Kinetics of the Reaction of Nitrous Acid with Model Compounds and Proteins, and the Conformational State of N-terminal Groups in the Chymotrypsin Family

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Abstract
The kinetics of the reaction of nitrous acid at 4° and pH 4.0 with various amino acids, peptides, and proteins were studied. The reaction with isoleucine methyl ester was found to have a linear dependence on the square of the HONO concentration showing that N2O3 was the reactive species. Third order nitrosation rate constants of primary amino groups showed a correlation with their pK values. They were calculated for the concentration of the unprotonated species to give intrinsic reactivities. The rate of nitrosation of acetyltryptophan to give N-nitrosoacetyltryptophan was found to be a linear function of the nitrous acid concentration. This nitrosation therefore follows a different mechanism. The reaction of nitrous acid with tyrosine residues was examined by spectrophotometry. The reaction was negligible compared to that of other groups. Acetylhistidine and imidazole did not react. Reactivities for α-amino groups, ε-amino groups, and other residues in proteins were compared. The conformational state of the N-terminal residues in serine proteinases, as revealed from their reactivities, is discussed in detail. It is concluded that nitrous acid reacts preferentially with "surface" residues and is a useful tool for exploring conformational states of reactive groups in proteins, especially α-amino groups and indole rings.

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