NMR R1ρ Rotating-Frame Relaxation with Weak Radio Frequency Fields

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Abstract
NMR spin relaxation in the rotating frame (R1ρ) is one of few methods available to characterize chemical exchange kinetic processes occurring on μs−ms time scales. R1ρ measurements for heteronuclei in biological macromolecules generally require decoupling of 1H scalar coupling interactions and suppression of cross-relaxation processes. Korzhnev and co-workers demonstrated that applying conventional 1H 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 R1ρ 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 R1ρ 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 R1ρ 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 R1ρ experiments. The new experiments also extend the capability of the R1ρ technique to study exchange processes outside the fast exchange limit.