Spectroscopic studies on molecular interactions. VI. Mechanism of metachromasy of 2-(4'-hydroxyphenylazo)benzoic acid by serum albumin.

Abstract

Mechanism of the albumin-induced metachromasy of 2-(4'-hydroxyphenylazo) benzoic acid (HABA) has been investigated with regard to its interactions with monomeric amino acids and some dipeptides, spectral effect of polarity of the dye environment, and azohydrazone tautomerism of HABA. The metachromasy is concerned not with the monomeric amino acids or the primary structure of serum albumin, but with its intramolecular environment of lower polarity corresponding to that of isopropanol. HABA, once bound to serum albumin by electrostatic forces etc., is buried in such an environment of lower polarity in the interior of the protein molecule, and is converted into the hydrazone form which has an absorption maximum at about 480 mμ. Thus, the metachromasy may occur. The azo-hydrazone tautomerism is supported by the infrared spectra, and is compatible with the results of quantum chemical calculations on 4-hydroxyazobenzene.