Deciphering Spectroscopic and Structural Insights into the Photophysical Behavior of 2,2'-Dipyridylamine: An Efficient Environment Sensitive Fluorescence Probe

Excited state deactivation properties and the effects of solvent hydrogen bonding (HB) on the photophysical behavior of 2,2'-dypyridylamine (DPyA) were investigated by steady state and time-resolved fluorescence experiments, molecular docking, and density functional theory (DFT) calculations. In addition to the polarity effect, the contributions of solvent HB donation (HBD) acidity and HB acceptance (HBA) basicity to modulate the solvatochromic spectral properties were estimated from multiparametric linear regression analysis using Kamlet-Taft (KT) and Catalán formalisms. The importance of C-N bond torsion, leading to the trans → cis conversion, was manifested by substantial increase in DPyA fluorescence yield in the presence of cyclodextrin (CD) and glycerol. The unusually low fluorescence yield in aqueous medium was explained on the basis of synergistic effect of solvent hydrogen bonding combined with excited state conformational isomerization, which renders DPyA to be an excellent environment sensitive fluorescence reporter. The experimental results were verified with structural insights obtained from DFT calculations at B3LYP/6-311++G(d,p) level and construction of potential energy surface (PES) in the ground state as well as in the excited states.

Targeting the heme protein hemoglobin by (−)-epigallocatechin gallate and the study of polyphenol–protein association using multi-spectroscopic and computational methods

(−)-Epigallocatechin gallate binds to BHb and exhibits anti-glycating as well as antioxidant behaviors towards glycation and photo-oxidation of BHb.

Lysozyme-luteolin binding: molecular insights into the complexation process and the inhibitory effects of luteolin towards protein modification

In the proposed work, the complexation of bioactive flavonoid luteolin with hen egg white lysozyme (HEWL) along with its inhibitory influence on HEWL modification has been explored with the help of multi-spectroscopic and computational methods. The binding affinity has been observed to be moderate in nature (in the order of 10⁴ M^-1) and the static quenching mechanism was found to be involved in the fluorescence quenching process. The binding constant (K_b) shows a progressive increase with the increase in temperature from (4.075 ± 0.046 × 10⁴ M^-1) at 293 K to (6.962 ± 0.024 × 10⁴ M^-1) at 313 K under experimental conditions. Spectroscopic measurements along with molecular docking calculations suggest that Trp62 is involved in the binding site of luteolin within the geometry of HEWL. The positive changes in enthalpy (ΔH = +19.99 ± 0.65 kJ mol^-1) as well as entropy (ΔS = +156.28 ± 2.00 J K^-1 mol^-1) are indicative of the presence of hydrophobic forces that stabilize the HEWL-luteolin complex. The micro-environment around the Trp residues showed an increase in hydrophobicity as indicated by synchronous fluorescence (SFS), three dimensional fluorescence (3D) and red edge excitation (REES) studies. The % α-helix of HEWL showed a marked reduction upon binding with luteolin as indicated by circular dichroism (CD) and Fourier-transform infrared spectroscopy (FTIR) studies. Moreover, luteolin is situated at a distance of 4.275 ± 0.004 nm from the binding site as indicated by FRET theory, and the rate of energy transfer k_ET (0.063 ± 0.004 ns^-1) has been observed to be faster than the donor decay rate (1/τ_D = 0.606 ns^-1), which is indicative of the non-radiative energy transfer during complexation. Leaving aside the binding study, luteolin showed promising inhibitory effects towards the d-ribose mediated glycation of HEWL as well as towards HEWL fibrillation as studied by fluorescence emission and imaging studies. Excellent correlation with the experimental observations as well as precise location and dynamics of luteolin within the binding site has been obtained from molecular docking and molecular dynamics simulation studies.

Therapeutic potency of substituted chromones as Alzheimer's drug: Elucidation of acetylcholinesterase inhibitory activity through spectroscopic and molecular modelling investigation

Introduction: Documentation on the potency of chromones as acetylcholinesterase (AChE) antagonists has paved the way for the design and usage of new chromone analogues as inhibitors of AChE modelled on the hypothesis based on cholinergic pathway of Alzheimer's disease (AD). Here, 2 minimally substituted chromones, namely 3-cyanochromone (CyC) and 7-amino-3- methylchromone (AMC), were checked for their AChE inhibition efficacies and plasma protein modulation. Methods: Colorimetric enzymatic assay as well as fluorescence measurements were performed for obtaining the experimental results, which were further corroborated by molecular docking and simulation studies. Results: The investigated systems exhibited strong inhibition activities against AChE, with CyC (IC50= 85.12 ± 6.70 nM) acting as better inhibitor than AMC (IC50 = 103.09 ± 11.90 nM) and both having IC50 values in the range of FDA approved cholinergic drug Donepezil (IC50 = 74.13 ± 8.30 nM). Non-competitive inhibition was observed in both the cases with the inhibitors binding near the peripheral anionic site (PAS) of the enzyme. Having one planar nitrile group in CyC as compared to sp3 hybridised substituents in AMC facilitated stacking interactions in the former, accounting for its higher inhibitory efficacy. A significant decrease in the inhibition potency of CyC (~32%) was noted in comparison with AMC (~5%) when the experiments were performed in presence of human serum albumin (HSA) instead of pure aqueous buffer. Conclusion: This comparative study affirms the importance of meticulous substitution in the chromone scaffold to promote maximum inhibition potency, while considering their usage as AD drugs.

Fluorescence solvatochromism and modulated anticholinergic activity of novel coumarin compounds sequestered in human serum albumin nanocavities

The antagonistic effect of the HSA matrix towards the acetylcholinesterase inhibition potencies of novel coumarin compounds is probed by multi-spectroscopic techniques.

An efficient synthesis of triazolium ion based NHC precursors using diaryliodonium salts and their photophysical properties

Copper-catalysed N-arylation of fused triazoles using diaryliodonium salts as an aryl source is described. This scalable protocol displayed good compatibility towards diverse sensitive functional groups like ester, alkyl and nitro groups and halogens (F, Cl, Br). The synthetic usefulness of the prepared triazolium salts was proved by preparing α-hydroxyketone through benzoin condensation. Photophysical studies of these compounds showed promising Stokes-shifted fluorescence emission in aqueous medium, so this molecular framework could be a proficient probe for biological applications.

Molecular recognition of bio-active flavonoids quercetin and rutin by bovine hemoglobin: an overview of the binding mechanism, thermodynamics and structural aspects through multi-spectroscopic and molecular dynamics simulation studies

The binding of two bio-active flavonoids, quercetin and rutin, with bovine hemoglobin (BHb) was investigated by multi-spectroscopic and computational (molecular docking and molecular dynamics simulation) studies. The two flavonoids were found to quench the intrinsic fluorescence of BHb through a static quenching mechanism. The binding constants at 288 K were observed to be (14.023 ± 0.73) × 104 M-1 and (7.848 ± 0.20) × 104 M-1, respectively for quercetin and rutin binding with BHb. Both rutin and quercetin were observed to increase the polarity around the Trp residues of BHb as indicated by synchronous and 3D spectral studies. No significant alterations in the secondary structural components of the protein were caused during the binding of the flavonoids as studied by CD and FTIR studies. The negative molar Gibbs free energies indicated the spontaneity of the interaction processes while the binding processes were characterized by a negative enthalpy change (ΔH) and a positive entropy change (ΔS). The possibility of energy transfer from the donor (BHb) to the acceptor molecules (flavonoids) was indicated by the FRET studies. According to the fluorescence studies, the flavonoids interact near to the β2-Trp37 residue of BHb. Excellent correlations with the experimental studies were observed from the molecular docking and molecular dynamics (MD) simulation studies. Further investigations established that these flavonoids are efficient in the inhibition of glucose mediated glycation of BHb.

Correlation of cholinergic drug induced quenching of acetylcholinesterase bound thioflavin-T fluorescence with their inhibition activity

The development of new acetylcholinesterase inhibitors (AChEIs) and subsequent assay of their inhibition efficiency is considered to be a key step for AD treatment. The fluorescence intensity of thioflavin-T (ThT) bound in the active site of acetylcholinesterase (AChE) quenches substantially in presence of standard AChEI drugs due to the dynamic replacement of the fluorophore from the AChE active site as confirmed from steady state emission as well as time-resolved fluorescence anisotropy measurement and molecular dynamics simulation in conjunction with docking calculation. The parametrized % quenching data for individual system shows excellent correlation with enzyme inhibition activity measured independently by standard Ellman AChE assay method in a high throughput plate reader system. The results are encouraging towards design of a fluorescence intensity based AChE inhibition assay method and may provide a better toolset to rapidly evaluate as well as develop newer AChE-inhibitors for AD treatment.