Electrochemical and spectral characterization of blue copper protein models Harmeet Dhillon, Kavita Sharma, Rakhee Gehlot, Sunita Kumbhat * Biosensor Laboratory, Department of Chemistry, J.N.V. University, Bhagat ki kothi, Jodhpur, Rajasthan 342033, India article info Article history: Received 25 December 2008 Received in revised form 10 February 2009 Accepted 11 February 2009 Available online 20 February 2009 Keywords: Type I blue copper 2,2 0 -dithiodianiline Model compounds Electrochemical behavior abstract Blue copper proteins play a central role in various enzymatic anabolic/catabolic pathways in living cells by virtue of the integrated metal ions. These ions may exist in variable oxidation states, with suitable reduction potentials and fast electron-transfer rates which in turn is a manifestation of their unusual geometry and co-ordination. We report the electrochemical and spectral characterization of three novel complexes of copper (II) with N 2 S type tridentate chelating agent 2,2 0 -dithiodianiline (dta), having struc- tural similarities to the active site of Type I copper proteins. High positive redox potentials in the range of 0.5–0.6 V vs Ag/AgCl electrode of the complexes and the absorption maxima at 550 nm, with high extinction coefficients, correspond well with typical blue copper proteins. The IR and EPR studies support the assigned pseudo tetrahedral structures to the complexes. The diffusion coefficient and rate constant for heterogeneous charge transfer for Cu 2+ /Cu + coordinated in a potentially bio-mimetic Type I site is reported. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Enzymes and metalloproteins are an important class of bioac- tive compounds, performing a variety of biochemical functions involving electron transfer and oxidative catalysis, by virtue of the integrated metal ions, which may exist in variable oxidation states [1,2]. Suitable redox potentials and fast charge transfer kinetics of Type I (Blue) proteins play a central role in various bio- chemical functions, which are in-turn a manifestation of their unu- sual geometry and co-ordination [3–6]. The classic blue copper sites have a single Cu (II) ion in a distorted tetrahedral environ- ment with a strong Cu–S bond to the thiolate of cysteine residue, two typical Cu–N (Histidine) bonds and a long Cu–S bond to a methionine thioether group [7]. The highly covalent Cu 2+ –S (Cys) bond provides a strong electronic coupling into protein pathways to facilitate rapid long-range electron transfer [8,9]. The shift of Cu 2+ /Cu + reduction potentials of Type I copper sites towards more positive potential may vary considerably from 184 mV (stellacya- nin) to 680 mV (rusticyanin) and to 1000 mV (ceruloplasmin) in comparison to that of ionic Cu 2+ /Cu + (154 mV) in aqueous medium [10]. The bioinorganic chemists have long been trying to unveil the intrinsic features of such proteins by modeling the active site through the synthesis of relatively simple model compounds, which are expected to mimic biological templates. In earlier at- tempts to mimic active sites of Type I copper protein, copper com- plexes with ligands derived from Schiff bases have dominated the literature [11–14], though reports on other multidentate ligands having N, S and/or O atoms are also available [15–17]. A versatile suphur–nitrogen chelating agent, 2,2 0 -dithiodianiline, is known for synthesis of metal complexes [18] and has been reported as a selective ionophore for copper under specified conditions in recent past [19]. Herein, we introduce a novel ligand system based on 2,2 0 - dithiodianiline (dta) for the formation of Cu (II) complexes with Type I site properties. Voltammetry and spectroscopy data are re- ported for a set of three new complexes. 2. Experimental 2.1. Apparatus and reagents Cyclic voltammetric and chronocoulometric measurements were carried out with Electrochemical Analyzer, BAS 27 (Bioanalyt- ical Systems, USA) equipped with C1 cell stand, with platinum disc electrode (PtE) as the working electrode, a platinum wire as the auxillary electrode and Ag/AgCl (3 M KCl) as the reference electrode. The area (A) of the electrode was determined by performing chronocoulometry of ferricyanide and was found to be 2.6  10 2 cm 2 . The electrochemical studies were carried out in acetonitrile, and 0.1 M tetrabutylammonium perchlorate (Fluka) was the supporting electrolyte. The electronic absorption spectra were recorded on a Shimadzu UV-160 spectrometer. Infra red (IR) spectra of KBr pellet samples were made on a Perkin Elmer 537 spectrometer. EPR spectra were recorded at RSIC, IIT-M, Chennai on a Varian E-112 X/Q band EPR spectrometer at 77 K in DMF; diphenylpicrylhydrazyl (DPPH) was used as an internal sealed marker. Magnetic susceptibility 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.02.017 * Corresponding author. Tel.: +91 291 2720840; fax: +91 291 2720436. E-mail address: skumbhat@rediffmail.com (S. Kumbhat). Electrochemistry Communications 11 (2009) 878–880 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom