This journal is c The Royal Society of Chemistry 2012 Mol. BioSyst. Cite this: DOI: 10.1039/c2mb25368b Role of block copolymer-micelle nanocomposites in dye–bovine serum albumin binding: a combined experimental and molecular docking study† Anamika Manna and Sankar Chakravorti* The role of a nanocomposite (NC), composed of intercalation of the diblock copolymer polyethylene- b-polyethylene glycol (PE-b-PEG) with the anionic surfactant sodium dodecyl sulphate (SDS), on the binding characteristics of bovine serum albumin (BSA) with a dye (1,8-naphthalimide, NAPMD) compared to the interaction between the same players in aqueous solution has been examined comprehensively in this paper. Static quenching due to complex formation in both NC medium and in buffer solution has been inferred on the basis of considerable changes in the absorption spectra of BSA on addition of NAPMD, of which the interaction is found to be stronger in NC medium. Temperature dependent fluorescence data also confirm an effective static quenching and stronger binding of NAPMD with BSA in NC medium. Peptide chain unfolding and denaturing of BSA in NC medium have been confirmed from steady state and time-resolved emission and circular dichroism data. This exposes both the tyrosine and tryptophan moieties as a unique case. Increased energy transfer between NAPMD and the tryptophan residue in the unfolded form of BSA helps in the appearance of tyrosine fluorescence in NC medium by quenching the tryptophan band. Ionization of the hydroxyl group in the aromatic ring of the tyrosine residue by the PEG group present in the NC medium produces a downshift of the tyrosine fluorescence band. The use of site selective markers confirms that NAPMD is near tryptophan in Sudlow’s site I in NC medium and in buffer solution it is away from tryptophan in Sudlow’s site II. The theoretical docking studies also vindicate the results of binding of NAPMD with BSA in site I or site II in NC and buffer media, as observed from different emission experiments including the site selective markers study. 1 Introduction Proteins are involved in all vital processes of transmitting information between specific cells, immune systems, in the control of gene expression and are crucial components of muscle and other systems for converting chemical energy into mechanical energy. Serum albumin is the most abundant protein found in the blood stream of the human body which has a remarkable ability to bind and transport a number of endogenous and exogenous ligands like drugs, amino acids, fatty acids, bilirubin, bile acids and thyroxin. 1–3 This makes these systems very interesting for the development of novel therapeutic agents, drug delivery pharmacokinetics, and phar- macodynamic modulations. 4–6 Bovine serum albumin (BSA), one of the important consti- tuent of the bovine serum protein, consists of 585 residues and is composed of a single peptide chain 7,8 with three a-helical domains I–III, each containing two subdomains A and B. The 17 disulphide bridges of BSA stabilize the protein and it is known to bind to different ligands efficiently. The main bind- ing sites of the protein are located in the hydrophobic cavity of subdomains IIA and IIIA. These binding sites are known as Sudlow’s I and Sudlow’s II 9,10 and the sole tryptophan residue is located in Sudlow I (Trp-214). 11 Naphthalimides comprise an environment sensitive special class of chromophores whose spectroscopic behavior depends on the properties of the surrounding medium so they may be used as solvatochromic probes. 12 1,8-Naphthalimide (NAPMD) photophysics 13 was established in terms of environment sensitive intramolecular charge transfer (ICT), and in water it Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India. E-mail: spsc@iacs.res.in; Fax: +91-33-2473-2805; Tel: +91-33-24734971 (ext. 250) † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c2mb25368b Received 12th September 2012, Accepted 6th November 2012 DOI: 10.1039/c2mb25368b www.rsc.org/molecularbiosystems Molecular BioSystems PAPER Downloaded by Indian Association for the Cultivation of Science on 23 November 2012 Published on 07 November 2012 on http://pubs.rsc.org | doi:10.1039/C2MB25368B View Article Online View Journal