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1. Thermodynamic origin of the affinity, selectivity, and domain specificity of metallothionein for essential and toxic metal ions

   https://doi.org/10.1093/mtomcs/mfae041  

   Quinn, Colette_F; Wilcox, Dean_E (September 2024, Metallomics)

   Abstract The small Cys-rich protein metallothionein (MT) binds several metal ions in clusters within two domains. While the affinity of MT for both toxic and essential metals has been well studied, the thermodynamics of this binding has not. We have used isothermal titration calorimetry measurements to quantify the change in enthalpy (ΔH) and change in entropy (ΔS) when metal ions bind to the two ubiquitous isoforms of MT. The seven Zn2+ that bind sequentially at pH 7.4 do so in two populations with different coordination thermodynamics, an initial four that bind randomly with individual tetra-thiolate coordination and a subsequent three that bind with bridging thiolate coordination to assemble the metal clusters. The high affinity of MT for both populations is due to a very favourable binding entropy that far outweighs an unfavourable binding enthalpy. This originates from a net enthalpic penalty for Zn2+ displacement of protons from the Cys thiols and a favourable entropic contribution from the displaced protons. The thermodynamics of other metal ions binding to MT were determined by their displacement of Zn2+ from Zn7MT and subtraction of the Zn2+-binding thermodynamics. Toxic Cd2+, Pb2+, and Ag+, and essential Cu+, also bind to MT with a very favourable binding entropy but a net binding enthalpy that becomes increasingly favourable as the metal ion becomes a softer Lewis acid. These thermodynamics are the origin of the high affinity, selectivity, and domain specificity of MT for these metal ions and the molecular basis for their in vivo binding competition. 

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2. Low concentrations of Cu ²⁺ in synthetic nutrient containing wastewater inhibit MgCO ₃ -to-struvite transformation

   https://doi.org/10.1039/D0EW01035A  

   Barčauskaitė, Karolina; Drapanauskaitė, Donata; Silva, Manoj; Murzin, Vadim; Doyeni, Modupe; Urbonavicius, Marius; Williams, Clinton F.; Supronienė, Skaidrė; Baltrusaitis, Jonas (March 2021, Environmental Science: Water Research & Technology)

   null (Ed.)

   Simultaneous major nutrient nitrogen (N) and phosphorus (P) recovery from wastewater is key to achieving food–energy–water sustainable development. In this work, we elucidate the reaction kinetics, crystalline structure and chemical composition of the resulting solid precipitate obtained from simulated N and P containing wastewater solution using widely abundant low solubility magnesite (MgCO 3 ) particles in the presence of common transition metal ions, such as zinc (Zn 2+ ) or copper (Cu 2+ ). We show that up to 100 ppm Zn 2+ from the simulated wastewater can be incorporated into the struvite lattice as isolated distorted Zn 2+ while even at very low concentrations of ∼5 ppm Cu 2+ ions almost completely inhibit struvite crystal formation. The resulting solid precipitate distinctly affects soil microbial biomass carbon and soil dehydrogenase enzyme activity. These results show a cautionary case where abundant natural mineral MgCO 3 exhibits very different chemistry in Cu 2+ containing simulated wastewater and does not readily adsorb or retain NH 4 + and PO 4 3− ions, unlike less sustainable but more water-soluble magnesium sources, such as MgCl 2 , at the equivalent [Mg 2+ ] : [NH 4 + ] : [PO 4 3− ] molar ratio of 1.4 : 1 : 1. 

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3. Thermodynamic and kinetic studies of H ₂ and N ₂ binding to bimetallic nickel-group 13 complexes and neutron structure of a Ni(η ² -H ₂ ) adduct

   https://doi.org/10.1039/C9SC02018G  

   Cammarota, Ryan C.; Xie, Jing; Burgess, Samantha A.; Vollmer, Matthew V.; Vogiatzis, Konstantinos D.; Ye, Jingyun; Linehan, John C.; Appel, Aaron M.; Hoffmann, Christina; Wang, Xiaoping; et al (July 2019, Chemical Science)

   Understanding H 2 binding and activation is important in the context of designing transition metal catalysts for many processes, including hydrogenation and the interconversion of H 2 with protons and electrons. This work reports the first thermodynamic and kinetic H 2 binding studies for an isostructural series of first-row metal complexes: NiML, where M = Al ( 1 ), Ga ( 2 ), and In ( 3 ), and L = [N( o -(NCH 2 P i Pr 2 )C 6 H 4 ) 3 ] 3− . Thermodynamic free energies (Δ G °) and free energies of activation (Δ G ‡ ) for binding equilibria were obtained via variable-temperature 31 P NMR studies and lineshape analysis. The supporting metal exerts a large influence on the thermodynamic favorability of both H 2 and N 2 binding to Ni, with Δ G ° values for H 2 binding found to span nearly the entire range of previous reports. The non-classical H 2 adduct, (η 2 -H 2 )NiInL ( 3 -H 2 ), was structurally characterized by single-crystal neutron diffraction—the first such study for a Ni(η 2 -H 2 ) complex or any d 10 M(η 2 -H 2 ) complex. UV-Vis studies and TD-DFT calculations identified specific electronic structure perturbations of the supporting metal which poise NiML complexes for small-molecule binding. ETS-NOCV calculations indicate that H 2 binding primarily occurs via H–H σ-donation to the Ni 4p z -based LUMO, which is proposed to become energetically accessible as the Ni(0)→M( iii ) dative interaction increases for the larger M( iii ) ions. Linear free-energy relationships are discussed, with the activation barrier for H 2 binding (Δ G ‡ ) found to decrease proportionally for more thermodynamically favorable equilibria. The Δ G ° values for H 2 and N 2 binding to NiML complexes were also found to be more exergonic for the larger M( iii ) ions. 

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4. Probing the Ni ²⁺ ‐selective Response of Fluorescent Probe NiSensor‐1 with the NiCast Photocaged Complex ^{† ‡}

   https://doi.org/10.1111/php.13567  

   Hickey, Erin E.; Kennedy, Daniel P.; Gwizdala, Celina; Basa, Prem N.; Müller, Peter; MacDonald, John; Burdette, Shawn C. (December 2021, Photochemistry and Photobiology)

   Abstract CTEA (N,N‐bis[2‐(carboxylmethyl)thioethyl]amine) is a mixed donor ligand that has been incorporated into multiple fluorescent sensors such as NiSensor‐1 that was reported to be selective for Ni²⁺. Other metal ions such as Zn²⁺do not produce an emission response in aqueous solution. To investigate the coordination chemistry and selectivity of this receptor, we prepared NiCast, a photocage containing the CTEA receptor. Cast photocages undergo a photoreaction that decreases electron density on a metal‐bound aniline nitrogen atom, which shifts the binding equilibrium toward unbound metal ion. The unique selectivity of CTEA was examined by measuring the binding affinity of NiCast and the CTEA receptor for Ni²⁺, Zn²⁺, Cd²⁺and Cu²⁺under different conditions. In aqueous solution, Ni²⁺binds more strongly to the aniline nitrogen atom than Cd²⁺; however, in CH₃CN, the change in affinity virtually disappears. The crystal structure of [Cu(CTEA)], which exhibits a Jahn–Teller–distorted square pyramidal structure, was also analyzed to gain more insight into the underlying coordination chemistry. These studies suggest that the fluorescence selectivity of NiSensor‐1 in aqueous solution is due to a stronger interaction between the aniline nitrogen atom and Ni²⁺compared to other divalent metal ions except Cu²⁺. 

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5. Cu(II) EPR Reveals Two Distinct Binding Sites and Oligomerization of Innate Immune Protein Calgranulin C

   https://doi.org/https://doi.org/10.1007/s00723-018-1053-7  

   Ghosh, S; Garcia, V; Singewald, K; Damo, S; Saxena, S. (January 2018, Applied magnetic resonance)

   S100A12 or Calgranulin C is a homodimeric antimicrobial protein of the S100 family of EF-hand calcium-modulated proteins. S100A12 is involved in many diseases like inflammation, tumor invasion, cancer and neurological disorders like Alzheimer’s disease. The binding of transition metal ions to the protein is important as the sequestering of the metal ion induces conformational changes in the protein, inhibiting the growth of various pathogenic microorganisms. In this work, we probe the Cu(II) binding properties of Calgranulin C. We demonstrate that the two Cu(II) binding sites in Calgranulin C show different coordination environments in solution. Electron spin resonance (ESR) spectra of Cu(II)-bound protein clearly show two distinct components at higher Cu(II):protein ratios, which is indicative of the two different binding environments for the Cu(II) ions. The g|| and A|| values are also different for the two components, indicating that the number of directly coordinated nitrogens in each site differs. Furthermore, we perform Continuous Wave (CW)-titrations to obtain the binding affinity of the Ca(II)-loaded protein to Cu2+ ions. We observe a positive cooperativity in binding of the two Cu(II) ions. In order to further probe the Cu2+ coordination, we also perform Electron Spin Echo Envelope Modulation (ESEEM) experiment. We perform ESEEM at two different fields where one Cu(II) binding site dominates over the other. At both sites we see distinct signatures of Cu(II)-histidine coordination. However, we clearly see that the ESEEM spectra corresponding to the two Cu2+ binding sites are significantly different. There is clear change in the intensity of the double quantum (DQ) peak with respect to the nuclear quadrupole interaction (NQI) peak at the two different fields. Furthermore, ESEEM along with Hyperfine Sublevel Correlation (HYSCORE) show that only one of the two Cu(II) binding sites has backbone coordination, confirming our previous observation. Finally, we perform Double Electron Electron Resonance (DEER) spectroscopy to probe if the difference in binding environment is due to the Cu(II) binding to different sites in the protein. We obtain a distance distribution with a sharp peak at ~ 3 nm and a broad peak at ~ 4 nm. The shorter distance agrees with the Cu(II)-Cu(II) distance expected for a dimer from the crystal structure. The longer distance is consistent with the Cu(II)-Cu(II) distance when oligomerization occurs. 

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