Study of Solution Structure of Vanadium (III) Complexes by FT Raman Spectroscopy

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Raman spectra of several vanadium compounds, mainly in the +3 oxidation state, were measured with a view toward exploring the chemical structure and the physiological and biochemical functions of vanadium in the blood cells of ascidians (marine invertebrate), and some fundamental knowledge concerning a characterization and structure of vanadium compounds in solution. Firstly, Raman spectra of a homogenate of ascidian blood cells were obtained. The blood cells of ascidia samea were separated into two fractions, namely, giant cell rich (GC) and signet-ring cell rich (SRC) fractions, and the Raman spectrum of each fractions was measured. Both fractions exhibited the bands which are assignable to hydrosulfate anion or alkyl sulfonates and also the band due to free sulfate. The band due to vanadium ion could not be measured for both fractions. The concentration of vanadium in ascidia samea would be lower than the detection limit of the present FT Raman spectrometer. On the other hand, a blood cell homogenate of ascidia gemmata did show the band due to vanadyl group in addition to the band of free sulfate. In order to examine the interaction between vanadium (III) or- (IV) and sulfate ion, the Raman spectra of aqueous solution of V_2(SO_4)_3 and VOSO_4 were measured. The Raman spectra of both systems showed the bands whichi are assignable to sulfate anion coordinated to vanadium center, in addition to the band due to free sulfate, indicating that sulfate anion derectly bounds to vanadium ion in aqueous solution. Ascidians sequester vanadium (V) ions from seawater, reduce them to +3 oxidation state, and accumulate in their blood cells. Then, it is of importance to correlate the solution structure of vanadate species with their vibrational frequencies. We, therefore, examined the pH dependence of the Raman spectra of vanadate (V) and assigned the observed Raman bands to each oligomeric species.