A 13 C-detected 15 N double-quantum NMR experiment to probe arginine side-chain guanidinium 15 N η chemical shifts.

Arginine side-chains are often key for enzyme catalysis, protein-ligand and protein-protein interactions. The importance of arginine stems from the ability of the terminal guanidinium group to form many key interactions, such as hydrogen bonds and salt bridges, as well as its perpetual positive charge. We present here an arginine 13 Cζ -detected NMR experiment in which a double-quantum coherence involving the two 15 Nη nuclei is evolved during the indirect chemical shift evolution period. As the precession frequency of the double-quantum coherence is insensitive to exchange of the two 15 Nη ; this new approach is shown to eliminate the previously deleterious line broadenings of 15 Nη resonances caused by the partially restricted rotation about the Cζ -Nε bond. Consequently, sharp and well-resolved 15 Nη resonances can be observed. The utility of the presented method is demonstrated on the L99A mutant of the 19 kDa protein T4 lysozyme, where the measurement of small chemical shift perturbations, such as one-bond deuterium isotope shifts, of the arginine amine 15 Nη nuclei becomes possible using the double-quantum experiment.