Structural characterization of Cr(V)-D-glucuronic acid species by X band EPR spectroscopy at room temperature.

LUIS F. SALA; SANDRA SIGNORELLA; JUAN CARLOS GONZÁLEZ; SILVIA GARCÍA; ANA MARÍA ATRIA; SEBASTIÁN BELLÚ; NADIA MAMANA; MARÍA J. CERUTI; ANTAL ROCKENBAUER; LAZLO KORECZ

Atenas (Grecia)

Congreso; 8th FIGIPAS Meeting in Inorganic Chemistry; 2005

NCSR Demokritos - Institute of Physical Chemistry

  The most common means of characterising Cr(V) complexes in solution is EPR spectroscopy. The EPR spectral parameters, giso and Aiso values, together with the proton shf coupling, are usefull in determining the binding modes of sugars to the Cr(V) Center. In this work the assignament of the structures of new oxo-Cr(V)-Glucuronic acid species in solution was made according to the isotropic EPR parameters and the shf pattern of the signal. It is known that the five-membered Cr(V) chelates are the most favourably formed and D-Glucuronic acid is well suited for stabilization of five-membered Cr(V) chelates through the 2-hydroxycarboxylato and vic-diolato potential chelation sites.   In the range of pH of 1-4, the EPR spectra of the intermediate-Cr(V) species show two triplets at giso=1.9781 and 1.9784 and two doublets at giso=1.9753 and 1.9764. The giso values for the triplets correspond to those calculated for five-coordinate oxo-Cr(V) bischelate with two carboxylato and two alcoholato ligands, and th shf pattern of these signals is that expected for two equivalent carbinolic protons coupled to the Cr(V) electronic spin.These carbinolic protons must belont to the a-hydroxocarboxylate donor site of two D-glucuranuronic acid molecules bound to Cr(V), to yield two geometric isomers of the [CrvO(O6,O5-glucufuranuronate)2]-. Doublet at giso=1,9753 and 1,9764 correspond to those calculated for five-coordinate oxo-Cr(V) monochelates with one carboxylato and one alcoholato ligands and two alcoholate ligands, respectively. The shf pattern of these signals indicates that only one carbinolic proton is coupled to the Cr(V) electronic spin and can be assigned to the two-monochelates:[Cr(V)(O)(OH2)2(O6,O5-glucufuranuronate]+ and [Cr(V)(O)(OH2)2(O1, O2-glucupyranuronate]+ with one D-glucuronic acid molecule acting as bidentate ligand through the 2-hydroxocarboxylate moiety of the furanosic form or through the cis diolate moiety of the pyranosic form. Two molecules of water complete the coordination sphere of Cr(V). The present results show that the furanosic form of glucuronic acid is preferred in complexation of Cr(V), even when in aqueous solution it is less stable than the pyranosic one-. At pH 7,5 EPR spectra show tow triplets at giso 1.9796 and 1.9798. These values are in agreement with those calculated for Cr(V) bound to four alcoholato ligands and can be assigned to the two geometric isomers of the Cr(V) bis chelate [Cr(V)(O)(O1,O2 glucupyranuronate)2]-. This result indicates that at his pH the Cr(V)-diolate binding mode is preferred over the Cr(V)-2-hydroxyacid coordination mode, like previously observed for D-galacturonic acid and quinic acid.