The measurement of relaxation rates of degenerate 1H transitions in methyl groups of proteins using acute angle radiofrequency pulses

Abstract

A simple NMR experiment for the measurement of transverse relaxation rates of degenerate 1H spins in 13CH3 methyl groups of deuterated proteins, is described. The experiment relies on the use of acute-angle 1H radio-frequency pulses, whereby a series of methyl 1H R2 decays is acquired with different values of the 1H pulse flip-angle, which modulates the relative contributions of the slow- and fast-relaxing components of 1H magnetization during the relaxation delay. These are subsequently analyzed simultaneously to extract the transverse relaxation rates of both components (RS and RF), along with the rate of cross-relaxation between these components, δ. The dipolar 1H–1H cross-correlated relaxation rate η = (RF – RS)/2 and the cross-relaxation rate δ, extracted from such acute-angle-pulse R2 decay series are compared with those derived from well-established methodology that uses relaxation-violated methyl 1H coherence transfer (so-called ‘forbidden’ experiments). Good agreement is achieved for the rates η derived from the two techniques, while slightly more accurate values of δ are obtained from analysis of acute-angle-pulse R2 series. Recording of acute-angle-pulse R2 series represents an attractive alternative to existing methodologies for quantifying methyl 1H relaxation and dynamics of the methyl three-fold symmetry axis in selectively [13CH3]-methyl-labeled, deuterated proteins – particularly so for very high-molecular-weight proteins, where the measurements of RF rates or the build-up of methyl 1H magnetization in relaxation-violated coherence transfer experiments, are problematic due to low sensitivity.