Effect of mercuric salts on the ultraviolet spectra of aromatic amino acids and proteins

Rapid changes occur in the ultraviolet spectra of tyrosine and tryptophan when they are interacted with mercuric acetate or chloride in aqueous solution. The changes result from an intensification and bathochromic shift in the spectra, as evidenced by peaks in the difference spectra at 286–778 mμ for tyrosine and at 297 mμ and 285–286 mμ for tryptophan. The spectral changes can be attributed to the formation of complexes of the amino acids with the mercuric ions. Additional but much slower changes occur when phosphate is present, and under these conditions changes occur in the spectra of compounds which showed no change with mercuric chloride in the absence of phosphate, as exemplified by N-acetyltryptophan. These slower changes are probably due to the formation of complexes involving both mercuric chloride and phosphate, and the rate of absorbance increase can be expressed by a rate law which is first-order in each of indole derivative, mercuric chloride, and phosphate. Third-order rate constants have been determined for N-acetyltryptophan and indole-3-n-propionic acid. Proteins such as β-lactoglobulin and ovalbumin also show spectral changes in the presence of mercuric salts even when phosphate is absent. These changes are not related to the reaction of mercuric ion with sulfhydryl groups but are probably due to structural changes resulting from the binding of mercuric ions to various sites in the proteins.