Impact of two-bond 15 N- 15 N scalar couplings on 15 N transverse relaxation measurements for arginine side chains of proteins.

NMR relaxation of arginine (Arg) 15 Nε nuclei is useful for studying side-chain dynamics of proteins. In this work, we studied the impact of two geminal 15 N-15 N scalar couplings on measurements of transverse relaxation rates (R 2 ) for Arg side-chain 15 Nε nuclei. For 12 Arg side chains of the DNA-binding domain of the Antp protein, we measured the geminal 15 N-15 N couplings ( 2 J NN ) of the 15 Nε 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 15 N 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 15 Nε R 2 measurements. However, our experimental data show that the expected modulation via two 2 J NN couplings vanishes during the 15 N 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 15 Nε 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.