Biotransformation of 6:2 Fluorotelomer Thioether Amido Sulfonate in Aqueous Film-Forming Foams under Nitrate-Reducing Conditions.

Despite the prevalence of nitrate reduction in groundwater, the biotransformation of per- and polyfluoroalkyl substances (PFAS) under nitrate-reducing conditions remains mostly unknown compared with aerobic or strong reducing conditions. We constructed microcosms under nitrate-reducing conditions to simulate the biotransformation occurring at groundwater sites impacted by aqueous film-forming foams (AFFFs). We investigated the biotransformation of 6:2 fluorotelomer thioether amido sulfonate (6:2 FtTAoS), a principal PFAS constituent of several AFFF formulations using both quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS) and qualitative high-resolution mass spectrometry analyses. Our results reveal that the biotransformation rates of 6:2 FtTAoS under nitrate-reducing conditions were about 10 times slower than under aerobic conditions, but about 2.7 times faster than under sulfate-reducing conditions. Although minimal production of 6:2 fluorotelomer sulfonate and the terminal perfluoroalkyl carboxylate, perfluorohexanoate was observed, fluorotelomer thioether and sulfinyl compounds were identified in the aqueous samples. Evidence for the formation of volatile PFAS was obtained by mass balance analysis using the total oxidizable precursor assay and detection of 6:2 fluorotelomer thiol by gas chromatography-mass spectrometry. Our results underscore the complexity of PFAS biotransformation and the interactions between redox conditions and microbial biotransformation activities, contributing to the better elucidation of PFAS environmental fate and impact.

Biotransformation of AFFF Component 6:2 Fluorotelomer Thioether Amido Sulfonate Generates 6:2 Fluorotelomer Thioether Carboxylate under Sulfate-Reducing Conditions.

The fate of per and polyfluoroalkyl substances (PFASs) in aqueous filmforming foams (AFFFs) under anaerobic conditions has not been well characterized, leaving major gaps in our understanding of PFAS fate and transformation at contaminated sites. In this study, the biotransformation of 6:2 fluorotelomer thioether amido sulfonate (6:2 FtTAoS), a component of several AFFF formulations, was investigated under sulfate-reducing conditions in microcosms inoculated with either pristine or AFFF-impacted solids. To identify the transformation products, we used high-resolution mass spectrometry and employed suspect-screening and nontargeted compound identification methods. These analyses demonstrated that 6:2 FtTAoS was transformed primarily to a stable polyfluoroalkyl compound, 6:2 fluorotelomer thioether propionate (6:2 FtTP). It did not undergo further reactions to produce the perfluoroalkyl carboxylates and fluorotelomer sulfonates and carboxylates that were observed during aerobic transformations. Here, the 6:2 FtTP was recalcitrant to biotransformation, indicating the stability of the thioether group under sulfate reducing conditions. The total oxidizable precursor (TOP) assay was used to assess the presence of other PFASs. Although nearly all of the PFAS mass initially present was recovered from the pristine microcosms, only 67% of the initial PFAS mass was recovered from the contaminated microcosms, suggesting the formation of volatile biotransformation products or those that could not be detected by the TOP assay.

Redox-based reagents for chemoselective methionine bioconjugation.

Cysteine can be specifically functionalized by a myriad of acid-base conjugation strategies for applications ranging from probing protein function to antibody-drug conjugates and proteomics. In contrast, selective ligation to the other sulfur-containing amino acid, methionine, has been precluded by its intrinsically weaker nucleophilicity. Here, we report a strategy for chemoselective methionine bioconjugation through redox reactivity, using oxaziridine-based reagents to achieve highly selective, rapid, and robust methionine labeling under a range of biocompatible reaction conditions. We highlight the broad utility of this conjugation method to enable precise addition of payloads to proteins, synthesis of antibody-drug conjugates, and identification of hyperreactive methionine residues in whole proteomes.

Syntheses and structural analyses of variable-stoichiometric Au-Pt-Ni carbonyl/phosphine clusters, Pt3(Pt(1-x)Ni(x))(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 and Pt2(Pt(2-y)Ni(y))(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2, with ligation-induced site-specific Pt/Ni substitutional disorder within butterfly-based Pt3(Pt(1-x)Ni(x))Au2 and Pt2(Pt(2-y)Ni(y))Au2 core-geometries.

In ongoing attempts of directed synthesis of high-nuclearity Au-Pt carbonyl/phosphine clusters with [Ni6(CO)12]2- used as reducing agent and CO source, we have isolated and characterized two new closely related variable-stoichiometric trimetallic clusters, Pt3(Pt(1-x)Ni(x))(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 (1) and Pt2(Pt(2-y)Ni(y))(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2). Their M4Au2 cores may be envisioned as substitutional disordered butterfly-based M4Au2 frameworks (M = Pt/Ni) formed by connections of the two basal M(B) atoms with both (Au-Au)-linked Au(PPh3) moieties. Based upon low-temperature CCD X-ray diffraction studies of eight crystals obtained from different samples, ligation-induced site-specific Pt/Ni substitutional disorder (involving formal insertion of Ni in place of Pt) in a given crystal was found to occur only at the one OC-attached basal M(B) site in 1 or at both OC-attached basal M(B) sites in 2 corresponding to a crystal composite of the Pt3(Pt(1-x)Ni(x))Au2 core in 1 or of the Pt2(Pt(2-y)Ni(y))Au2 core in 2; the Ph3P-attached M(B) site (M(B) = Pt) in 1 and two wingtip M(w) sites (M(w) = Pt) in 1 and 2 were not substitutionally disordered. The resulting variable stoichiometry of the M4Au2 core in 1 may be viewed as a crystal composite of two superimposed individual stereoisomers, Pt4(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 (1a) and Pt3Ni(AuPPh3)2(mu2-CO)4(CO)(PPh3)3 (1b), in the averaged unit cell of a given crystal. Likewise, 2 represents the crystal-averaged composite of three individual stereoisomers, Pt4(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2a), Pt3Ni(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2b), and Pt2Ni2(AuPPh3)2(mu2-CO)4(CO)2(PPh3)2 (2c). Formal Ni substitution for Pt at only the basal M(B) site(s) in the four crystal composites each of 1 and 2 was found to vary widely from 17% to 79% Ni in 1 and from 21% to 95% Ni in 2. Nevertheless, reasonably close Pt/Ni occupancy factors were found within each of the four pairs of composite crystals selected from samples obtained from duplicate syntheses. Both 1 and 2 may be formally derived from the electronically equivalent classic butterfly Pt4(mu2-CO)5(PPh3)4 cluster by replacement of its bridging mu2-CO ligand spanning the basal M(B)-M(B) edge with two one-electron donating (Au-Au)-linked AuPPh3 moieties along with the substitution of a terminal CO in place of one or both M(B)-attached PPh3 ligands in 1 and 2, respectively; site-specific Pt/Ni substitutional disorder occurs only at the CO-attached M(B) sites. The variable-stoichiometric 1 and 2 re also electronically equivalent and geometrically related to the crystal-ordered butterfly-based Pt4(mu2-CO)4(PR3)4(mu3-HgX)2 clusters (R3 = Ph3, MePh2; X = CF3, Br, I).