Spectrochimica Acta Part A 79 (2011) 1240–1246 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy jou rn al hom epa ge: www.elsevier.com/locate/saa Spectrofluorometric studies on the binding interaction of bioactive imidazole with bovine serum albumin: A DFT based ESIPT process Jayaraman Jayabharathi ∗ , Venugopal Thanikachalam, Kanagarathinam Saravanan, Marimuthu Venkatesh Perumal Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India a r t i c l e i n f o Article history: Received 7 March 2011 Received in revised form 26 March 2011 Accepted 15 April 2011 Keywords: Bioactive imidazole ESIPT PES FRET Binding distance Binding constant a b s t r a c t Bioactive imidazole derivatives were synthesized and characterized by NMR spectra, mass and CHN analysis. An excited state intramolecular proton transfer (ESIPT) process in hydroxy imidazole has been studied using emission spectroscopy. In hydrocarbon solvent, the tautomer emission predominates over the normal emission and in alcoholic solvent like ethanol; a dramatic enhancement of normal emission is observed which was due to increased solvation. DFT calculation on energy, charge distribution of the rotamers in the ground and excited states of the imidazole derivative were performed and discussed. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than in the ground state. The interaction between bioactive imidazole derivative and bovine serum albumin (BSA) was investigated. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Bio-active imidazole derivatives attracted considerable atten- tion because of their unique optical properties [1,2] and the imidazole nucleus forms the main structure of human organ- isms, i.e., the amino acid histidine, Vitamin B 12 , a component of DNA base structure and also have analytical applications uti- lizing their fluorescence and chemiluminescence properties [3]. Excited state intramolecular proton transfer (ESIPT) phenomena have been investigated [4–8] in the past decades due to the practical applications of ESIPT exhibiting molecules as laser dyes [4], photo stabilizers [5], fluorescent probes in biology [6] and light-emitting materials for electroluminescent devices [7–9]. ESIPT occurs in aromatic molecules having a phenolic hydroxy group with an intramolecular hydrogen bond to the nearby hetero atom of the same chromophore. Serum albumins are the most abundant proteins in plasma [10,11], among the serum albumins, BSA has a wide range of phys- iological functions involving the binding, transport and delivery of fatty acids, porphyrins, steroids, etc. Steady state and time-resolved fluorescence spectroscopy are used for analyzing the binding prop- erties of BSA and drugs and also to study the structural information of protein cavities [12–14]. Fluorescence quenching is an impor- ∗ Corresponding author. Tel.: +91 9443940735. E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi). tant method to study the interaction of substances with protein because of its accuracy, sensitivity, rapidity and convenience of usage. In the present paper, we will focus the light on the photophys- ical studies of 2-aryl imidazole derivatives (1–6), ESIPT process of hydroxy imidazole, density functional theory (DFT) calculation on energy, dipole moment and PES studies of various rotamers. We also exploited the detailed investigation of BSA–imidazole associa- tion (binding parameters and the effect of imidazole on the protein conformation) using fluorescence and UV–vis absorption studies. 2. Experimental 2.1. Materials and methods Butane-2,3-dione (Sigma–Aldrich Ltd.), 4-fluoro benzaldehyde (S.D. fine.), 4-fluoro salicylaldehyde, 4-fluoro-2-methoxy benzalde- hyde, 3-methyl aniline, 2,5-dimethyl aniline and all other reagents used without further purification. Bovine serum albumin (BSA) was obtained from Sigma–Aldrich Company, Bangalore. All BSA solu- tion was prepared in the Tris–HCl buffer solution (0.05 mol L -1 Tris, 0.15 mol L -1 NaCl, pH 7.4) and was kept in the dark at 303 K. Tris base (2-amino-2-(hydroxymethyl)-1,3-propanediol) had a purity of not less than 99.5% and NaCl, HCl and other starting materials were all of analytical purity and doubly distilled water was used throughout. 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.04.049