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.

Copper nanoclusters (CuNCs) exhibit a high tendency to undergo oxidation particularly at the subnanometer size regime. In the light of overcoming this bottleneck, we have been successful in developing tripeptide (glutathione, GSH) templated CuNCs which show high biocompatibility and stability, in spite of being ultrafine in size. These blue-emitting CuNCs possess very promising optical features such as significant quantum yield (QY) and excellent photostability. Our cell-imaging studies reveal that the CuNCs localize primarily in nuclear membranes of the different cancerous (Hela, MDAMB-231, and A549) cells, and the cell viability assay conclusively established their nontoxic nature. Apart from their biological significances, these CuNCs also illustrate their ability to serve as a metal ion sensor, selectively detecting Fe 3+ ions in solution at the nanomolar concentration regime. This unique luminescent property of the NCs will enable them to serve as label-free and versatile probes having several biological and analytical applications.

Herein we report the effect of different nucleobase pair compositions on the association-induced fluorescence enhancement property of Thioflavin T (ThT), upon binding with 20 base pair long double-stranded DNA (dsDNA). Analysis of binding and decay constants along with the association (K ass ) and dissociation (K diss ) rate constants obtained from the fluctuation in the fluorescence intensity of ThT after binding with different DNA revealed selective affinity of ThT toward AT-rich dsDNA. Molecular docking also substantiates the experimental results. We also observed that addition of orange-emitting ethidium bromide (EtBr) to cyan-emitting ThT−DNA complexes leads to bright white light emission (WLE) through Forster resonance energy transfer. Additionally, the emission of white light is far greater in the case of intra-DNA strands. Besides endorsing the binding insights of ThT to AT-rich dsDNA, the present investigations open a new perspective for realizing promising WLE from two biomarkers without labeling the DNA.

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.

The relationship between protein conformational dynamics and enzymatic reactions has been a fundamental focus in modern enzymology. Using single-molecule Fluorescence Resonance Energy Transfer (FRET) with a combined statistical data analysis approach, we have identified the intermittently appearing coherence of the enzymatic conformational state from the recorded single molecule intensity-time trajectories of enzyme 6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) in catalytic reaction. The coherent conformational state dynamics suggests that the enzymatic catalysis involves a multi-step conformational motion along the coordinates of substrate-enzyme complex formation and product releasing, presenting as an extreme dynamic behavior intrinsically related to the time bunching effect that we have reported previously. The coherence frequency, identified by statistical results of the correlation function analysis from single-molecule FRET trajectories, increases with the increasing substrate concentrations. The intermittent coherence in conformational state changes at the enzymatic reaction active site is likely to be common and exist in other conformation regulated enzymatic reactions. Our results of HPPK interaction with substrate support a multiple-conformational state model, being consistent with a complementary conformation selection and induced-fit enzymatic loop-gated conformational change mechanism in substrate-enzyme active complex formation.