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 degrees or acute-angle (<90 degrees) 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.

Automated summary

Degenerate spin systems are challenging for CPMG relaxation dispersion NMR experiments. A simple alternative method called the steady-state CPMG scheme is proposed, which utilizes acute-angle 1H radiofrequency pulses to establish a steady-state for effective rates of magnetization decay. The method is applied to two protein systems with different exchange regimes.