Differential rates of proton exchange for the guanidinium nitrogens of L-arginine determined by natural-abundance nitrogen-15 nuclear magnetic resonance spectroscopy.

Differential rates of NH proton-exchange reactions have been determined for the guanidinium nitrogens of l-arginine, $\text{1}\text{?}$, in the pH range of 0.5 to 11.5 by natural-abundance nitrogen-15 NMR spectroscopy. Base-catalyzed NH proton exchange of the NH group is found to be two times faster than for the guanidino NH₂ groups. The results can be rationalized by consideration of the contributions of various valence-bond structures to the resonance hybrid of $\text{1}\text{?}$.