The present contribution reports a detailed characterization of the binding interaction of a potential anticancer, anti-HIV drug 1-phenylisatin (1-PI) with a model transport protein Bovine Serum Albumin (BSA) using fluorescence spectroscopic techniques. The thermodynamic parameters e.g., ΔH, ΔS and ΔG for the binding phenomenon have been evaluated on the basis of the van't Hoff equation to reveal that the binding process is principally driven by ionic interactions mediated by charge transfer interaction. This line of argument has been substantiated by frontier molecular orbital analysis of 1-PI. However, the drug-induced quenching of the intrinsic tryptophanyl fluorescence of the protein is found not to abide by a linear Stern–Volmer regression (displaying an upward curvature) when an extensive time-resolved fluorescence spectroscopic characterization of the quenching process has been undertaken to unveil the actuating quenching mechanism. Based on the constancy of the fluorescence lifetime of the protein as a function of drug concentration the observed quenching is inferred to proceed through a static mechanism between the quenching partners. Constant wavelength synchronous fluorescence, excitation–emission matrix fluorescence and circular dichroic (CD) spectroscopic techniques have been exploited to unravel the tertiary and secondary conformational changes in the protein (BSA) induced by drug (1-PI)-binding. The probable binding location of the drug molecule within the protein cavity (hydrophilic subdomain I) has been explored by AutoDock-based blind docking simulation and the inference is further substantiated by site-competitive replacement experiments with specific site-markers. Light is also cast on the drug–protein binding kinetics using the stopped-flow fluorescence technique which reveals an association rate constant of k_a (± 5%) = 1.471 × 10⁻³ s⁻¹ for the interaction of 1-PI with BSA.

The binding interaction of an antimicrobial protein lysozyme (chicken egg white lysozyme; abbreviated as Lz from now and onwards) with intramolecular charge transfer (ICT) probe N,N-dimethylaminonaphthyl-(acrylo)-nitrile (DMANAN) has been explored using fluorescence spectroscopic techniques. The thermodynamic parameters like ΔH, ΔS and ΔG for the binding phenomenon have been evaluated on the basis of van't Hoff equation to reveal that the binding is characterized by negative enthalpy and positive entropy changes substantiating the predominant role of hydrophobic forces in the binding process. Quenching of intrinsic tryptophanyl fluorescence of Lz with increasing DMANAN concentration is the actuating tool in the analysis. Particular focus has been rested on fluorescence resonance energy transfer (FRET) between excited tryptophan in the protein and protein-bound DMANAN. Synchronous fluorescence, three-dimensional excitation-emission matrix fluorescence and circular dichroism (CD) spectroscopic techniques have been exploited to unravel the conformational changes in Lz induced by DMANAN binding. Further, molecular docking has been employed to explore the binding site of the probe in Lz based on the AutoDock-based blind docking strategy.

A new Schiff base compound 2-((benzylimino)-methyl)-naphthalen-1-ol (2BIMN1O) has been synthesized and characterized by ¹H NMR, ¹³C NMR, DEPT, FT-IR and mass spectroscopic techniques. The significantly low fluorescence yield of the compound has been rationalized in connection with photo-induced electron transfer (PET) from the imine receptor moiety to the naphthalene fluorophore unit. Subsequently, an evaluation of the transition metal ion-induced modification of the fluorophore–receptor communication reveals a promising prospect for the title compound to function as a fluorosensor for Cu²⁺ and Zn²⁺ ions selectively, through remarkable fluorescence enhancement. While perturbation of the PET process in 2BIMN1O has been argued to be the responsible mechanism behind the fluorescence enhancement, the selectivity for these two metal ions has been interpreted on the grounds of an appreciably strong binding interaction. Particularly notable aspects regarding the chemosensory activity of the compound are its ability to detect the aforesaid transition metal ions down to the level of micromolar concentration (detection limit being 0.82 and 0.35 μM respectively), along with a simple and efficient synthetic procedure. Also the spectral modulation of 2BIMN1O in the presence of the transition metal ions paves the way for the construction of a calibration curve in the context of its fluorescence signaling potential.

The understanding of the interaction of nanomaterials with relevant biological targets e.g., proteins is of paramount importance in biological and pharmaceutical fields of research. In a biological fluid, proteins can associate with nanomaterials which can subsequently exert a significant impact on the conformation and functionality of the protein. Here we report the binding interaction of a model plasma protein Bovine Serum Albumin (BSA) with a magnetic nanoparticle of mixed spinel origin (Ni_0.5Zn_0.5Fe₂O₄, abbreviated as NZFO from now and onwards). The thermodynamic parameters (ΔH, ΔS and ΔG) for the protein–nanoparticle binding interaction have been evaluated from the van't Hoff equation to unveil that the binding interaction is enthalpically as well as entropically driven (ΔH < 0 and ΔS > 0), with an overall favorable Gibbs free energy change (ΔG < 0). Also the thermodynamic parameters delineate the predominant role of electrostatic interaction in the BSA–NZFO binding process. The results of temperature dependent fluorescence quenching and time-resolved fluorescence decay measurements indicate a static quenching mechanism in the present case. Steady-state absorption, synchronous fluorescence, three-dimensional (3D) fluorescence and circular dichroism (CD) spectroscopic techniques have been employed to unveil the conformational changes in BSA induced by the binding of NZFO. Disruption of the native conformation of the protein upon binding with NZFO is reflected through a reduced functionality (in terms of esterase activity) of the protein–NZFO conjugate system in comparison to the native protein. Based on the experimental findings the probable binding location of NZFO is argued to be the hydrophilic domain IB. This seems physically realizable since domain I of BSA is characterized by a net negative charge and hence can serve as a favorable binding site for NZFO carrying a positive surface charge. The key role of electrostatic forces in the BSA–NZFO interaction process is further substantiated from chemical denaturation study and measurement of the effect of ionic strength on the interaction process.