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1. Coordinated Anions Tune Z-Type Ligand Displacement from Colloidal CdSe and InP Nanocrystal Surfaces

   https://doi.org/10.1021/acs.inorgchem.4c03534  

   Degbevi, Mawuli; Stoffel, Jonathan T; Tsui, Emily Y (December 2024, Inorganic Chemistry)

   Neutral metal salts coordinate to the surfaces of colloidal semiconductor nanocrystals (NCs) by acting as Lewis acid acceptors for the NC surface anions. This ligand coordination has been associated with increased emission due to passivation of surface hole traps. Here, variation of the anionic ligands of metal salts is used to study anion effects on metal complex Lewis acidity and surface coordination at CdSe and InP NCs. To resolve dynamic ligand exchange processes, the tetracarbonylcobaltate anion, [Co(CO)4]–, is used as a monoanionic ligand for which IR spectroscopy can readily identify displacement of neutral M[Co(CO)4]x species (M = Cd or In; x = 2 or 3, respectively) upon addition of neutral donor ligands. Notably, although Cd[Co(CO)4]2 is more Lewis acidic than cadmium oleate, the former is more readily displaced from the NC surfaces. Lewis acidity and X-type anion exchange are therefore factors to be considered when performing post-synthetic addition of metal salts for NC photoluminescence emission enhancement. 

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2. Reactivity of Thiol-Rich Zn Sites in Diacylglycerol-Sensing PKC C1 Domain Probed by NMR Spectroscopy

   https://doi.org/10.3389/fmolb.2021.728711  

   Cole, Taylor R.; Igumenova, Tatyana I. (August 2021, Frontiers in Molecular Biosciences)

   null (Ed.)

   Conserved homology 1 (C1) domains are peripheral zinc finger domains that are responsible for recruiting their host signaling proteins, including Protein Kinase C (PKC) isoenzymes, to diacylglycerol-containing lipid membranes. In this work, we investigated the reactivity of the C1 structural zinc sites, using the cysteine-rich C1B regulatory region of the PKCα isoform as a paradigm. The choice of Cd 2+ as a probe was prompted by previous findings that xenobiotic metal ions modulate PKC activity. Using solution NMR and UV-vis spectroscopy, we found that Cd 2+ spontaneously replaced Zn 2+ in both structural sites of the C1B domain, with the formation of all-Cd and mixed Zn/Cd protein species. The Cd 2+ substitution for Zn 2+ preserved the C1B fold and function, as probed by its ability to interact with a potent tumor-promoting agent. Both Cys 3 His metal-ion sites of C1B have higher affinity to Cd 2+ than Zn 2+ , but are thermodynamically and kinetically inequivalent with respect to the metal ion replacement, despite the identical coordination spheres. We find that even in the presence of the oxygen-rich sites presented by the neighboring peripheral membrane-binding C2 domain, the thiol-rich sites can successfully compete for the available Cd 2+ . Our results indicate that Cd 2+ can target the entire membrane-binding regulatory region of PKCs, and that the competition between the thiol- and oxygen-rich sites will likely determine the activation pattern of PKCs. 

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3. 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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4. High-resolution structures with bound Mn2+ and Cd2+ map the metal import pathway in an Nramp transporter

   https://doi.org/10.7554/eLife.84006  

   Ray, Shamayeeta; Berry, Samuel P; Wilson, Eric A; Zhang, Casey H; Shekhar, Mrinal; Singharoy, Abhishek; Gaudet, Rachelle (April 2023, eLife)

   Transporters of the Nramp (Natural resistance-associated macrophage protein) family import divalent transition metal ions into cells of most organisms. By supporting metal homeostasis, Nramps prevent diseases and disorders related to metal insufficiency or overload. Previous studies revealed that Nramps take on a LeuT fold and identified the metal-binding site. We present high-resolution structures ofDeinococcus radiodurans(Dra)Nramp in three stable conformations of the transport cycle revealing that global conformational changes are supported by distinct coordination geometries of its physiological substrate, Mn²⁺, across conformations, and by conserved networks of polar residues lining the inner and outer gates. In addition, a high-resolution Cd²⁺-bound structure highlights differences in how Cd²⁺and Mn²⁺are coordinated by DraNramp. Complementary metal binding studies using isothermal titration calorimetry with a series of mutated DraNramp proteins indicate that the thermodynamic landscape for binding and transporting physiological metals like Mn²⁺is different and more robust to perturbation than for transporting the toxic Cd²⁺metal. Overall, the affinity measurements and high-resolution structural information on metal substrate binding provide a foundation for understanding the substrate selectivity of essential metal ion transporters like Nramps. 

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5. QM-cluster model study of CO ₂ hydration mechanisms in metal-substituted human carbonic anhydrase II

   https://doi.org/10.1088/2516-1075/acb02c  

   Summers, Thomas J; DeYonker, Nathan J (January 2023, Electronic Structure)

   Abstract Human carbonic anhydrase (CA) metalloenzymes utilize a Zn 2+ -containing active site to catalyze the interconversion of carbon dioxide to bicarbonate. The Zn 2+ ion may be replaced with other divalent transition metals, though the catalytic efficiency of the enzyme will be reduced. In this work, quantum mechanical cluster models of the active site are used to map the reaction profile for the hydration mechanism of carbon dioxide. The Lipscomb proton transfer and Lindskog rotation mechanisms were examined for the native Zn 2+ -enzyme along with variants where the metal was substituted with Cd 2+ , Ni 2+ , Fe 2+ , and Fe 3+ . The findings highlight the impact the metal coordination geometry has on the reaction profile. The results also suggest Fe 2+ , which is the functional metal for a prototypical CA of an anaerobic bacterium, might also be functional for human CA if cultured within an anaerobic environment. 

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