LAUSR

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 (R2) 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 (2JNN) of the 15Nε nuclei and found that the magnitudes of the 2JNN 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 2JNN couplings of this magnitude could in principle cause significant modulation in signal intensities during the Carr–Purcell-Meiboom–Gill (CPMG) scheme for Arg 15NεR2 measurements. However, our experimental data show that the expected modulation via two 2JNN 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 R2 relaxation rates for Arg side-chain 15Nε nuclei despite the presence of two 2JNN couplings. Although the so-called recoupling conditions may cause overestimate of R2 rates for very mobile Arg side chains, such conditions can readily be avoided through appropriate experimental settings.