Fluorescence and Circular Dichroism Studies on the Interaction of Brmocresol Purple with Bovine Serum Albumin

The mechanism of interaction of bromocresol purple (BCP) with bovine serum albumin (BSA) has been investigated by spectrofluorometric and circular dichroism methods. Association constant for the BCP-BSA system showed thatthe interaction is non-covalent in nature and that there occurs only a partial occupation of a binding site. Binding studies in the presence of hydrophobic probe, 8-anilino-1-naphthalene sulphonic acid, sodium salt (ANS) showed that there is hydrophobic interaction between BCP and ANS and they may share common sites in BSA. Stern-Volmer analysis of fluorescence quenching data showed that the fraction of fluorophore (protein) accessible to the quencher (BCP), was close to unity, indicating thereby that both tryptophan residues of BSA are involved in dye-protein interaction. The rate constant for quenching, greater than 10 1 0 M - 1 s - 1 , indicated that the dye binding site is in close proximity to tryptophan residue of BSA. Thermodynamic parameters, obtained from data at different temperatures, showed that the binding of BCP to BSA involves hydrophobic bonds predominantly. Fluorescence intensity data in the presence of additives showed that hydrophobic interaction plays a prominent role. Significant decrease in concentration of free dye was observed for BCP in presence of paracetamol. Circular dichroism studies revealed the change in helicity of BSA, due to binding of BCP to BSA.