Two classes of binding sites, a single high-affinity site with an association constant of 4·8×10⁶ M⁻¹ and two low-affinity sites with association constant of about 0·05×10⁶ M⁻¹ have been observed in the interaction of Naproxen with bovine serum albumin (BSA). Chemical modification of two tryptophan residues in BSA with 2-hydroxy-5-nitrobenzyl bromide has led to a reduction in the association constant of the high-affinity site by 89% and its number of binding sites by 66% suggesting the involvement of tryptophan residues in the high-affinity site. In contrast, the two low-affinity sites were not affected by the modification. Binding of Naproxen to the low-affinity sites of BSA induces microdisorganisation of the albumin structure leading to conformational changes as evident from fluorescence measurements with 1-anilino-8-naphthalenesulphonic acid as the probe.

The binding of rose bengal (RB) to bovine serum albumin (BSA) occurs with both the folded (atpH 7·4) and the unfolded (pH 12·7) forms of BSA. Absorption spectroscopy has revealed an identical red-shift of 15nm in λ_max of RB in presence of BSA both atpH 7·4 and 12·7. The affinity constants (K) atpH 12·7 have been reduced only by 50% in magnitude from those atpH 7·4. These lead us to infer that neither disulphide loops nor buried residues are involved but that the binding of RB occurs at the sites near the surface of BSA. Moreover, the drastic alterations in the near-UV circular dichroism suggest tertiary structural changes induced by RB on binding to BSA. The conformational changes at the binding sites of BSA atpH 7·4 and the affinity of RB particularly towards the exposed residues in BSA atpH 12·7 are the significant factors in the binding of RB to BSA.