Abstract
NMR relaxation of arginine (Arg) (15)N(ε) 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)N(ε) 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)N(ε) 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)N(ε) 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)N(ε) 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.