NATURE AND LIGATION OF VANADIUM WITHIN WHOLE-BLOOD CELLS AND HENZE SOLUTION FROM THE TUNICATE ASCIDIA CERATODES, AS INVESTIGATED BY USING X-RAY-ABSORPTION SPECTROSCOPY

Vanadium K-edge X-ray absorption spectroscopy (XAS) studies have been carried out at 10 K on packed whole blood cell samples and on Henze so lution from the tunicate Ascidia ceratodes (A. ceratodes). High energy -resolution vanadium K-edge spectra exhibit pre-edge transitions at 54 64.9 +/- 0.1, 5466.8 +/- 0.1, and 5468.8 +/- 0.1 eV for both whole blo od cell samples and Henze solution. The whole blood vanadium K-edge sp ectrum is very similar to that of vanadium(III) in aqueous sulfuric ac id solution. Both spectra exhibit a feature at 5476 eV indicative of a n endogenous vanadium(III)-sulfate interaction. This represents the fi rst direct spectroscopic evidence for intracellular [(VSO4)(H2O)(4-5)] (+). Absence of the vanadium(III)-sulfate feature in the vanadium X-ra y absorption edge spectrum of Henze solution indicates loss of the sul fate ligand on dilution of the intravacuolar contents following whole cell lysis. Vanadium K-edge EXAFS analysis of the whole blood samples revealed about six nearest neighbor oxygen (or nitrogen) atoms at 1.99 +/- 0.02 Angstrom. No evidence either for the more distant carbon she lls of an intracellular vanadium chelate or for (VOV)(4+) dimers, was found in the EXAFS spectra of whole blood cells. Since the XAS spectra were taken at 10 K, the likelihood that significant amounts of such s pecies remained undetected is remote. Taken together, the results are consistent with the proposition that blood cell vanadium, at least wit hin the tunicate A. ceratodes, is >90% monomeric, existing as a mixtur e of the hexaaquovanadium(III) ion and the [(VSO4)(H2O)(4-5)](+) compl ex ion. The possible presence of up to 10% of, e.g., a tris-chelated t unichrome-vanadium(III) complex is not excluded. Within Henze solution , vanadium K-edge and EXAFS spectral analysis likewise indicated unoxi dized, monomeric V(III) with six to seven first shell oxygens at 1.99 +/- 0.02 Angstrom but with no indication of a sulfate interaction.