Studies on the chemical modification of arginine. I. The reaction of 1,2-cyclohexanedione with arginine and arginyl residues of proteins.

The guanido group of arginine condenses with 1, Z-cyclohexanedione in alkaline aqueous medium to form a new a-amino acid. The product (1 ,2-cyclohexanedione-arginine) was shown by independent synthesis to be N5-(4-0x0-1,3diazaspiro[4.4]non 2 ylidene) L ornithine. At hydroxide concentrations of 0.2 N or higher, this was the principal product, and the reaction with free arginine and with the arginyl residues of model proteins was completed in 3 hours. At lower base concentration or with salmine at high base concentrations, a heterogeneous mixture of ninhydrin-positive products was observed. The reaction appears to be specific for the guanido group. Peptide maps of tryptic digests of 1,2-cyclohexanedione-treated chains of hemoglobin showed blocking of hydrolysis at arginyl bonds with no change in positions of tryptic peptides not adjacent to arginyl residues.