Fluorescent Schiff Bases with Phenothiazine and Coumarin Moieties: Synthesis, Characterization, Photophysical, Electrochemical, Computational and Biological Studies.

We present the design and synthesis of four new schiff bases viz., two each of phenothiazine and coumarin derivatives. The structures were proposed based on FT-IR, 1H NMR, 13C NMR and mass spectral data. The photophysical, solvatochromic and electrochemical studies of all the compounds were carried out. Furthermore, theoretical studies such as density functional theory (DFT) were carried out to gain a better understanding of the compounds' intramolecular charge transfer properties and electronic structures. Good agreement was noticed between the HOMO-LUMO energy gap obtained from DFT studies and that calculated from absorption threshold wavelengths. All the compounds showed Stokes shifts in the range of 6345-11,405 cm-1. These findings showed that the new schiff bases could be considered as attractive candidates for use in the development of OLEDs, organic electrical devices and optoelectronic devices. Newly synthesized compounds were tested for biological activities. When compared to conventional standards, gallic acid and indomethacin, the schiff bases showed better antioxidant and antiinflammatory activities, respectively.

Elucidating the binding mechanism of an antimigraine agent with a model protein: insights from molecular spectroscopic, calorimetric and computational approaches.

Sumatriptan (SUM), a serotonin activator used to treat migraines and cluster headaches. Molecular spectroscopic methods including fluorescence quenching, time dependent fluorescence, FRET, absorption, circular dichroism, differential scanning calorimetric and computational approaches were employed to unravel the interaction between sumatriptan and bovine serum albumin (BSA). The fluorescence quenching studies suggested the interaction between SUM and BSA with a moderate binding with the binding constant (Kb) in the order of 104. The findings of temperature and time dependent fluorescence quenching studies confirmed the role of static quenching mechanism. Thermodynamic parameters suggested the key role of electrostatic force in the interaction of SUM with BSA. Absorption and CD spectral studies revealed the bioenvironmental changes around the Trp in BSA upon binding of SUM. Calorimetric based thermal denaturation results confirmed that the thermal stability of BSA was improved in the presence of SUM. resulted in the this decreased flexibility of protein chain. Site competitive studies indicated SUM was located in the hydrophobic cavity of site I which was further confirmed by the docking and dynamic simulation studies. Additionally, molecular dynamics simulations inferred the microenvironmental condition around the SUM and the amino acids and forces involved in the binding of SUM with BSA.Communicated by Ramaswamy H. Sarma.

In Vitro and Computational Approaches to Untangle the Binding Mechanism of Galangin with Calf Thymus DNA.

Flavonoids have potential applications in the nutraceutical, medicinal, pharmaceutical and cosmetic fields. The binding of flavonoids with DNA could unravel essential information required for the design of novel and effective chemical agents. The present paper describes the interaction of a flavonoid and a potent anticancer drug, galangin (GAL) with calf thymus DNA (ct-DNA) by fluorescence, UV absorption, melting studies, viscosity measurements and molecular docking studies. A hyperchromic effect was noticed in the absorption spectra of ct-DNA in the presence of the GAL system, indicating the presence of a groove mode of binding. Furthermore, GAL persuaded the minor changes in ct-DNA viscosity, indicating a non-intercalative mode of binding. Fluorescence studies revealed that the GAL quenched the fluorescence intensity of ct-DNA-Hoechst, thereby indicating the interaction between GAL and ct-DNA. Fluorescence results obtained at 298, 308 and 318 K revealed that the fluorescence quenching of ct-DNA-Hoechst-GAL occurred through the static quenching mechanism. Thermodynamic parameters for ct-DNA-Hoechst-GAL were computed and suitable conclusions were drawn. The changes noticed in the conformation of ct-DNA upon interaction with GAL were evaluated in terms of molar ellipticity. It indicated a plausible interaction between ct-DNA and GAL. The molecular docking studies also confirmed the groove mode of binding in the ct-DNA-GAL system. Thus, this work helped to unravel the binding mechanism between GAL and ct-DNA.

Exploring the binding of two potent anticancer drugs bosutinib and imatinib mesylate with bovine serum albumin: spectroscopic and molecular dynamic simulation studies.

Bosutinib (BST) and imatinib mesylate (IMT) are tyrosine kinase inhibitors (TKIs). In view of the importance of these inhibitors in cancer treatment, we investigated the mechanism of interaction between BST/IMT and bovine serum albumin (BSA) using various spectroscopic and molecular docking methods. Fluorescence studies indicated that BST/IMT interacted with BSA without affecting the microenvironment around the residue Trp213 of BSA. The quenching mechanism associated with the BST-BSA and IMT-BSA interactions was determined by performing fluorescence measurements at different temperatures. These results suggested that BST and IMT quenched the fluorescence intensity of BSA through static and dynamic processes, respectively, which was confirmed by time-resolved fluorescence measurements. Evaluation of the thermodynamic parameters ∆H°, ∆S°, and ∆G° suggested that hydrophobic and electrostatic interactions played significant roles in the BST-BSA interaction, while IMT-BSA was stabilized by hydrophobic forces. Competitive experimental results revealed that the primary binding sites for BST and IMT on BSA were sites II and I, respectively. This was supported by the results of molecular docking and dynamic simulation studies. The change in the secondary structure of BSA upon binding with BST/IMT was investigated by 3D fluorescence, absorption, and CD spectroscopic studies. In addition, the influences of ß-cyclodextrin and metal ions (Cu2+ and Zn2+) on the binding affinities of BST and IMT to BSA were examined. Graphical abstract Binding of BST and IMT in BSA at site II and site I respectively.

Spectroscopic and molecular modeling approaches to investigate the binding of proton pump inhibitors to human serum albumin.

The interaction between two proton pump inhibitors viz., omeprazole (OME) and esomeprazole (EPZ) with human serum albumin (HSA) was studied by fluorescence, absorption, circular dichroism (CD), Fourier transform infrared spectroscopy (FT-IR), voltammetry, and molecular modeling approaches. The Stern-Volmer quenching constants (Ksv) for OME-HSA and EPZ-HSA systems obtained at different temperatures revealed that both OME and EPZ quenched the intensity of HSA through dynamic mode of quenching mechanism. The binding constants of OME-HSA and EPZ-HSA increased with temperature, indicating the increased stability of these systems at higher temperatures. Thermodynamic parameters viz., ∆H°, ∆S°, and ∆G° were determined for both systems. These values revealed that both systems were stabilized by hydrophobic forces. The competitive displacement and molecular docking studies suggested that OME/EPZ was bound to Sudlow's site I in subdomain IIA in HSA. The extent of energy transfer from HSA to OME/EPZ and the distance of separation in tryptophan (Trp214) Trp214-OME and Trp214-EPZ was determined based on the theory of fluorescence resonance energy transfer. UV absorption, 3D fluorescence, and CD studies indicated that the binding of OME/EPZ to HSA has induced micro environmental changes around the protein which resulted changes in its secondary structure.

Synthesis, crystal studies, anti-tuberculosis and cytotoxic studies of 1-[(2E)-3-phenylprop-2-enoyl]-1H-benzimidazole derivatives.

Series of 1-[(2E)-3-phenylprop-2-enoyl]-1H-benzimidazole derivatives were synthesized and characterized by spectral methods. Among 21 derivatives, single crystals of 3a and 3l were grown and their structural parameters were evaluated. Newly synthesized compounds were screened for anti-tubercular activity and the MIC was determined against Mycobacterium tuberculosis H37Rv by Microplate Alamar Blue Assay (MABA) method. Majority of the compounds exhibited a promising inhibition of M. tuberculosis and the molecules functionalized with electron-donating groups at C-2 carbon of benzimidazole moiety were found to be more active in inhibiting M. tuberculosis. Further, more promising compounds viz., 3b, 3i and 3l were tested for their cytotoxic activity. Compound 3l was found to display excellent activity (IC50 < 10 µg mL(-1)) with 100% cell lysis at 30 µg mL(-1) concentration against A549 (Human lung carcinoma) and 8E5 (Human; Acute Lymphoblastic Leukemia) cell lines.

Fluorescence and circular dichroism studies on binding and conformational aspects of an anti-leukemic drug with DNA.

In the present study, we have explored the mode of binding of an anti-leukemic drug, imatinib (IMT) mesylate with DNA and resulting conformational changes in DNA double helix. UV-Vis absorption, fluorescence and circular dichroism spectroscopic techniques were employed to study these interactions. Spectroscopic results revealed that the intercalation was the primary mode of interaction between IMT and DNA. The binding constant value of 6.62 × 10(3 )M(-1) indicated the moderate interaction between IMT and DNA. Melting temperature of DNA increased from 75 to 80 °C upon interaction with IMT.

Probing the site-selective binding of an antiretroviral drug, Stavudine to calf thymus DNA.

The interaction of an anti-HIV drug, stavudine (STV) with calf thymus deoxyribonucleic acid (DNA) was investigated employing acridine orange (AO) as a fluorescence probe. Spectroscopic investigations revealed the intercalative mode of binding of STV to DNA. The analysis of fluorescence data indicated the presence of static quenching mechanism between STV and DNA. Thermodynamic parameters indicated the presence of van der Waals forces in addition to intercalative mode of binding. CD data revealed the partial B → A conformational transition of DNA upon intercalative mode of binding with STV.

Elucidation of binding mechanism and identification of binding site for an anti HIV drug, stavudine on human blood proteins.

The binding of stavudine (STV) to two human blood proteins [human hemoglobin (HHb) and human serum albumin (HSA)] was studied in vitro under simulated physiological conditions by spectroscopic methods viz., fluorescence, UV absorption, resonance light scattering, synchronous fluorescence, circular dichroism (CD) and three-dimensional fluorescence. The binding parameters of STV-blood protein were determined from fluorescence quenching studies. Stern-Volmer plots indicated the presence of static quenching mechanism in the interaction of STV with blood proteins. The values of n close to unity indicated that one molecule of STV bound to one molecule of blood protein. The binding process was found to be spontaneous. Analysis of thermodynamic parameters revealed the presence of hydrogen bond and van der Waals forces between protein and STV. Displacement experiments indicated the binding of STV to Sudlow's site I on HSA. Secondary structures of blood proteins have undergone changes upon interaction with STV as evident from the reduction of α-helices (from 46.11% in free HHb to 38.34% in STV-HHb, and from 66.44% in free HSA to 52.26% in STV-HSA). Further, the alterations in secondary structures of proteins in the presence of STV were confirmed by synchronous and 3D-fluorescence spectral data. The distance between the blood protein (donor) and acceptor (STV) was found to be 5.211 and 5.402 nm for STV-HHb and STV-HSA, respectively based on Föster's non-radiative energy transfer theory. Effect of some metal ions was also investigated. The fraction of STV bound to HSA was found to be 87.8%.

Investigations to reveal the nature of interactions of human hemoglobin with curcumin using optical techniques.

Curcumin (CUR) is an important bioactive compound present in the rhizome of Curcuma longa. Herein, we report the interaction of CUR with human hemoglobin (Hb) using various biophysical methods viz., fluorescence, UV absorption, resonance light scattering spectra (RLS), synchronous fluorescence, fluorescence anisotropy, circular dichroism (CD) and three-dimensional fluorescence. There was a considerable quenching of the intrinsic fluorescence of Hb upon binding to CUR through dynamic quenching mechanism. The distance (r) between the donor and acceptor was obtained from the Forster's theory of fluorescence resonance energy transfer (FRET) and found to be 1.55 nm. Alterations in the conformation of Hb due to its interaction with CUR were confirmed by UV absorption and CD spectroscopic methods. The α-helicity of Hb was found to decrease upon binding with CUR.

Spectral characterization of the binding and conformational changes of serum albumins upon interaction with an anticancer drug, anastrozole.

The present study employed different optical spectroscopic techniques viz., fluorescence, FTIR, circular dichroism (CD) and UV-vis absorption spectroscopy to investigate the mechanism of interaction of an anticancer drug, anastrozole (AZ) with transport proteins viz., bovine serum albumin (BSA) and human serum albumin (HSA). The drug, AZ quenched the intrinsic fluorescence of protein and the analysis of results revealed the presence of dynamic quenching mechanism. The binding characteristics of drug-protein were computed. The thermodynamic parameters, enthalpy change (ΔH°) and entropy change (ΔS°) were calculated to be +92.99 kJ/mol and +159.18 J/mol/K for AZ-BSA and, +99.43 kJ/mol and +159.19 J/mol/K for AZ-HSA, respectively. These results indicated that the hydrophobic forces stabilized the interaction between the drug and protein. CD, FTIR, absorption, synchronous and 3D fluorescence results indicated that the binding of AZ to protein induced structural perturbation in both serum albumins. The distance, r between the drug and protein was calculated based on the theory of Förster's resonance energy transfer and found to be 5.9 and 6.24 nm, respectively for AZ-BSA and AZ-HSA.

Interaction of antioxidant flavonoids with calf thymus DNA analyzed by spectroscopic and electrochemical methods.

Mechanism of interaction of bioactive flavonoids, hesperitin (HES) and naringenin (NAR) with calf thymus deoxyribonucleic acid (DNA) was studied employing UV absorption, fluorescence, circular dichroism, melting temperature, fluorescence anisotropy and differential pulse voltammetric methods. The observed fluorescence quenching of DNA-ethidium bromide system by the flavonoid indicated the intercalative mode of binding between the flavonoid and DNA. Stern-Volmer plots have revealed the presence of static quenching mechanism. Binding and thermodynamic characteristics of interaction were evaluated. Melting temperature of DNA was found to be increased up to 5 °C in the presence of the flavonoid indicating the stabilization of DNA double helix upon binding. CD and fluorescence anisotropic results have revealed the conformational changes in DNA upon binding to the flavonoid. The observed positive shift in peak potential and decreased peak current of the flavonoid in the presence of DNA further supported the intercalative mode of binding.

Optical, structural and thermodynamic studies of the association of an anti-leucamic drug imatinib mesylate with transport protein.

The interaction of an anti-leukemic drug, imatinib mesylate (IMT) with human serum albumin (HSA) was investigated by fluorescence, synchronous fluorescence, three-dimensional fluorescence, circular dichroism and UV-vis absorption techniques under physiological condition. The process of binding of IMT on HSA was observed to be through a spontaneous molecular interaction procedure. IMT effectively quenched the intrinsic fluorescence of HSA via static quenching. The values of binding constant, number of molecules that interact simultaneously with the binding site and thermodynamic parameters were evaluated by carrying out the interactions at three different temperatures. Based on thermodynamic parameters and displacement studies with site probes, it was proposed that the drug bound at Sudlow's site I of subdomain IIA. The change in the conformation of HSA was evident from synchronous, three-dimensional fluorescence and circular dichroism studies. The distance between the donor (protein) and acceptor (drug) was calculated based on the Foster's theory of resonance energy stransfer and it was found to be 1.30 nm. The effect of different metal ions on the binding of the drug to protein was also investigated.

Interaction of triprolidine hydrochloride with serum albumins: thermodynamic and binding characteristics, and influence of site probes.

The interaction between triprolidine hydrochloride (TRP) to serum albumins viz. bovine serum albumin (BSA) and human serum albumin (HSA) has been studied by spectroscopic methods. The experimental results revealed the static quenching mechanism in the interaction of TRP with protein. The number of binding sites close to unity for both TRP-BSA and TRP-HSA indicated the presence of single class of binding site for the drug in protein. The binding constant values of TRP-BSA and TRP-HSA were observed to be 4.75 ± 0.018 × 10(3) and 2.42 ± 0.024 × 10(4)M(-1) at 294 K, respectively. Thermodynamic parameters indicated that the hydrogen bond and van der Waals forces played the major role in the binding of TRP to proteins. The distance of separation between the serum albumin and TRP was obtained from the Förster's theory of non-radioactive energy transfer. The metal ions viz., K(+), Ca(2+), Co(2+), Cu(2+), Ni(2+), Mn(2+) and Zn(2+) were found to influence the binding of the drug to protein. Displacement experiments indicated the binding of TRP to Sudlow's site I on both BSA and HSA. The CD, 3D fluorescence spectra and FT-IR spectral results revealed the changes in the secondary structure of protein upon interaction with TRP.

Evaluation of binding and thermodynamic characteristics of interactions between a citrus flavonoid hesperitin with protein and effects of metal ions on binding.

The mechanism of interaction of a non-glycosidic citrus flavonoid, hesperitin (HES) with bovine serum albumin (BSA) was studied by UV-vis absorption, fluorescence, FT-IR, circular dichroism, fluorescence anisotropy and synchronous fluorescence spectroscopy in phosphate buffer of pH 7.4. Fluorescence data revealed that the fluorescence quenching of BSA by HES was the result of the formed complex of HES-BSA. The binding constants and thermodynamic parameters at four different temperatures, the location of binding, and the nature of binding force were determined. The hydrogen bonds interactions were found to be the predominant intermolecular forces to stabilize the complex. The conformation of BSA was discussed by synchronous fluorescence and CD methods. The alterations of protein secondary structure upon complexation with HES were evident from the gradual decrease in α-helicity. The distance between the donor (BSA) and acceptor (flavonoid) was calculated from the fluorescence resonance energy transfer and found to be 1.978 nm. Common ions viz., Zn(2+), K(+), Cu(2+), Ni(2+), Mn(2+) and Co(2+) were found to influence the binding of flavonoid to protein.

Interaction of an antidepressant buzepide methiodide with DNA immobilized on the glassy carbon electrode.

In the present study, a DNA-biosensor was prepared using immobilization technique to investigate the interaction between an antidepressant, buzepide methiodide (BZP) and calf thymus DNA. BZP showed a quasireversible peak in Britton-Robinson (BR) buffer of pH 5 at bare glassy carbon electrode (GCE). At DNA modified GCE, the peak potential of BZP was observed to be shifted towards positive potential revealing intercalative mode of binding. The binding constant and stoichiometry between DNA and BZP are calculated to be 1.908×10(5)M(-1) and 0.982, respectively. The spectroscopic techniques viz., spectrofluorescence and UV-vis absorption have also been employed to understand the interaction between BZP and DNA. The results serve as a reference for the interaction of BZP with DNA base pairs in the natural environment of living cells.

Characterization of interaction and the effect of carbamazepine on the structure of human serum albumin.

The binding of carbamazepine (CBZ) to human serum albumin (HSA) was investigated under simulative physiological conditions. In this study, intrinsic fluorescence of tryptophan-214 in HSA was monitored upon the addition of CBZ. Binding constant of CBZ-HSA was calculated by the remarkable static quenching effect of CBZ and found to be (2.081+/-0.023)x10(4)M(-1). The fluorimetric results revealed that the hydrophobic interaction was a predominant intermolecular force for stabilizing the complex, which is also in agreement with the results obtained from voltammetric approach. Three site probes, warfarin, ibuprofen and digitoxin, were employed in fluorescence displacement experiments to locate the exact binding site for CBZ in HSA. The alteration in secondary structure of protein in the presence of CBZ was confirmed by the evidences from circular dichroism and FT-IR spectroscopy. Further, the distance r between donor (Trp-214) and acceptor (CBZ) was obtained according to fluorescence resonance energy transfer (FRET).

Mechanism of interaction between human serum albumin and N-alkyl phenothiazines studied using spectroscopic methods.

The binding characteristics of human serum albumin (HSA) with N-alkyl phenothiazines derivatives (NAP) viz., levomepromazine monomaleate (LMM) and propericiazine (PPC) have been studied by employing fluorescence, absorption, circular dichroism and FT-IR techniques. The Stern-Volmer quenching constant, K(SV) values were found to decrease with increase in temperature thereby indicating the presence of static quenching mechanism in the interactions of NAP with HSA. The number of binding sites, n and the binding constant, K were noticed to be, respectively, 1.11 and (5.188+/-0.034)x10(4) M(-1) for LMM and 1.06 and (4.436+/-0.066)x10(4) M(-1) for PPC at 298 K. The negative value of enthalpy change and positive value of entropy change in the present study indicated that the hydrophobic forces played a major role in the binding of NAP to HSA. The circular dichroism and FT-IR spectral data revealed the conformational changes in the structure of protein upon its interaction with NAP. The binding distances and the energy transfer efficiency between NAP and protein were determined based on Förster's theory of energy transfer. The decreased binding constants of HAS-LMM and HAS-PPC systems in presence of common ions indicated the availability of higher concentration of free drug in plasma.

In vitro study on the binding of anti-coagulant vitamin to bovine serum albumin and the influence of toxic ions and common ions on binding.

The mechanism of binding of vitamin K(3) (VK(3)) with bovine serum albumin (BSA) was investigated by fluorescence, absorption and circular dichroism (CD) techniques under physiological conditions. The analysis of fluorescence data indicated the presence of static quenching mechanism in the binding. Various binding parameters have been evaluated. Thermodynamic parameters, the standard enthalpy change, DeltaH(0) and the standard entropy change, DeltaS(0) were observed to be -164.09 kJ mol(-1) and -465.08 J mol(-1)K, respectively. The quantitative analysis of CD spectra confirmed the change in secondary structure of the protein upon interaction with VK(3). The binding average distance, r between the donor (BSA) and acceptor (VK(3)) was determined based on the Förster's theory and it was found to be 3.3 nm. The effects of toxic ions and common ions on VK(3)-BSA system were also investigated.