MAGNETIC RESONANCE IN CHEMISTRY Magn. Reson. Chem. 2007; 45: 846–849 Published online 9 August 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/mrc.2057 The cis/trans isomerization of Cu(II) – bis-(glycinato) complex in solution: a computer aided multifrequency EPR and DFT/PCM calculation study M. Pezzato, G. Della Lunga, M. C. Baratto, A. Sinicropi, R. Pogni and R. Basosi * Department of Chemistry, University of Siena, Via A. De Gasperi, I-53100 Siena, Italy Received 9 March 2007; Revised 11 June 2007; Accepted 19 June 2007 In this paper the Cu(II)–bis-(glycinato) complex has been analysed in solution by applying a combined approach of multifrequency EPR and DFT/PCM calculations. The accuracy in the determination of magnetic parameters has been reached by the use of a unique simulation program (COSMOS) for the whole range of temperatures analysed and by the error analysis. A change in magnetic parameters was envisaged in the 243–253 K range of temperature, and was interpreted in terms of stabilization, near the freezing point of the solution, of one of the isomers of the complex. A DFT/PCM computational model was crucial in assigning, on the basis of the experimental superhyperfine interaction value, the isomer to the trans form. Copyright  2007 John Wiley & Sons, Ltd. KEYWORDS: CW-multifrequency EPR; DFT/PCM; Cu(II)-bis-(glycinato); cis/trans isomerization; EPR spectra simulation INTRODUCTION The computer program COSMOS developed in our labo- ratory gives the possibility of simulating EPR spectra of copper complexes virtually in all motional conditions. 1–3 This program was developed and implemented to over- come the problem of simulation of copper complexes in a slow motional condition. It arises from a program orig- inally conceived for the simulation of the EPR spectra of nitroxides in liquids and is based on the Stochastic Liouville Equation (SLE). 4–6 Information on the microscopic models of rotational dynamics for copper systems of biological interest has been for a long time precluded by the lack of a full program capable of simulating the EPR spectra containing more than one hyperfine interaction. Copper compounds are ubiquitously present in nature, where they form an important class of coordination com- plexes. For example, copper(II) amino acid complexes rep- resent the catalytic site in many important proteins, such as the blue copper enzymes. 7–11 Coordination geometry of the copper(II) ion is dictated by the nature and size of the ligands, which explains the wide variety of observed coor- dination complexes for this metal ion. In the literature many studies have been reported on the analysis of different cop- per(II) amino acid complexes, and some of them are based on EPR analysis. 12 – 22 Glycine is an ˛-amino acid that exists in aqueous solution in three different forms, with the deprotonated species predominating at alkaline pH. Cu(II) – bis-(glycinato) complexes may be present as cis and trans isomers which are L Correspondence to: R. Basosi, Department of Chemistry, University of Siena, Via A. De Gasperi 2, I-53100 Siena, Italy. E-mail: basosi@unisi.it in equilibrium with each other. 19 An X-ray absorption study was also performed on the Cu(II)–glycinate complexes in aqueous solution, but the applied EXAFS technique could not give conclusive information about the presence of the cis or trans isomers of the Cu(II) – bis-⊲glycinato⊳ Ð 2H 2 O complex. 23 Moreover, the geometry and the detailed electronic structure of the solid state complex do not need to be identical to the structure of the complex in solution because of the difference between the crystal lattice and solute–solvent interactions. In this paper, the Cu(II) – bis-(glycinato) complex has been analysed in a water/glycerol solution by a multifrequency EPR approach. The experimental spectra were recorded at X (9.58 GHz) and S (3.86 GHz) bands at different temperatures in the range 213 – 293 K. All the EPR spectra were simulated using the COSMOS program, converging to a reliable and unique set of magnetic parameters. Furthermore, an error analysis on the magnetic parameters was performed by a non-linear least-squares fit- ting method. 24,25 The use of a unique simulation program to simulate all the spectra in different motional conditions paired with density functional theory and polarizable contin- uum model (DFT/PCM) computations, 26,27 was synergistic in assigning with confidence the presence of different iso- mers of the Cu(II)–bis-(glycinato) complex, changing the physical state of the sample. MATERIALS AND METHODS Isotopically enriched 63 CuO was obtained from Oak Ridge National Laboratory, Oak Ridge, TN. Glycine was purchased from Sigma and used without further purification. The Cu(II)–bis-(glycinato) solution for EPR experiments was prepared in water with a final concentration of 12 mmol. The metal to ligand molar ratio was 1:4 in order to Copyright  2007 John Wiley & Sons, Ltd.