NMR R1ρ rotating-frame relaxation with weak radio frequency fields

NMR spin relaxation in the rotating frame (R-1rho) is one of few methods available to characterize chemical exchange kinetic processes occurring on mus-ms time scales. R-1rho measurements for heteronuclei in biological macromolecules generally require decoupling of H-1 scalar coupling interactions and suppression of cross-relaxation processes. Korzhnev and co-workers demonstrated that applying conventional H-1 decoupling schemes while the heteronuclei are spin-locked by a radio frequency (rf) field results in imperfect decoupling [Korzhnev, Skrynnikov, Millet, Torchia, Kay. J. Am. Chem. Soc. 2002, 124, 10743-10753]. Experimental NMR pulse sequences were presented that provide accurate measurements of R-1rho rate constants for radio frequency field strengths > 1000 Hz. This paper presents new two-dimensional NMR experiments that allow the use of weak rf fields, between 150 and 1000 Hz, in R-1rho experiments. Fourier decomposition and average Hamiltonian theory are employed to analyze the spin-lock sequence and provide a guide for the development of improved experiments. The new pulse sequences are validated using ubiquitin and basic pancreatic trypsin inhibitor (BPTI). The use of weak spin-lock fields in R-1rho experiments allows the study of the chemical exchange process on a wider range of time scales, bridging the gap that currently exists between Carr-Purcell-Meiboom-Gill and conventional R-1rho experiments. The new experiments also extend the capability of the R-1rho technique to study exchange processes outside the fast exchange limit.