It shows that T:, offers potentially dierent information about the spectral densities than either r:, TF, or the steady-state NOEX. We present a 2D heteronuclear NMR pulse sequence for measuring site-specific T$‘s in biomolecules. The sequence is based on a double-INEPT transfer and applies a spin lock to the heteronuclei for variable delays. If a weak on-resonance spin lock is used, and if the spectral density functions are assumed to be Lorentzians, then rc is theoretically indistinguishable from r:. We conclude with an application of the pulse sequence to the uniformly ‘5N-enriched protein eglin c. The rc, data reflect the differential mobility in the molecule. o 199 t Academic PESS, h.