A nitrogen-15 NMR study of the barriers to isomerization about guanidinium and guanidino carbon-nitrogen bonds in L-arginine

Abstract

^(15)N nuclear magnetic resonance (NMR) spectroscopy has been employed to investigate barriers to isomerization about the C-N2 bond in guanidinium and guanidino groups of L-arginine by line-shape analyses of the N1 and N3 resonances in a 50% (v/v) dimethyl sulfoxide/water solution from -52 to 25 ºC. The free energies of activation were found to be 12.9 and 10.4 kcal/mol, and the chemical-shift differences between the nonequivalent nitrogens, N1 and N3, were 2.4 and 42.3 ppm for the guanidinium and guanidino groups of L-arginine, respectively. For the guanidino form of L-arginine, the three possible tautomers with different arrangements of the guanidino protons are interconverted rapidly on the NMR time scale over the whole of the temperature range studied. Approximately one-third of the guanidino form is present as the R-N=C(NH_2)_2 tautomer. The results are compared with those previously reported for tetramethylguanidine derivatives.

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