PFAS in municipal landfill leachate: Occurrence, transformation, and sources.

To understand sources and processes affecting per- and polyfluoroalkyl substances (PFAS), 32 PFAS were measured in landfill leachate from 17 landfills across Washington State in both pre-and post-total oxidizable precursor (TOP) assay samples, using an analytical method that was the precursor to EPA Draft Method 1633. As in other studies, 5:3FTCA was the dominant PFAS in the leachate, suggesting that carpets, textiles, and food packaging were the main sources of PFAS. Total PFAS concentrations (Σ32PFAS) ranged from 61 to 172,976 ng/L and 580-36,122 ng/L in pre-TOP and post-TOP samples, respectively, suggesting that little or no uncharacterized precursors remained in landfill leachate. Furthermore, due to chain-shortening reactions, the TOP assay often resulted in a loss of overall PFAS mass. Positive matrix factorization (PMF) analysis of the combined pre- and post-TOP samples produced five factors that represent sources and processes. Factor 1 consisted primarily of 5:3FTCA (intermediate of 6:2 fluorotelomer degradation and characteristic of landfill leachate), while factor 2 was dominated by PFBS (degradant of C-4 sulfonamide chemistry) and, to a lesser extent, by several PFCAs and 5:3FTCA. Factor 3 consisted primarily of both short-chain PFCAs (end-products of 6:2 fluorotelomer degradation) and PFHxS (derived from C-6 sulfonamide chemistry), while the main component of factor 4 was PFOS (dominant in many environmental media but minor in landfill leachate, perhaps reflecting a production shift from longer to shorter chain PFAS). Factor 5, highly loaded with PFCAs, was dominant in post-TOP samples and therefore represented the oxidation of precursors. Overall, PMF analysis suggests that the TOP assay approximates some redox processes which occur in landfills, including chain-shortening reactions which yield biodegradable products.

Influence of Rare-Earth Ion Radius on Metal-Metal Charge Transfer in Trinuclear Mixed-Valent Complexes.

We report the synthesis and characterization of a highly conjugated bisferrocenyl pyrrolediimine ligand, Fc2PyrDIH (1), and its trinuclear complexes with rare earth ions─(Fc2PyrDI)M(N(TMS)2)2 (2-M, M = Sc, Y, Lu, La). Crystal structures, nuclear magnetic resonance (NMR) spectra, and ultraviolet/visible/near-infrared (UV/vis-NIR) data are presented. The latter are in good agreement with DFT calculations, illuminating the impact of the rare earth ionic radius on NIR charge transfer excitations. For [2-Sc]+, the charge transfer is at 11,500 cm-1, while for [2-Y]+, only a d-d transition at 8000 cm-1 is observed. Lu has an ionic radius in between Sc and Y, and the [2-Lu]+ complex exhibits both transitions. From time-dependent density functional theory (TDDFT) analysis, we assign the 11,500 cm-1 transition as a mixture of metal-to-ligand charge transfer (MLCT) and metal-to-metal charge transfer (MMCT), rather than pure metal-to-metal CT because it has significant ligand character. Typically, the ferrocenes moieties have high rotational freedom in bis-ferrocenyl mixed valent complexes. However, in the present (Fc2PyrDI)M(N(TMS)2)2 complexes, ligand-ligand repulsions lock the rotational freedom so that rare-earth ionic radius-dependent geometric differences increasingly influence orbital overlap as the ionic radius falls. The Marcus-Hush coupling constant HAB trends as [2-Sc]+ > [2-Lu]+ > [2-Y]+.

Bis-Ferrocenyl-Pyridinediimine Trinuclear Mixed-Valent Complexes with Metal-Binding Dependent Electronic Coupling: Synthesis, Structures, and Redox-Spectroscopic Characterization.

A family of metal dichloride complexes having a bis-ferrocenyl-substituted pyridinediimine ligand was systematically synthesized ((Fc2PDI)MCl2, M = Mg, Zn, Fe, and Co) and characterized crystallographically, spectroscopically, electrochemically, and computationally. Electronic coupling between the ligand ferrocene units is switched on upon binding to a MCl2 fragment, as evidenced by both sequential oxidation of the ferrocenes in cyclic voltammetry (ΔEox ≈ 200 mV) and by Inter-Valence Charge Transfer electronic excitations in the near IR. Additionally, UV-vis spectra are used to directly observe orbital mixing between the ferrocenyl units and the imine π system since breaking of the orbital symmetry results in allowed transitions (ϵ = 2800 M-1cm-1 vs ϵ ≈ 200 M-1cm-1 in free ferrocene) as well as broadening and red-shifting of the ferrocenyl transitions-indicating organic character in formerly pure metal-centered transitions. DFT analysis reveals that interaction between the ferrocenes and the MCl2 fragment is small and suggests that communication is mediated by better energy matching between the ferrocene and organic π* orbitals upon coordination, allowing superexchange coupling through the LUMO. Furthermore, single crystal diffraction data obtained from oxidation of one and both ferrocenes show distortions, introducing the empty dxy/dx2-y2 orbitals into the secondary coordination sphere of the MCl2 fragment. Such structural rearrangements are infrequent in ferrocenyl mixed-valent compounds, and implications for catalysis as well as electronic communication are discussed.

Tyrosine-Coordinated P-Cluster in G. diazotrophicus Nitrogenase: Evidence for the Importance of O-Based Ligands in Conformationally Gated Electron Transfer.

The P-cluster is a unique iron-sulfur center that likely functions as a dynamic electron (e(-)) relay site between the Fe-protein and the catalytic FeMo-cofactor in nitrogenase. The P-cluster has been shown to undergo large conformational changes upon 2-e(-) oxidation which entail the coordination of two of the Fe centers to a Ser side chain and a backbone amide N, respectively. Yet, how and if this 2-e(-) oxidized state (P(OX)) is involved in catalysis by nitrogenase is not well established. Here, we present the crystal structures of reduced and oxidized MoFe-protein (MoFeP) from Gluconacetobacter diazotrophicus (Gd), which natively possesses an Ala residue in the position of the Ser ligand to the P-cluster. While reduced Gd-MoFeP is structurally identical to previously characterized counterparts around the FeMo-cofactor, oxidized Gd-MoFeP features an unusual Tyr coordination to its P-cluster along with ligation by a backbone amide nitrogen. EPR analysis of the oxidized Gd-MoFeP P-cluster confirmed that it is a 2-e(-) oxidized, integer-spin species. Importantly, we have found that the sequence positions corresponding to the Ser and Tyr ligands are almost completely covariant among Group I nitrogenases. These findings strongly support the possibility that the P(OX) state is functionally relevant in nitrogenase catalysis and that a hard, O-based anionic ligand serves to stabilize this state in a switchable fashion.

Evidence for Functionally Relevant Encounter Complexes in Nitrogenase Catalysis.

Nitrogenase is the only enzyme that can convert atmospheric dinitrogen (N2) into biologically usable ammonia (NH3). To achieve this multielectron redox process, the nitrogenase component proteins, MoFe-protein (MoFeP) and Fe-protein (FeP), repeatedly associate and dissociate in an ATP-dependent manner, where one electron is transferred from FeP to MoFeP per association. Here, we provide experimental evidence that encounter complexes between FeP and MoFeP play a functional role in nitrogenase catalysis. The encounter complexes are stabilized by electrostatic interactions involving a positively charged patch on the ß-subunit of MoFeP. Three single mutations (ßAsn399Glu, ßLys400Glu, and ßArg401Glu) in this patch were generated in Azotobacter vinelandii MoFeP. All of the resulting variants displayed decreases in specific catalytic activity, with the ßK400E mutation showing the largest effect. As simulated by the Thorneley-Lowe kinetic scheme, this single mutation lowered the rate constant for FeP-MoFeP association 5-fold. We also found that the ßK400E mutation did not affect the coupling of ATP hydrolysis with electron transfer (ET) between FeP and MoFeP. These data suggest a mechanism where FeP initially forms encounter complexes on the MoFeP ß-subunit surface en route to the ATP-activated, ET-competent complex over the αß-interface.