Pinki Saha Sardar

A Comparative Study of Interaction of Tetracycline with Several Proteins Using Time Resolved Anisotropy, Phosphorescence, Docking and FRET

A comparative study of the interaction of an antibiotic Tetracycline hydrochloride (TC) with two albumins, Human serum albumin (HSA) and Bovine serum albumin (BSA) along with Escherichia Coli Alkaline Phosphatase (AP) has been presented exploiting the enhanced emission and anisotropy of the bound drug. The association constant at 298 K is found to be two orders of magnitude lower in BSA/HSA compared to that in AP with number of binding site being one in each case. Fluorescence resonance energy transfer (FRET) and molecular docking studies have been employed for the systems containing HSA and BSA to find out the particular tryptophan (Trp) residue and the other residues in the proteins involved in the binding process. Rotational correlation time (θc) of the bound TC obtained from time resolved anisotropy of TC in all the protein-TC complexes has been compared to understand the binding mechanism. Low temperature (77 K) phosphorescence (LTP) spectra of Trp residues in the free proteins (HSA/BSA) and in the complexes of HSA/BSA have been used to specify the role of Trp residues in FRET and in the binding process. The results have been compared with those obtained for the complex of AP with TC. The photophysical behaviour (viz., emission maximum, quantum yield, lifetime and θc) of TC in various protic and aprotic polar solvents has been determined to address the nature of the microenvironment of TC in the protein-drug complexes.

Interaction of multitryptophan protein with drug: an insight into the binding mechanism and the binding domain by time resolved emission, anisotropy, phosphorescence and docking.

The interaction of antibiotic Tetracycline hydrochloride (TC) with Alkaline Phosphatase (AP) from Escherichia coli, an important target enzyme in medicinal chemistry, having tryptophan (Trp) residues at 109, 220 and 268 has been studied using the steady state and time resolved emission of the protein and the enhanced emission of the bound drug. The association constant at 298 K (≈10(6) [M](-1)) and the number of binding site (=1) were estimated using the quenched Trp emission of AP, the enhanced emission and the anisotropy of the bound drug. The values of ΔH(0) and ΔS(0) are indicative of electrostatic and H-bonding interaction. The low temperature phosphorescence of free AP and the protein- drug complex and molecular docking comprehensively prove the specific involvement of partially exposed Trp 220 in the binding process without affecting Trp 109 and Trp 268. The Förster energy transfer (ET) efficiency and the rate constant from the Trp residue to TC=0.51 and ≈10(8) s(-1) respectively. Arg 199, Glu 219, Trp 220, Lys 223, Ala 231, Arg 232 and Tyr 234 residues are involved in the binding process. The motional restriction of TC imposed by nearby residues is reflected in the observed life time and the rotational correlation time of bound TC.

Mono- and di-nuclear photoluminescent complexes of zinc(II), cadmium(II) and mercury(II) of a chiral diimine ligand

Reaction of α-pyridoin and N-phenyl-o-phenylenediamine affords 2-(2-(phenylamino)phenylimino)-1,2-di(pyridin-2-yl)ethanol (L) which undergoes cyclization to a chiral diimine, 2-methoxy-1-phenyl-2,3-di(pyridin-2-yl)-1,2-dihydroquinoxaline, L(OMe) (conjugated 14πe system) in the presence of zinc(II), cadmium(II) and mercury(II) ions affording [Zn(L(OMe))Cl2] (1), [Cd2(L(OMe))2Cl4] (2) and [Hg2(L(OMe))2Cl4] (3) complexes. Ligand L and complexes 1-3 are substantiated by elemental analyses, mass, IR, (1)H NMR and UV-vis spectra including the single-crystal X-ray structures of 1 and 3. The possibility of the atropisomerism of L is restricted in cyclic L(OMe). L and complexes 1-3 are fluorescent in fluid solutions at 298 K (CH2Cl2: 1, λ(ex) = 470 nm, λ(em) = 627 nm, Φ = 0.014, τ(avg) = 2.5 ns; 2, λ(ex) = 430 nm, λ(em) = 599 nm, Φ = 0.08, τ(avg) = 7.6 ns; 3, λ(ex) = 415 nm, λ(em) = 600 nm, Φ = 0.021, τ(avg) = 2.8 ns). Time-resolved emission spectra (TRES) established that the two-component lifetimes of 1-3 are due to the existence of two conformers. Density functional theory (DFT) and time dependent (TD) DFT calculations authenticated that 1-3 complexes are fluorescent due to intra-ligand charge transfer (ILCT) to the π(diimines)* orbital.

Interaction of serum albumins with fluorescent ligand 4-azido coumarin: spectroscopic analysis and molecular docking studies

Steady state fluorescence and time resolved fluorescence studies at 298 K and low temperature phosphorescence (LTP) studies at 77 K of the interaction of bovine serum albumin (BSA) and human serum albumin (HSA) with ligand 4-azido-2H-chromen-2-one or 4-azidocoumarin (4-AC) have been carried out to visualize the location of the binding site and perturbation of the binding site of the tryptophan (Trp)/tyrosine (Tyr) of the protein(s) by monitoring the emission maxima of Trp residue(s) in proteins. The fluorescence quenching study of Trp estimated that the binding constant for both protein–ligand complexes is in the order of ∼106 with binding site 1. Perturbation in the secondary structures of serum albumins due to binding of 4-AC is also observed from circular dichroism (CD) studies. An energy transfer (ET) study further demonstrated that the non-radiative singlet–singlet ET that takes place from the Trp singlet states of proteins to the singlet state of ligands is greater in the case of BSA. This is supported by the distance and orientation of the donor–acceptor pair obtained from molecular docking studies. The molecular docking studies were also fruitfully exploited to understand the involvement of Trp213 in BSA and Trp214 in HSA in the ET process along with the perturbation of the residues around 5 A from the ligand 4-AC. Phosphorescence spectra at 77 K of the Trp residues in the free proteins (BSA/HSA) and in the complexes of BSA/HSA have also been utilized to specify the role of Trp residues in ET and the binding process.

Zinc(II), iron(II/III) and ruthenium(II) complexes of o-phenylenediamine derivatives: oxidative dehydrogenation and photoluminescence

Reactions of benzoyl pyridine, o-phenylenediamine and anhydrous ZnX2 in methanol afford imine complexes [Zn(L1)X2] (X = Cl, 1; X = Br, 2) in good yields (L1 = (E)-N(1)-(phenyl(pyridin-2-yl)methylene)benzene-1,2-diamine). The reduction of 1 with NaBH4 affords (E)-N(1)-(phenyl(pyridine-2-yl)methylene)benzene-1,2-diamine (L2H). The reaction of L2H with [Ru(II)(PPh3)3Cl2] results in the oxidative dehydrogenation to L1 generating cis-[Ru(II)(L1)(PPh3)Cl2] (3). The reaction of L2H with salicylaldehyde affords (E)-2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)imino)methyl)phenol (L3H2). The reaction of L3H2 with anhydrous FeCl3 in CH3OH affords cis-[Fe(III)(L3H(-))Cl2] (4). Reaction of L3H2 with [Ru(II)(PPh3)3Cl2] results in the oxidative dehydrogenation to diimine, L4H, affording trans-[Ru(II)(L4(-))(PPh3)2](+), which is isolated as trans-[Ru(II)(L4(-))(PPh3)2]PF6 (5(+)PF6(-)) (L4H = 2-((E)-(2-((E)-phenyl(pyridin-2-yl)methyleneamino)phenylimino)methyl)phenol). The reduction of L3H2 with NaBH4 produces 2-(((2-((phenyl(pyridin-2-yl)methyl)amino)phenyl)amino)methyl)phenol (L5H3). With iron(III) L5H3 undergoes oxidative dehydrogenation to L3H2 affording 4, while with [Ru(II)(PPh3)3Cl2], L5H3 undergoes 4e + 4H(+) transfer giving 5(+). A fluid solution of L3H2 at 298 K exhibits an emission band at 470 nm (λ(ex) = 330 nm, τ1 = 3.70 ns) and a weaker band at 525 nm (λ(ex) = 330, 390 nm, τ1 = 1.1 ns) at higher concentrations due to molecular aggregation, which are temperature dependent. 4 is brightly emissive (λ(ex) = 330 nm, λ(em) = 450 nm, Φ = 0.586, τ1 = 3.70 ns). Time resolved emission spectra (TRES) and lifetime measurements confirm that the lower energy absorption band of L3H2 at 390 nm, which is absent in complex 4, has a larger non-radiative rate constant (k(nr)). The redox innocent Al(III) adduct of L3H2 is fluorescent (λ(ex) = 330 nm, λ(em) = 450 nm, τ1 = 3.70 ns). On the contrary, the cis-[Fe(II)(L3H(-))Cl2](-) and cis-[Co(L3H(-))Cl2](-) analogues are non emissive. Density function theory (DFT) calculations, redox potentials and the near infra-red (NIR) absorption data prove that 4 is emissive due to the stable [Fe(III)(L3H(-)*)] state, while 3, 5(+), cis-[Fe(II)(L3H(-))Cl2](-) and cis-[Co(L3H(-))Cl2](-) are non-emissive due to transformations of the [M(II)(L*)] to [M(III)(L˙(-)*)] states.

Investigation on the interaction of Rutin with serum albumins: Insights from spectroscopic and molecular docking techniques

The binding interaction of Rutin, a flavonoid, with model transport proteins, bovine serum albumin (BSA) and human serum albumin (HSA), were investigated using different spectroscopic techniques, such as fluorescence, time-resolved single photon counting (TCSPC) and circular dichroism (CD) spectroscopy as well as molecular docking method. The emission studies revealed that the fluorescence quenching of BSA/HSA by Rutin occurred through a simultaneous static and dynamic quenching process, and we have evaluated both the quenching constants individually. The binding constants of Rutin-BSA and Rutin-HSA system were found to be 2.14 × 106 M-1 and 2.36 × 106 M-1 at 298 K respectively, which were quite high. Further, influence of some biologically significant metal ions (Ca2+, Zn2+ and Mg2+) on binding of Rutin to BSA and HSA were also investigated. Thermodynamic parameters justified the involvement of hydrogen bonding and weak van der Waals forces in the interaction of Rutin with both BSA and HSA. Further a site-marker competitive experiment was performed to evaluate Rutin binding site in the albumins. Additionally, the CD spectra of BSA and HSA revealed that the secondary structure of the proteins was perturbed in the presence of Rutin. Finally protein-ligand docking studies have also been performed to determine the probable location of the ligand molecule.

Dependence of photoinduced energy transfer on orientation of acceptor lanthanide ions with respect to π-plane of naphthalene in naphthalene-linked six-member crown ethers

The photoinduced energy transfer (ET) from naphthalene (N) to rare earth ions Eu3+/Tb3+ has been studied in the lanthanide ion complexes of 1,4-naptho-22-crown-6 (system-I) using as counter ion. The ET efficiency and rate constants have been calculated and the values compared with the metal ion complexes of three other naptho-crown-6 systems (II to IV), which have different orientation of the naphthalene unit with respect to the crown ring. Geometry optimization by DFT method indicates a subtle difference in the geometries of systems I and II that is reflected in substantial differences in the ET efficiencies of the two systems. In all the systems ET occurs by an exchange mechanism involving the lowest triplet state of the naphthalene moiety. The ratios of the overlap integral of the Eu3+/Tb3+ complexes in different crown ethers involved in such an exchange mechanism have been estimated.

Photoacidic behavior of small phenolics in micellar media

ABSTRACT The present work demonstrates the photophysical characterization of the interaction of two antibacterial and antimicrobial compounds, thymol and its isomer, carvacrol with different biomimetic micellar nanocavities having varying surface charge characteristics. These types of molecules are reported to be photoacids and this behavior is remarkably exhibited upon interaction with the macromolecular assemblies of micellar systems in the presence of inorganic and organic quencher molecules (silver nitrate and diethylamine) and is manifested through significant modulations of the bimolecular quenching constant values.

Features of partial encapsulation of an ESIPT probe 3-hydroxy-2-naphthoic acid (3HNA) in the nano cavities of β- and γ-cyclodextrin: comparative study with sequestered 3HNA in micelles and reverse micelle

The role of intramolecular H-bonding within an excited state intramolecular proton transfer (ESIPT) probe, the intermolecular H-bonding ability of the microenvironment with the probe in different organized assemblies, the polarity of the medium after encapsulation and the motional restriction of the probe have been investigated by monitoring the ESIPT emission as well as the localized π–π* emission of 3-hydroxy-2-naphthoic acid (3HNA) in various confined media like cyclodextrins, micelles and reverse micelle. The steady state and time-resolved fluorescence and also the time resolved anisotropy measurements of the encapsulated probe indicate the formation of a 1 : 1 complex between the β- and γ-cyclodextrin hosts and the guest. The values of the binding constants and different thermodynamic parameters for complexation have also been reported. The results in various confined media have been correlated using control experiments on the ESIPT emission of 3HNA in mixed solvents consisting of dimethyl sulfoxide (DMSO) and water (mixture of an aprotic polar and a protic polar solvent) and also of acetonitrile (ACN) and dichloromethane (DCM) (mixture of an aprotic polar and a non-polar solvent). Theoretical calculation on the orientation of 3HNA within β- and γ-cyclodextrins has been carried out in order to explain the observed rotational correlation time of the probe in confined environments.

Role of tryptophan 135 of Chandipura virus phosphoprotein P in dimerization and complex formation with leader RNA: structural aspect using time resolved anisotropy and simulation

The aggregation of phosphoprotein P of Chandipura virus (CHPV) and its interaction with viral leader (le) RNA in aqueous buffer and 40% ethylene glycol (EG)-buffer have been characterised using two single tryptophan (Trp/W) mutants W105F and W135F. The longer rotational correlation time [(θC)T] originating from overall motion observed at 300 nM concentration conforms to a globular structure of the monomer of WT and both the mutants. The (θC)T values also indicate that W135F does not form a dimer at 1500 nM concentration; while a dimer with disordered structure is predicted for both WT and W105F. The complexes of WT and W105F with le RNA at monomeric and dimeric conditions are indicated to have tight core packing. Dimerization and complex formation at both the concentrations enhance the correlation time arising from the localised motion of Trp side chain [(θC)S] for WT and W105F predicting hindered localized rotation of Trp 135. Comparative protein modelling and molecular dynamics (MD) simulations using the amino acids domain ranging from 105–168 of the full length CHPVP based on the vesicular stomatitis virus phosphoprotein (VSVP) also indicate that formation of dimers are more feasible for WT and W105F compared to W135F.