A comparative study on the binding of single and double chain surfactant–cobalt(III) complexes with bovine serum albumin G. Vignesh a , K. Sugumar a , S. Arunachalam a,⇑ , S. Vignesh b , R. Arthur James b a School of Chemistry, Bharathidasan University, Tiruchirapalli 620 024, Tamil Nadu, India b Department of Marine Science, Bharathidasan University, Tiruchirapalli 620 024, Tamil Nadu, India highlights  Studies on the binding of surfactant– cobalt(III) complexes with BSA were carried out.  Hydrophobic attraction plays a major role in the binding process.  Binding induces considerable amount of conformational changes in the protein.  Surfactant–cobalt(III) complexes show good antibacterial and antifungal activities. graphical abstract article info Article history: Received 20 November 2012 Received in revised form 25 April 2013 Accepted 29 April 2013 Available online 22 May 2013 Keywords: Bovine serum albumin Surfactant–cobalt(III) complexes Binding parameters Antimicrobial activities abstract The comparative binding effect of single and double aliphatic chain containing surfactant–cobalt(III) complexes cis-[Co(bpy) 2 (DA) 2 ](ClO 4 ) 3 2H 2 O (1), cis-[Co(bpy) 2 (DA)Cl](ClO 4 ) 2 2H 2 O (2), cis-[Co(phen) 2 (- CA) 2 ](ClO 4 ) 3 2H 2 O (3), and cis-[Co(phen) 2 (CA)Cl](ClO 4 ) 2 2H 2 O (4) with bovine serum albumin (BSA) under physiological condition was analyzed by steady state, time resolved fluorescence, synchronous, three-dimensional fluorescence, UV–Visible absorption and circular dichroism spectroscopic techniques. The results show that these complexes cause the fluorescence quenching of BSA through a static mech- anism. The binding constants (K b ) and the number of binding sites were calculated and binding constant values are found in the range of 10 4 –10 5 M 1 . The results indicate that compared to single chain complex, double chain surfactant–cobalt(III) complex interacts strongly with BSA. Also the sign of thermodynamic parameters (DG°, DH°, and DS°) indicate that all the complexes interact with BSA through hydrophobic force. The binding distance (r) between complexes and BSA was calculated using Förster non-radiation energy transfer theory and found to be less than 7 nm. The results of synchronous, three dimensional fluorescence and circular dichroism spectroscopic methods indicate that the double chain surfactant– cobalt(III) complexes changed the conformation of the protein considerably than the respective single chain surfactant–cobalt(III) complexes. Antimicrobial studies of the complexes showed good activities against pathogenic microorganisms. Ó 2013 Elsevier B.V. All rights reserved. Introduction Studies on the interaction between metal complexes and bio- macromolecules have attracted great interest in the past decades [1–3]. Among the bio-macromolecules studied serum albumins are mostly used as model biopolymers. They are the major soluble protein constituents of the circulatory systems, contributing signif- icantly in the physiological functions as carrier proteins. Among these proteins bovine serum albumin (BSA) has been studied extensively [4–7]. This protein consists of 585 amino acids in a sin- gle polypeptide chain cross-linked with 17 disulfide bonds. It is 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.04.123 ⇑ Corresponding author. Tel.: +91 431 2407053. E-mail address: arunasurf@yahoo.com (S. Arunachalam). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 415–422 Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa