Swarna Kamal Samanta

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.

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.