Thermodynamic and spectroscopic studies of Cu(II) and Ni(II) complexes with a new proline–threonine dipeptide ligand Ste ´phanie Boudesocque a,b , Ziad Damaj a , Laurent Dupont a , Jean-Bernard Behr c , Emmanuel Guillon a,b, * a GRECI (Groupe de Recherche en Chimie Inorganique), Universite ´ de Reims Champagne-Ardenne, BP 1039, 51687 Reims Cedex 2, France b ESIEC (Ecole Supe ´rieure d’Inge ´nieurs en Emballage et Conditionnement), Po ˆle Technologique Henri Farman, BP 1029, F-51686 Reims Cedex 2, France c Laboratoire Re ´actions Se ´lectives et Applications, UMR CNRS 6519, UFR Sciences, BP 1039, 51687 Reims Cedex 2, France Received 16 October 2007; received in revised form 17 January 2008; accepted 18 January 2008 Available online 1 February 2008 Abstract The interactions between a new proline–threonine dipeptide ligand with two metallic cations were investigated in aqueous solution. The metallic cations studied were the copper(II) and the nickel(II), which are involved in many biological processes. The combination of potentiometry, UV–visible spectrophotometry, EPR, and mass spectrometry was used to determine the formation constants of the com- plexes and their structure in solution. The complexation sites were identified using electronic absorption and EPR spectroscopies. Copper complexes were obtained as square planar or square pyramidal mononuclear species, whereas nickel complexes were obtained as dinu- clear species with an octahedral geometry. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Copper(II) and nickel(II) complexes; Dipeptide; UV–Vis spectrophotometry; EPR; Potentiometry 1. Introduction The interactions between the metals of the first row tran- sition and dipeptides have been widely studied [1–3]. The dipeptide complexes with metal ions have been extensively studied in order to mimic and to understand specific metal- loprotein structures and functions [4]. The amino-acids proline and threonine are both biologically important. The proline (Pro) is a non-essential, genetically coded amino-acid. It is the only protein-forming amino-acid with a secondary amino group. This amino-acid is a critical res- idue for secondary structure in oligopeptides and proteins. It also has a predominant action in the interaction of the peptides with metal ions [5]. Whereas the threonine (Thr) is an essential, genetically coded amino-acid, which is important for the human nutrition. There is a great number of studies dealing with the simple systems proline–metal ion [6,7] and threonine–metal ion [8]. But, to our knowl- edge the interaction between the dipeptide proline–threo- nine (Pro-Thr) and metal ions has never been studied. In metalloprotein systems the amide linkage is usually a criti- cal binding site of many metal ions, which induces specific structural and thermodynamic features of the complexes. Many studies have shown that Cu(II) and Ni(II) ions are efficient to deprotone the amidic nitrogen atom by forming stable chelated species [9–11]. Moreover, due to their impli- cation in various biological processes, Ni(II) and Cu(II) complexes with dipeptide ligands are the most studied sys- tems among the complexes with the first row transition metals. Copper(II) is involved in the metabolism of dioxy- gen (transport, storage, and activation) [12] and plays an essential role in the superoxide deactivation and in the respiratory chain [13]. Moreover, copper is an essential element found in relatively high concentrations in the brain 0162-0134/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2008.01.021 * Corresponding author. Address: GRECI (Groupe de Recherche en Chimie Inorganique), Universite ´ de Reims Champagne-Ardenne, BP 1039, 51687 Reims Cedex 2, France. Tel.: +33 (0) 3 26 91 31 42; fax: +33 (0) 3 26 91 32 43. E-mail address: emmanuel.guillon@univ-reims.fr (E. Guillon). www.elsevier.com/locate/jinorgbio Available online at www.sciencedirect.com Journal of Inorganic Biochemistry 102 (2008) 1514–1522 JOURNAL OF Inorganic Biochemistry