A Large Reservoir of Sulfate and Sulfonate Resides within Plasma Cells from Ascidia ceratodes, Revealed by X-ray Absorption Near-Edge Structure Spectroscopy

The study of sulfur within the plasma cells of Ascidia ceratodes [Carlson, R.M.K. (1975) Proc. Natl. Acad. Sci. U.S.A. 72, 2217-2221; Frank, P., Carlson, R.M.K., & Hodgson, K.O. (1986) Inorg. Chem. 25, 470-478; Hedman, B., Frank, P., Penner-Hahn, J.E., Roe, A.L., Hodgson, K.O., Carlson, R.M.K., Brown, G., Cerino, J., Hettel, R., Troxel, T., Winick, H., & Yang, J. (1986) Nucl. Instrum. Methods Phys. Res., Sect. A 246, 797–800] has been extended with X-ray absorption near-edge structure (XANES) spectroscopy. An intense absorption feature at 2482.4 eV and a second feature at 2473.7 eV indicate a large endogenous sulfate concentration, as well as smaller though significant amounts of thiol or thioether sulfur, respectively. A strong shoulder was observed at 2481.7 eV on the low-energy side of the sulfate absorption edge, deriving from a novel type of sulfur having a slightly lower oxidation state than sulfate sulfur. The line width of the primary transition on the sulfur edge of a vanadium(III) sulfate solution was found to be broadened relative to that of sodium sulfate, possibly deriving from the formation of the VS0₄⁺ complex ion [Britton, H.T.S., & Welford, G. (1940) J. Chem. Soc., 761-764; Duffy, J.A., & Macdonald, W.J.D. (1970) J. Chem. Soc., 977-980; Kimura, T., Morinaga, M., & Nakano, J. (1972) Nippon Kagaku Zaishi, 664–667]. Similar broadening appears to characterize the oxidized sulfur types in vanadocytes. A very good linear correlation between oxidation state and peak position (in electronvolts) was found for a series of related sulfur compounds. This correlation was used to determine a 5+ oxidation state for the additional sulfur type at 2481.7 eV. Construction of a cellular sulfur minus vanadium(III) sulfate difference spectrum, along with comparison with spectra of known compounds, identified the novel sulfur(V) as an aliphatic sulfonic acid analogous to cysteic acid. The overall sulfonic acid concentration is comparable to that of sulfate in plasma cells and appears to be unprecedented in marine organisms.