A "steady-State" Relaxation Dispersion Nuclear Magnetic Resonance Experiment for Studies of Chemical Exchange in Degenerate ¹H Transitions of Methyl Groups

Degenerate spin-systems consisting of magnetically equivalent nuclear spins, such as a 1H3 spin-system in selectively 13CH3-labeled proteins, present considerable challenges for the design of Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion NMR experiments to characterize chemical exchange on the micro-to-millisecond time-scale. Several approaches have been previously proposed for the elimination of deleterious artifacts observed in methyl 1H CPMG relaxation dispersion profiles obtained for (13C)1H3 groups. We describe an alternative, experimentally simple solution and design a "steady-state"methyl 1H CPMG scheme, where 90° or acute-angle (<90°) 1H radiofrequency pulses are applied after each CPMG echo in-phase with methyl 1H magnetization, resulting in the establishment of a "steady-state"for effective rates of magnetization decay. A simple computational procedure for quantitative analysis of the "steady-state"CPMG relaxation dispersion profiles is developed. The "steady-state"CPMG methodology is applied to two protein systems where exchange between major and minor species occurs in different regimes on the chemical shift time-scale.