Journal of the Brazilian Chemical Society
On-line version ISSN 1678-4790
SCARPELLINI, Marciela et al. EPR and semi-empirical studies as tools to assign the geometric structures of FeIII isomer models for transferrins. J. Braz. Chem. Soc. [online]. 2006, vol.17, n.8, pp.1617-1626. ISSN 1678-4790. http://dx.doi.org/10.1590/S0103-50532006000800020.
The reaction of Fe(ClO4)3.nH2O with N,N'-bis[(2-hydroxybenzyl)-N,N' -bis(1-methylimidazole-2-yl-methyl)]ethylenediamine (H2bbimen) affords two geometric isomers, A and B, of the complex cation [Fe(bbimen)]+, which were fully characterized by IR and UV-visible spectroscopies, ESI mass spectrometry, molar conductivity measurements, cyclic voltammetry, spectroelectrochemistry and EPR spectroscopy. The geometry of one of these isomers has been clearly demonstrated through X-ray crystallographic analysis. It crystallizes in the monoclinic system, space group P21/c, a = 14.104 (3), b = 15.626 (3), c = 13.291 (3) Å, b = 98.07 (3)º, Z = 4, R1 = 6.35% and wR2 = 20.57%. The electrochemical properties of [Fe(bbimen)]+ (-0.58 V versus NHE) are quite similar to those of transferrins (-0.52 V versus NHE) and indicate that it is a good model for the redox potential of these metalloenzymes. EPR spectroscopy was the only spectroscopic technique able to differentiate the isolated isomers A and B. EPR studies revealed that isomer A is better described as a rhombically distorted (E/D @ 0.33) high-spin FeIII complex (g1 @ 4.1 with a shoulder at g2 @ 9.0), while the spectrum of B has a set of lines at g1 @ 3.0, g2 @ 3.6 and g3 @ 5.1, in addition to the line at g1 @ 4.2 and a shoulder at g2 @ 9.0, which have been ascribed to FeIII complexes in axial symmetry (E/D @ 0.22). Semi-empirical theoretical studies, combined with EPR data, were essential to the proposition of the geometric structures of A and B.
Keywords : FeIII complex; geometric isomers; EPR; transferrins; semi-empirical studies.