The siderophore enterobactin (Ent) is produced by enteric bacteria to mediate iron uptake. Ent scavenges iron and is taken up by the bacteria as the highly stable ferric complex [FeIII(Ent)]3−. This complex is also a specific target of the mammalian innate immune system protein, Siderocalin (Scn), which acts as an anti-bacterial agent by specifically sequestering siderophores and their ferric complexes during infection. Recent literature suggesting that Scn may also be involved in cellular iron transport has increased the importance of understanding the mechanism of siderophore interception and clearance by Scn; Scn is observed to release iron in acidic endosomes and [FeIII(Ent)]3− is known to undergo a change from catecholate to salicylate coordination in acidic conditions, which is predicted to be sterically incompatible with the Scn binding pocket (also referred to as the calyx). To investigate the interactions between the ferric Ent complex and Scn at different pH values, two recombinant forms of Scn with mutations in three residues lining the calyx were prepared: Scn-W79A/R81A and Scn-Y106F. Binding studies and crystal structures of the Scn-W79A/R81A:[FeIII(Ent)]3− and Scn-Y106F:[FeIII(Ent)]3− complexes confirm that such mutations do not affect the overall conformation of the protein but do weaken significantly its affinity for [FeIII(Ent)]3−. Fluorescence, UV-Vis and EXAFS spectroscopies were used to determine Scn/siderophore dissociation constants and to characterize the coordination mode of iron over a wide pH range, in the presence of both mutant proteins and synthetic salicylate analogs of Ent. While Scn binding hinders salicylate coordination transformation, strong acidification results in the release of iron and degraded siderophore. Iron release may therefore result from a combination of Ent degradation and coordination change.

Radioactive core samples containing elevated concentrations of Cr from a high level nuclear waste plume in the Hanford vadose zone were studied to asses the future mobility of Cr. Cr(VI) is an important subsurface contaminant at the Hanford Site. The plume originated in 1969 by leakage of self-boiling supernate from a tank containing REDOX process waste. The supernate contained high concentrations of alkali (NaOH Ϸ 5.25 mol/L), salt (NaNO 3 /NaNO 2 Ͼ10 mol/L), aluminate [Al(OH) 4 Ϫ ϭ 3.36 mol/L], Cr(VI) (0.413 mol/L), and 137 Cs ϩ (6.51 ϫ 10 Ϫ5 mol/L). Water and acid extraction of the oxidized subsurface sediments indicated that a significant portion of the total Cr was associated with the solid phase. Mineralogic analyses, Cr valence speciation measurements by X-ray adsorption near edge structure (XANES) spectroscopy, and small column leaching studies were performed to identify the chemical retardation mechanism and leachability of Cr. While X-ray diffraction detected little mineralogic change to the sediments from waste reaction, scanning electron microscopy (SEM) showed that mineral particles within 5 m of the point of tank failure were coated with secondary, sodium aluminosilicate precipitates. The density of these precipitates decreased with distance from the source (e.g., beyond 10 m). The XANES and column studies demonstrated the reduction of 29-75% of the total Cr to insoluble Cr(III), and the apparent precipitation of up to 43% of the Cr(VI) as an unidentified, non-leachable phase. Both Cr(VI) reduction and Cr(VI) precipitation were greater in sediments closer to the leak source where significant mineral alteration was noted by SEM. These and other observations imply that basic mineral hydrolysis driven by large concentrations of OH Ϫ in the waste stream liberated Fe(II) from the otherwise oxidizing sediments that served as a reductant for CrO 4 2Ϫ. The coarse-textured Hanford sediments contain silt-sized mineral phases (biotite, clinochlore, magnetite, and ilmenite) that are sources of Fe(II). Other dissolution products (e.g., Ba 2ϩ) or Al(OH) 4 Ϫ present in the waste stream may have induced Cr(VI) precipitation as pH moderated through mineral reaction. The results demonstrate that a minimum of 42% of the total Cr inventory in all of the samples was immobilized as Cr(III) and Cr(VI) precipitates that are unlikely to dissolve and migrate to groundwater under the low recharge conditions of the Hanford vadose zone.

The siderophore enterobactin (Ent) is produced by many species of enteric bacteria to mediate iron uptake. This iron scavenger can be reincorporated by the bacteria as the ferric complex [Fe III (Ent)] 3-and is subsequently hydrolyzed by an esterase to facilitate intracellular iron release. Recent literature reports on altered protein recognition and binding of modified enterobactin increase the significance of understanding the structural features and solution chemistry of ferric enterobactin. The structure of the neutral protonated ferric enterobactin complex [Fe III (H 3 (14) Å 2 ). 1 H NMR spectroscopy was used to monitor the amide bond rotation between the catecholate and salicylate geometries using the gallic complexes of enterobactin; [Ga III (Ent)] 3-and [Ga III (H 3 Ent)] 0 . The ferric salicylate complexes display quasi-reversible reduction potentials from −89 mV to −551 mV (relative to the normal hydrogen electrode NHE) which supports the feasibility of a low pH iron release mechanism facilitated by biological reductants. Keywords

We evaluate the hypothesis that the reactivity trend for iodination of natural humic substances (HS) resembles that for the iodination of some substituted phenols. The hypothesis was tested by comparing the rates of reaction of I 2 (aq) with HS and a series of eight substituted phenolic compounds. Rates of iodination for all of the phenolic compounds, except salicylate, are described with an empirical rate law R ) k obs [phenolic compound] with the values of k obs related to the structure of the substituted phenol. The values of k obs , corresponding to iodination of the simple substituted phenols, range from 5.6 × 10 -8 to 4.7 × 10 -5 M s -1 at 25°C. These rate coefficients can be predicted over at least three orders-of-magnitude from a modified Hammett relation. The rates of iodination of HS fall within the range measured for substituted phenols, suggesting that iodination of the natural HS proceed by similar pathways. The humic substances differ markedly in their reactivity toward I 2 (aq) in several important ways. First, unlike the substituted phenols, the HS typically reacted in two stages. An initial stage involves rapid uptake of I 2 (aq) and is followed by a much slower reaction. Surprisingly, each stage of the reaction follows a similar rate law with respect to the reactants. Second, rates of iodination of the HS are characterized by noninteger rate orders with respect to the concentration of protons and the concentration of dissolved iodide [I -(aq)].

Aims: To describe the diversity and metabolic potential of microbial communities in uranium mine tailings characterized by high pH, high metal concentration and low permeability. Methods and Results: To assess microbial diversity and their potential to influence the geochemistry of uranium mine tailings using aerobic and anaerobic culture-based methods, in conjunction with next generation sequencing and clone library sequencing targeting two universal bacterial markers (the 16S rRNA and cpn60 genes). Growth assays revealed that 69% of the 59 distinct culturable isolates evaluated were multiple-metal resistant, with 15% exhibiting dual-metal hypertolerance. There was a moderately positive correlation coefficient (R = 0Á43, P < 0Á05) between multiple-metal resistance of the isolates and their enzyme expression profile. Of the isolates tested, 17 reduced amorphous iron, 22 reduced molybdate and seven oxidized arsenite. Based on next generation sequencing, tailings depth was shown to influence bacterial community composition, with the difference in the microbial diversity of the upper (0-20 m) and middle (20-40 m) tailings zones being highly significant (P < 0Á01) from the lower zone (40-60 m) and the difference in diversity of the upper and middle tailings zone being significant (P < 0Á05). Phylotypes closely related to well-known sulfate-reducing and iron-reducing bacteria were identified with low abundance, yet relatively high diversity. Conclusions: The presence of a population of metabolically-diverse, metalresistant micro-organisms within the tailings environment, along with their demonstrated capacity for transforming metal elements, suggests that these organisms have the potential to influence the long-term geochemistry of the tailings. Significance and Impact of the study: This study is the first investigation of the diversity and functional potential of micro-organisms present in low permeability, high pH uranium mine tailings.

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