NMR relaxation of arginine (Arg) 15Nε nuclei is useful for studying side-chain dynamics of proteins. In this work, we studied the impact of two geminal 15N–15N scalar couplings on measurements of transverse relaxation rates (R 2 ) for Arg side-chain 15Nε nuclei. For 12 Arg side chains of the DNA-binding domain of the Antp protein, we measured the geminal 15N–15N couplings ( 2 J NN ) of the 15Nε nuclei and found that the magnitudes of the 2 J NN coupling constants were virtually uniform with an average of 1.2 Hz. Our simulations, assuming ideal 180° rotations for all 15N nuclei, suggested that the two 2 J NN couplings of this magnitude could in principle cause significant modulation in signal intensities during the Carr–Purcell-Meiboom–Gill (CPMG) scheme for Arg 15Nε R 2 measurements. However, our experimental data show that the expected modulation via two 2 J NN couplings vanishes during the 15N CPMG scheme. This quenching of J modulation can be explained by the mechanism described in Dittmer and Bodenhausen (Chemphyschem 7:831–836, 2006). This effect allows for accurate measurements of R 2 relaxation rates for Arg side-chain 15Nε nuclei despite the presence of two 2 J NN couplings. Although the so-called recoupling conditions may cause overestimate of R 2 rates for very mobile Arg side chains, such conditions can readily be avoided through appropriate experimental settings.
Journal of Biomolecular NMR – Springer Journals
Published: May 29, 2018
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