Gamma-radiation-induced binding of tyrosine and dopa with albumin and mutually with each other was studied and compared with that of phenylalanine in aqueous solution. With albumin, these three amino acids were bound to the same degree in neutral solution, but in acid solution the binding yield decreased with decreasing numbers of OH groups on the aromatic ring. Increasing the number of OH groups lessened the effect of OH radical scavengers on the binding yield in neutral solution. Chromatographic studies of irradiated amino acid solutions showed the formation of a dimer-like product, but melanine was not formed. By the ninhydrin reaction, a radiation-induced modification of the aromatic amino acids much greater than the degradation of the alanyl side chain was observed in both N2 and O2 saturated solutions. The formation of phenoxyl type radicals does not appear to be due to the reactions of OH and <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math>. The substitutional binding of phenoxyl- and phenyl-type radicals, produced in different ratios to aromatic rings of other molecules, was suggested as the binding mechanism of tyrosine and dopa.

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