BINDING OF 2-(4'-HYDROXYPHENYLAZO) BENZOIC ACID WITH ARGININE, LYSINE, TYROSINE AND TRYPTOPHAN MODIFIED BOVINE SERUM ALBUMIN

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The two spectrally distinguishable bound forms of 2-(4'-hydroxyphenylazo) benzoic acid (HABA) on bovine serum albumin (BSA), i.e, the azo and hydrazone forms, were suggested to form ion pairs with basic amino acid residues in our previous study. Arginine (Arg), lysine (Lys), tyrosine (Tyr) and tryptophan (Trp) were modified to estimate the amino acid residues participating in the formation of an ion pair with HABA. Decrease of the binding of only hydrazone form was observed following mocification of Arg residues. The modification of Lys, Tyr and Trp residues caused no decrease in binding to either form. However, the induced circular dichroism (CD) spectra of HABA bound to N- and Oacetylated BSA were reversed at 360 and 440 nm, respectively. But two bands of these spectra were capable of taking on the same shape of the spectra of native BSA only by Odeacetylation. The induced CD spectra of bound HABA by Trp modified BSA changed as if the bound amounts of the azo form were increased. The binding sites of the azo form may possibly be situated in the vicinity of Trp 212 on the tertiary structure.
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