Medium-dependence of vanadium K-edge X-ray absorption spectra with application to blood cells from phlebobranch tunicates

Abstract

K-edge X-ray absorption spectroscopy (XAS) is a subtle probe of the chemical environment and oxidation state of the elements. Thus, the change in the energy position of the rising K-edge inflection in the XAS spectrum of [V(H 2O) 6] 3+ in pH 0, 1, 2 and 3 aqueous solutions produces a titration curve that can be fit ( r=0.999) with an unusual model involving two cooperative deprotonations, yielding p K a1=1.5±0.1 and p K a2=1.1±0.1. These pH effects on V III K-edge XAS spectra vary with the medium (40% aqueous methanol) and the counterion (Cl −, SO 4 2−). Applied to whole blood of the tunicate Ascidia ceratodes, as collected from Monterey Bay, California, fits to the vanadium K-edge XAS spectra produced a detailed speciation of the major endogenous cellular V III (complex, percent): [V(H 2O) 6] 3+, 23.6%; [V(SO 4)(H 2O) 5] +, 38.1%; [V(SO 4) 2(H 2O) 4] −, 19.8%, and; [V(SO 4)(OH) 2(H 2O) 3], 7.7%. Genus-associated differences in the distribution of blood cell vanadium appear on comparison with a sample of whole blood from Phallusia nigra, in which most of the vanadium is distributed among [V(H 2O) 6] 3+, 34%; tris-chelated V III, 33%, and; [V IVO(H 2O) 5] 2+, 30%, with no V III complex ions at all detected. Vanadium distribution in the blood cells of a single specimen of A. ceratodes from Bodega Bay, California is shown to vary significantly from the norm of animals collected from Monterey Bay, California. Finally, preliminary results are reported from in vitro experiments exposing A. ceratodes blood cells to vanadyl ion, showing active uptake and incorporation.