The mechanism by which the protein bovine serum albumin undergoes unfolding induced by the anionic surfactant sodium dodecyl sulphate (SDS) and then the subsequent refolding brought in by β-Cyclodextrin (β-CD) was studied by steady-state fluorescence, time resolved measurements and Circular Dichroism (CD) spectroscopy. The prominent findings of this investigation are (i) SDS unfolds the protein in a sequential manner passing through three different phases of binding of SDS followed by a saturation phase; (ii) the refolding process is initiated through inclusion/removal of SDS molecules by β-CD and hence this also seems to happen in a phased manner; (iii) the process of refolding seems to be reversible to the unfolding process but the protein does not regain all its structure on refolding; (iv) however, CD results reveal almost 100% recovery of the secondary structure lost during SDS induced unfolding. We have conclusively proved that there is a marginal structural gain of the native protein at low surfactant concentration and β-CD also induces a marginal structural loss to the native protein. The unfolding process induced by SDS seems to be spontaneous and the binding of SDS to BSA is rather strong, as revealed by thermodynamic parameters.

The effect of the added fluoroquinolone, Ciprofloxacin Hydrochloride (CpH), on structural properties of Bovine Serum Albumin (BSA) was investigated by Circular Dichroism (CD), steady-state, time-resolved and Dynamic Light Scattering (DLS) spectroscopic approaches. The intrinsic fluorescence of the Tryptophan (Trp) amino acid residue in the globular protein BSA was made use of and the effect of pH at two different temperatures was thoroughly investigated. CD results indicate that CpH induces some structural changes in BSA and this has been well-supported by steady-state, lifetime and DLS data. The fluorescence intensity of Trp gradually decreases with the rise in concentration of CpH and we have conclusively proved that at pH 7.4 and 9.2, the mechanism of fluorescence quenching is mostly dynamic in nature, whereas at pH 4.5 mainly static quenching is operational. Thermodynamic parameters have been studied to rationalize the nature of binding of CpH to BSA, and we have concluded that hydrophobic and van der Waals forces play an important role in the process of drug–protein interaction at three different pH values. The lifetime of Trp was found to decrease with the rise in CpH concentration and the percentage reduction in lifetime was found to be a function of the pH of the medium under investigation.