Organic thiols are highly reactive ligands and play an important role in the speciation of several metals and organic pollutants in the environment. Although small thiols can be isolated and their concentrations can be estimated using chromatographic and derivatization techniques, estimating concentrations of thiols associated with biomacromolecules and humic substances has been difficult. Here we present a fluorescence-spectroscopy-based method for estimating thiol concentrations in biomacromolecules and cell membranes using one of the soluble bromobimanes, monobromo(trimethylammonio)bimane (qBBr). The fluorescence of this molecule increases significantly when it binds to a thiol. The change in the sample fluorescence due to thiols reacting with qBBr is used to determine thiol concentration in a sample. Using this method, small thiols such as cysteine and glutathione can be detected in clean solutions down to ~50 nM without their separation and prior concentration. Thiols associated with dissolved organic matter (DOM) can be detected down to low micromolar concentration, depending on the DOM background fluorescence. The charge on qBBr prevents its rapid diffusion across cell membranes, so qBBr is ideal for estimating thiol concentration at the cell membrane-water interface. This method was successfully used to determine the thiol concentration on the cell envelope of intact Bacillus subtilis to nanomolar concentration without any special sample preparation. Among the chemical species tested for potential interferences (other reduced sulfides methionine and cystine, carboxylate, salt (MgCl(2))), carboxylates significantly influenced the absolute fluorescence signal of the thiol-qBBr complex. However, this does not affect the detection of thiols in heterogeneous mixtures using the presented method.

30We have examined the speciation of Hg(II) complexed with intact cell suspensions (10 13 31 cells L -1 ) of Bacillus subtilis, a common gram-positive soil bacterium, Shewanella oneidensis 32 MR-1, a facultative gram-negative aquatic organism, and Geobacter sulfurreducens, a gram-33 negative anaerobic bacterium capable of Hg-methylation at Hg(II) loadings spanning four orders 34 of magnitude (120 nM to 350 µM) at pH 5.5 (±0.2). The coordination environments of Hg on 35 bacterial cells were analyzed using synchrotron based X-ray Absorption Near Edge Structure 36 (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy at the Hg LIII 37 edge. The abundance of thiols on intact cells was determined by a fluorescence-spectroscopy 38 based method using a soluble bromobimane, monobromo(trimethylammonio)bimane (qBBr) to 39 block thiol sites, and potentiometric titrations of biomass with and without qBBr treatment. The 40 chemical forms of S on intact bacterial cells were determined using S k-edge XANES 41 spectroscopy. 42Hg(II) was found to complex entirely with cell bound thiols at low Hg:biomass ratios. 43 For Bacillus subtilis and Shewanella oneidensis MR-1 cells, the Hg-S stoichiometry changed 44 from Hg-S3 to Hg-S2 and Hg-S (where 'S' represents a thiol site such as is present on cysteine) 45 progressively as the Hg(II) loading increased on the cells. However, Geobacter sulfurreducens 46 did not form Hg-S3 complexes. Because the abundance of thiol was highest for Geobacter 47 sulfurreducens (75 µM/g wet weight) followed by Shewanella oneidensis MR-1 (50 µM/g wet 48 weight) and Bacillus subtilis (25 µM/g wet weight), the inability of Hg(II) to form Hg-S3 49 complexes on Geobacter sulfurreducens suggests that the density and reactivity of S-amino acid 50 containing cell membrane proteins on Geobacter sulfurreducens are different from those of 51Bacillus subtilis and Shewanella oneidensis MR-1. Upon saturation of the high affinity thiol sites 52 at higher Hg:biomass ratios, Hg(II) was found to form a chelate with -hydroxy carboxylate 53anion. The stoichiometry of cell envelope bound Hg-thiol complexes and the associated 54 abundance of thiols on the cell envelopes provide important insights for understanding the 55 differences in the rate and extent of uptake and redox transformations of Hg in the environment. Introduction 66Mercury is a common contaminant found in many terrestrial and aquatic systems, and its 67 bioaccumulation in organisms, including humans, is a major environmental concern (Mergler et 68 al, 2007). The solubility, speciation, toxicity and the ultimate fate of Hg within aquatic 69 ecosystems is dependent on a large number of chemical and biological variables (Morel et al., 70 1998;Barkay and Schaefer, 2001). In aquatic systems, Hg solubility is high under oxygen-rich 71 acidic conditions but it is significantly inhibited under anoxic sulfide-rich waters (Martell and 72 Smith, 1974). The Hg-sulfide complexes are among the strongest complexes of all known Hg 73 i...