The use of spin-label and spin-probe techniques is of considerable interest in Biology and Biophysics [1, 2]. The presence of different paramagnetic centers in biological systems, as nitroxide radicals are paramagnetic metals, and the resulting spin-spin interactions provide substantial structural structure and dynamic information on such systems. Studies of model systems containing a metal ion and a labeled biomolecule by application of ESR and magnetic methods are rare. Our investigations concern a new dimeric copper(II) complex with spin-labeled glycin. We report the synthesis, spectral and magnetic properties of this complex. Experimental Spin labeling of glycine was performed using ethyl ester glycine and 3,5-dibromo-4-oxo-2,2,6,6-tetramethyl-piperidin-1-oxide according to the method previously described by B.T. Golding [3]. Hydrolysis of this peptidic compound with an original protocol, resulted in a new paramagnetic N-substituted α-amino-acid: ▪ The complex was prepared by adding copper acetate monohydrate to an ethanolic solution of the labeled ligand in the ratio Cu/L = 1 2 . The green compound was purified by recrystallization in boiling water. Results and Discussion Elemental analysis shows a Cu/L ratio of 1 2 and the molecular weight determined by mass spectrometry is 1142.14. These results are in agreement with a dinuclear species: [Cu((NO)GLYO) 2] 2 · 3H 2O. The IR vCO band in the free ligand (1700 cm −1) suffers down shift upon complexation (1660 cm −1) supporting the coordination of the Cu(II) ion through the carboxylic oxygen atom. The position of this carbonyl absorption is characteristic of a copper carboxylate species [4]. The electronic spectrum exhibits a broad band (gl = 725 nm, ϵ = 175 · mol −1 · cm −1, similar to those observed in copper carboxylate dimers [4]. In pyridin solution, the shift of this absorption and its higher intensity (λ = 660 nm, ϵ =280 l · mol −1 · cm −1) indicate the dissociation of the 31dimer into a monomer. The ESR spectrum of the polycrystalline sample confirms the dinuclear character of the copper(II) complex. This spectrum consists of a copper(II) dimer typical signal with a large dipolar splitting (D = 0.3 cm −1 and lines expressing spin interactions between nitroxides or between nitroxides and copper(II) in monomeric species impurities ( g = 2.025) The EPR spectrum of the compound in solution indicates the complex is dimeric in DMF and monomeric in pyridin. For the monomeric species, in pyridin, the average line characteristics of the copper nitroxide interaction is observed at g = 2.025 [5]. The magnetic susceptibility measurements, in the range 300–4 K, show an antiferromagnetic interaction between both copper ions which is similar to that of the copper acetate dimer [4]. A fitting procedure of a theoretical equation based on the isotropic HDvV-Model leads to a very good agreement between calculated and experimental susceptibilities. The following values were obtained: J CuCu = −133 cm −1, J ṄOṄO = −1.9 cm −1, g Cu = 2.25, g NO = 2.01, x = 4.7% (monomeric impurities). The results obtained for this complex encourage new investigations with model systems including more complex biomolecules. Selective spin-labeling of macromolecules would be helpful to obtain information on structural and dynamic aspects in biological systems (calculation of distances between paramagnetic centers, biodisponibility of s metal).