The effect of the anionic surfactant sodium dodecyl sulfate (SDS) on the protein human serum albumin (HSA) was studied using steady-state spectroscopy, time-resolved measurements, and circular dichroism spectroscopy. The binding of SDS to the domain IIA of HSA, housing the single tryptophan amino acid residue (Trp214), was monitored, and it was found that this addition of the surfactant takes place in a sequential manner depending upon the concentration of the added surfactant. Both fluorescence intensity and lifetimes of HSA decreased with the increasing concentration of SDS, and the surfactant molecules serve the role of a quencher for the fluorescence of Trp214. Circular dichroism data also support the structural changes induced by SDS. The 17 disulfide bridges present in HSA provide the necessary structural rigidity to the protein. Stern-Volmer plots and thermodynamic parameters have been used to characterize the sequential binding of SDS to HSA, and these parameters not only confirm that the binding is spontaneous in nature but also is quite strong, depending on the concentration of the added surfactant.

The binding of the antibiotic tetracycline hydrochloride (TC) to three proteins was investigated by steady-state, time-resolved, and circular dichroism spectroscopy. The tryptophan (Trp) amino acid residues were used as an intrinsic fluorophore to decipher the structure-function relationship. As monitored by CD spectroscopy, the addition of TC causes the protein to alter some of its helical content although such changes are only marginal. The gradual decrease in fluorescence intensity of Trp can be ascribed to static quenching which takes place by the interaction of the drug with the protein. Besides Trp quenching, there is evidence of fluorescence resonance energy transfer (FRET) in all three proteins with different values of efficiency of energy transfer. Various quenching/binding and thermodynamic parameters associated with such drug-protein interactions have been estimated. The results thus obtained can provide guidelines to synthetic chemists to design and synthesize target-oriented drugs.

A very efficient protocol for synthesizing highly fluorescent, protein-templated silver nanoclusters (Ag/NCs) has been discussed. Two types of Ag/NCs (Ag9/HSA and Ag14/HSA), although showing significant differences in their photophysical properties, can be interconverted at will, which makes this study unique. The Ag/HSA NCs have been quantified by several spectroscopic techniques, and they find tremendous applications as photoluminescent markers. Besides their rather easy synthetic methodology, our Ag/HSA NCs show two-photon excitation properties that enable them to be used in bioimaging.

To address the concern of material chemists over the issue of stability and photoluminescent (PL) characteristics of Cu nanoclusters (NCs), herein we present an efficient protocol discussing PL Cu NCs (Cu/HSA) having blue emission and high photostability. These PL NCs were illustrated as efficient probes for Förster resonance energy transfer (FRET) with a compatible fluorophore (Coumarin 153). Our spectroscopic results were well complemented by our molecular docking calculations, which also favored our proposed mechanism for Cu NC formation. The beneficial aspect and uniqueness of these nontoxic Cu/HSA NCs highlights their temperature-dependent PL reversibility as well as the reversible FRET with Coumarin 153, which enables them to be used as a nanothermometer and a PL marker for sensitive biological samples.

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.