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, 1̂, 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 -NH_2 groups. The results can be rationalized by consideration of the contributions of various valence-bond structures to the resonance hybrid of 1.