THE INFLUENCE OF AMINO ACID SIDE CHAINS ON WATER BINDING TO THE COPPER(II) IN COPPER(II) COMPLEXES: AN EPR AND A MOLECULAR MECHANICS STUDY Krunoslav Mirosavljević 1, Jasmina Sabolović 2, Vesna Nöthig-Laslo 1 1 Ruđer Bošković Institute 2 Institute for Medical Research and Occupational Health Zagreb, Croatia INTRODUCTION Copper(II) complexes with amino acids and peptides are potentially good enzyme mimetics. Interactions of amino acid side chains in those complexes may have important role in their temperature dependent behaviour. The EPR studies of the Brownian motion of bis(N,N-dimethyl- L -  -valinato)copper(II) (Cu(Me 2 Val) 2 ) and bis(N,N-dimethyl- L -  -leucinato)copper(II) (Cu(Me 2 Leu) 2 ) dissolved in different solvents (deuterated methanol, ~ K, and deuterated chloroform, ~ K) were combined with molecular mechanics calculations. The conformational analysis of these compounds was performed in order to find the most stable conformations. The aim of this work was to find whether different EPR behaviour of these two copper(II) complexes could be connected with the conformational (sterical) reasons. Bis(N,N-dimethyl- L -  -alaninato)copper(II) (Cu(Me 2 Ala) 2 ) was used as reference complex. There are no interactions of alanine side chains because –CH 3 is small group and the apical water molecule must be present for the stability of the complex. The effective volume of the complex is constant in temperature interval examined. The EPR spectra of Cu(Me 2 Leu) 2 both in CDCl 3 and CD 3 OD suggest aqua-complex (D 2 O in coordination sphere of copper) in whole temperature interval examined. In each solvent the spectra were simulated with the same parameters (A 0 and g 0 ). The conformational analysis backs EPR results: the energy difference between the most stable conformer and first above it increases for aqua-complex. It means that the presence of the water molecule additionally stabilizes complex. Cu(Me 2 Val) 2 in CD 3 OD shows more complicated dependence of   to  /T. Since it is not linear (i.e. the volume of the complex changes in temperature interval examined), there are some conformational changes. The energy difference between the most stable conformer and first above it is smaller when water molecule is present which allows us to suggest some rearrangements of amino acid side chains and perhaps releasing of apical ligand (water) during the heating. THEORY AND RESULTS ΔB(m I ) = a + b m I + c m I 2 Because of the incomplete averaging of interaction between magnetic moments of an electron and a nuclear spin, line widths in EPR spectra,  B, depend on nuclear magnetic moment, m I. It provides that line widths obtained in EPR spectra can be expressed: Reorientation correlation time,  , depends on viscosity to temperature ratio,  /T, according to Stokes-Einstein relation: Parameter b is varied by temperature and it is connected with   : DISCUSION AND CONCLUSION gg g  gg A  /mTA  /mTA  /mT Cu(Me 2 Ala) 2 CD 3 OD2.117       0.01 Cu(Me 2 Leu) 2 CDCl       0.01 CD 3 OD2.133       0.01 Cu(Me 2 Val) 2 CDCl       0.01 CD 3 OD K       0.01 CD 3 OD K       0.01 Table 1. EPR parameters used for calculations of  . Figure 4. The most stable conformations of aqua-complexes of Cu(Me 2 Leu) 2 and Cu(Me 2 Val) 2 with both side chains in axial positions. Cu(Me 2 Leu) 2 ·4H 2 O OwOw OwOw Cu(Me 2 Val) 2 ·4H 2 0