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.
, Δ
H
, Δ
S
and Δ
G
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 present work explores the binding interaction, thermodynamics and kinetics of interaction of a potential anti-HIV, anti-cancer drug 1-phenylisatin with a model transport protein. The probable binding location and nature of binding forces are also argued from combined experimental and simulation techniques.
A simple intramolecular charge transfer (ICT) compound, 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid methyl ester (DPDAME), has been documented to be a potential molecular reporter for probing ...microheterogeneous environments of a model transport protein bovine serum albumin (BSA) using spectroscopic techniques. Meteoric modifications to the emission profile of DPDAME upon addition of BSA come out to be a result of its binding to hydrophobic subdomain IIA. The highly polarity-sensitive ICT emission of DPDAME is found to be a proficient extrinsic molecular reporter for efficient mapping of native, intermediate, unfolded, and refolded states of the protein. Experimental data coupled with a reinforcing support from theoretical simulation using CHARMM22 software confirm the binding site of the probe to be the subdomain IIA of BSA, while FRET study reveals a remarkably close approach of our extrinsic molecular reporter to Trp-212 (in domain IIA): the distance between DPDAME and Trp-212 is 1.437 nm. The caliber of DPDAME as an external fluorescence marker also extends to the depiction of protein−surfactant (BSA−SDS) interaction to commendable fruition. Additionally, the protective action of small amounts of SDS on urea-denatured protein is documented by polarity-sensitive ICT emission of the probe. The present study also reflects the enhancement of the stability of BSA with respect to chemically induced denaturation by urea as a result of binding to the probe DPDAME.
The present work demonstrates the effect of biological confinement on the photophysics and dynamics of a bio-active drug molecule viz., 5-chlorosalicylic acid (5ClSA). 5ClSA is a potential candidate ...exhibiting Excited-State Intramolecular Proton Transfer (ESIPT) reaction and thereby generating the phototautomer (i.e. proton transferred keto form) in the excited state. Given the pK(a) of 5ClSA (around 2.64), the anionic form of the drug molecule is expected to be the interacting species with the protein under the experimental conditions (buffered solution of pH 7.40). The ESIPT photophysics of the drug (5ClSA anion) is found to be remarkably modified within the confined bio-environment of a model transport protein Bovine Serum Albumin (BSA) in terms of remarkable emission intensity enhancement coupled with a discernible red-shift of the emission maximum wavelength. Such considerable modification of the ESIPT photophysics of the 5ClSA anion has been exploited to determine the drug-protein binding strength (as characterized by the binding constant K (±10%) = 6.11 × 10(2) M(-1)). The present work also delves into evaluation of the probable binding location of the drug within the biomacromolecular assembly of the protein by a blind docking simulation technique, which reveals hydrophobic subdomain IIA to be the probable binding site of the drug. Circular dichroism (CD) spectroscopy delineates the effect of drug binding on the protein secondary structure in terms of decrease of α-helical content of BSA with increasing drug concentration. Apart from this, the excitation-emission matrix fluorescence technique is found to hint at the effect on protein tertiary structure upon binding to the drug. Chaotrope-induced protein denaturation has been explored to complement the findings on the binding interaction process. The modulated dynamics of the proton transfer phototautomer of the 5ClSA anion within the biological confinement is also investigated in this context to explore the slower rate of solvent-relaxation dynamics.
Here, we report a Density Functional Theoretical (DFT) study on the photophysics of a potent Excited-State Intramolecular Proton Transfer (ESIPT) molecular system, viz., 10-hydroxybenzo
hquinoline ...(HBQ). Particular emphasis has been rendered on the assessment of the proton transfer reaction in HBQ in the ground and excited-states through elucidation and a careful perusal of the potential energy surfaces (PES). The non-viability of Ground-State Intramolecular Proton Transfer (GSIPT) process is dictated by a high-energy barrier coupled with no energy minimum for the proton transferred (K-form) form at the ground-state (S
0) PES. Remarkable reduction of the barrier along with thermodynamic stability inversion between the enol (E-form) and the keto forms (K-form) of HBQ upon photoexcitation from S
0 to the S
1-state advocate for the operation of ESIPT process. These findings have been cross-validated on the lexicon of analysis of optimized geometry parameters, Mulliken’s charge distribution on the heavy atoms, and molecular orbitals (MO) of the E- and the K-forms of HBQ. Our computational results also corroborate to experimental observations. From the modulations in optimized geometry parameters in course of the PT process a critical assessment has been endeavoured to delve into the movement of the proton during the process. Additional stress has been placed on the analysis of the intramolecular hydrogen bonding (IMHB) interaction in HBQ. The IMHB interaction has been explored by calculation of electron density
ρ(r) and the Laplacian ∇
2
ρ(r) at the bond critical point (BCP) using Atoms-In-Molecule (AIM) method and by calculation of interaction between σ⁎ of OH with the lone pair of the nitrogen atom using Natural Bond Orbital (NBO) analysis.
► Theoretical modelling of the photophysics of an ESIPT probe 10-hydroxybenzo
hquinoline (HBQ). ► Calculation of intramolecular hydrogen bond (IMHB) energy. ► Role of hyperconjugative charge transfer interaction in IMHB assessed by NBO perspective. ► Topological properties of IMHB analyzed from AIM view point. ► Computational results are assayed from direct comparison with experimental reports.
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(0.5)Zn(0.5)Fe(2)O(4), 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.
The present work demonstrates a detailed characterization of the interaction of a bio-active drug molecule 3,5-dichlorosalicyclic acid (3,5DCSA) with a model transport protein Bovine Serum Albumin ...(BSA). The drug molecule is a potential candidate exhibiting Excited-State Intramolecular Proton Transfer (ESIPT) reaction and the modulation of ESIPT photophysics within the bio-environment of the protein has been exploited spectroscopically to monitor the drug-protein binding interaction. Apart from evaluating the binding constant (K (±10%) = 394 M(-1)) the probable location of the neutral drug molecule within the protein cavity (hydrophobic subdomain IIA) is explored by AutoDock-based blind docking simulation. The rotational relaxation dynamics of the drug within the protein has been interpreted on the lexicon of the two-step and wobbling-in-cone model. Circular dichroism (CD) spectroscopy delineates the effect of drug binding on the protein secondary structure in terms of decrease of α-helical content of BSA with increasing drug concentration. Also the esterase activity of the drug:protein conjugate system is found to be reduced in comparison to the native protein.
The present account aims at amassing and recounting on our series of spectroscopic studies with a potential excited state intramolecular proton transfer (ESIPT) probe 1-hydroxy-2-naphthaldehyde ...(HN12). After a detailed investigation from experimental as well as theoretical viewpoints, a deeper insight into the photophysics of the selected molecular system is provided from thorough spectral deciphering of the effects of solvent, medium pH and temperature. In the following sections, the ESIPT emission of HN12 has been documented to be a potential avenue wherefrom characterization of a wide variety of biological, biomimetic and supramolecular assemblies has been executed to commendable fruition. Efforts are also invested to delineate the location, distribution and strength of interaction of the probe with various microheterogeneous environments.
► Excited-state intramolecular proton transfer probe 1-hydroxy-2-naphthaldehyde (HN12). ► ESIPT photophysics of HN12 in homogeneous solvents and computational support. ► Sensitive ESIPT spectral signatures to medium polarity/pH, temperature. ► Application of ESIPT emission as a probe for biological and biomimicking environments. ► Exploring protein denaturation, renaturation and phase-transition in lipid membranes.
A new Schiff base compound 2-((benzylimino)-methyl)-naphthalen-1-ol (2BIMN1O) has been synthesized and characterized by (1)H NMR, (13)C 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 Cu(2+) and Zn(2+) 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.
Intramolecular charge transfer (ICT) reaction has been investigated in 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid methyl ester (DPDAME) using spectroscopic techniques. The molecule DPDAME ...shows local emission in non-polar solvent and dual emission in polar solvents. Solvatochromic effects on the Stokes shifted emission band clearly demonstrate the charge transfer character of the excited state. Quantum chemical calculations have been performed at Hartree–Fock (HF) and density functional theoretical (DFT) levels to correlate the experimental findings. Potential energy curves (PECs) for the ICT reaction have been evaluated along the donor twist angle at DFT and time dependent density functional theory (TDDFT) levels for the ground and excited states, respectively, using B3LYP hybrid functional and 6-31G
⁎
⁎ basis set. The solvent effects on the spectral properties have been explored theoretically at the same level with time dependent density functional theory-polarized continuum model (TDDFT-PCM) and the theoretical results are found to well substantiate the solvent polarity dependent Stokes shifted emission of DPDAME. Huge enhancement of dipole moment (Δ
μ=16.42
D) of the molecule following photoexcitation dictates the highly polar character of the excited state. Although elucidation of PECs does not exactly predict the operation of ICT according to twisted intramolecular charge transfer (TICT) model in DPDAME, lowering of vertical transition energy as a function of the donor twist coordinate scripts the occurrence of red shifted emission as observed experimentally.
The excited-state intramolecular proton transfer (ESIPT) reaction of 1-hydroxy-2-naphthaldehyde (HN12) has been studied within the interior of the supramolecular assemblies of α-, β-, and ...γ-cyclodextrins (CD) and biomimicking environments of ionic (SDS) and non-ionic (TW-20) micelles. Fluorescence measurements are used to investigate the effect of various supramolecular assemblies on the ESIPT reaction by monitoring the large Stokes-shifted tautomer emission of HN12. Enhanced tautomer emission in the microencapsulated state predicts favorable ESIPT reaction in the supramoleuclar assemblies. Benesi−Hildebrand plots have been employed to ascertain that the stoichiometric ratios of the complexes formed between HN12 and CDs are 1:2, 1:1, and 1:1 for α-, β-, and γ-CD, respectively. The binding constants (K 1) and free-energy change (ΔG) for inclusion complexation are also determined from the linearized Benesi−Hildebrand plots. Steady-state fluorescence anisotropy, REES, excitation anisotropy, and fluorescence lifetime measurements are in line with other experimental findings. Differential action of urea on SDS and TW-20-bound probe has also been investigated.
