title: XAS spectroscopy, sulfur, and the brew within blood cells from Ascidia ceratodes.
abstract: We report the first use of K-edge X-ray absorption spectroscopy (XAS) as a direct spectroscopic probe of pH and cytosolic emf within living cells. A new accuracy metric of model-based fits to K-edge spectra is further developed. Sulfur functional groups in three collections of living blood cells and one sample of cleared blood plasma from the tunicate Ascidia ceratodes were speciated using K-edge XAS. Cysteine and cystine, the preferred thiol-disulfide model, averaged about 12% of total sulfur. Sulfate monoesters and cyclic diesters unexpectedly constituted 36% of blood cell sulfur. Soluble sulfate averaged about 25% across the three blood cell samples, while the ratio of SO4(2-) to HSO4(-) implied average signet ring vacuolar pH values of 0.85, 1.4, or 3.1. Intracellular (VSO4)(+) was unobserved, while [V(RSO3)n]((3-n)+) was detected in the two lowest pH blood cell samples. About 5% of sulfur was distributed as mono- or dibenzothiophene or ethylene-epi-sulfide, or as a thiadiazole reminiscent of the polycarpathiamines. Blood plasma was dominated by sulfate (83%), but with 15% of an alkylsulfate ester and about 2% of low-valent sulfur. Gravimetric analysis of soluble sulfate yielded average concentrations of blood cell sulfur. Average [cysteine] and [cystine] (ranging ~10-30 mM and ~20-90 mM, respectively) implied blood-cell cytosolic emf values of approximately -0.20 V. High cellular [cysteine] is consistent with the proposed model for enzymatic reduction of vanadate by endogenous thiol, wherein the trajectory of metal site-symmetry is controlled and directed through to a thermodynamically favored 7-coordinate V(III) product.
authors: Department of Chemistry, Stanford University, Stanford, CA 94305, USA; Stanford Synchrotron Radiation Lightsource, SLAC, Stanford University, Stanford, CA 94309, USA. Electronic address: pfrank@slac.stanford.edu.
authors: Patrick
authors: P
authors: Frank
authors: Stanford Synchrotron Radiation Lightsource, SLAC, Stanford University, Stanford, CA 94309, USA.
authors: Britt
authors: B
authors: Hedman
authors: Department of Chemistry, Stanford University, Stanford, CA 94305, USA; SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94309, USA.
authors: Keith O
authors: KO
authors: Hodgson
mesh_terms: D000818:Animals; D001773:Blood Cells / Q000737:chemistry* / Q000166:cytology; D003545:Cysteine / Q000737:chemistry; D003553:Cystine / Q000737:chemistry; D003600:Cytosol / Q000737:chemistry; D004220:Disulfides / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D015394:Molecular Structure; D013431:Sulfates / Q000032:analysis / Q000097:blood; D013438:Sulfhydryl Compounds / Q000737:chemistry; D013455:Sulfur / Q000032:analysis / Q000097:blood* / Q000737:chemistry; D014561:Urochordata / Q000737:chemistry* / Q000166:cytology; D014638:Vanadates / Q000378:metabolism; D056928:X-Ray Absorption Spectroscopy / Q000379:methods*
chemical_list: D004220:Disulfides; D013431:Sulfates; D013438:Sulfhydryl Compounds; D014638:Vanadates; D003553:Cystine; D013455:Sulfur; D003545:Cysteine
keywords: Ascidian; Blood cells; Speciation; Sulfur; XAS; emf