Chemical modification of canavanine with p-nitrophenylglyoxal. Factors influencing the chemistry and reactivity of alpha-dicarbonyl-guanidino reactions.

The role of structural features and deprotonation of guanidino derivatives on chemical reactions with p-nitrophenylglyoxal has been investigated. Canavanine, an arginine analog, reacts to form a yellow product, which absorbs maximally at 350 nm (epsilon = 6500) and at 278 nm (epsilon = 14 500). Elemental analysis, fast atom bombardment mass spectral analysis, n.m.r. and i.r. studies suggest that the product is a 5-(p-nitrophenyl)4-oxo-2 imidazoline derivative of canalaline. Kinetic studies show that the second order rate constant for the reaction increases with increasing pH in the range of pH 7-11.0. It is concluded that the pH dependence of the reaction can be explained by general base catalysis and not simply by a deprotonation of the guanidinoxy side chain. The reaction of arginine, polyarginine, and other derivatives differs markedly from that of canavanine. The results suggest that change in the tautomeric equilibria between the imino and amino forms of the guanidino group may partly account for differences in reaction of canavanine and arginine and the reactions of specific arginyl residues in proteins.