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1. Experimental and theoretical investigations of infrared multiple photon dissociation spectra of arginine complexes with Zn ²⁺ and Cd ²⁺

   https://doi.org/10.1039/c8cp03484b  

   Chalifoux, Aaron M. ; Boles, Georgia C. ; Berden, Giel ; Oomens, Jos ; Armentrout, P. B. ( January 2018 , Physical Chemistry Chemical Physics)

   Arginine (Arg) complexes with Zn 2+ and Cd 2+ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light from a free electron laser. Electrospray ionization generated complexes of deprotonated Arg with Zn 2+ , [Zn(Arg–H)] + , and Arg with CdCl + , CdCl + (Arg). Possible low-energy conformers of these species were found using quantum chemical calculations, and their calculated IR spectra were compared to experimentally measured IRMPD spectra. Calculations were performed at the B3LYP/6-311+G(d,p) level for Zn 2+ complexes and B3LYP/def2-TZVP with an SDD effective core potential on cadmium for CdCl + complexes. [Zn(Arg–H)] + was found to adopt a charge-solvated, tridentate [N,CO − ,N ω ′] structure where Zn 2+ binds to the backbone amine, carbonyl oxygen, and side-chain terminal guanidine nitrogen (N ω ′). The CdCl + (Arg) species was suggested to be a mixture of a dominant (∼85%) charge-solvated, tridentate [N,CO,N ω ′] structure where the CdCl + binds to the backbone amine, carbonyl, and side-chain imine (N ω ′) and a minor (∼15%) bidentate [N,CO − ](N ω ′H 2 + ) zwitterionic structure where the metal center binds to the backbone amine and carbonyl oxygen with intramolecular proton migration frommore »the hydroxyl to the N ω ′ guanidine nitrogen (as designated in parenthesis).« less
2. Infrared multiple photon dissociation action spectroscopy of protonated glycine, histidine, lysine, and arginine complexed with 18-crown-6 ether

   https://doi.org/10.1039/c9cp02265a  

   McNary, Christopher P. ; Nei, Y.-W. ; Maitre, Philippe ; Rodgers, M. T. ; Armentrout, P. B. ( June 2019 , Physical Chemistry Chemical Physics)

   Complexes of 18-crown-6 ether (18C6) with four protonated amino acids (AAs) are examined using infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by the infrared free electron laser at the Centre Laser Infrarouge d’Orsay (CLIO). The AAs examined in this work include glycine (Gly) and the three basic AAs: histidine (His), lysine (Lys), and arginine (Arg). To identify the (AA)H + (18C6) conformations present in the experimental studies, the measured IRMPD spectra are compared to spectra calculated at the B3LYP/6-311+G(d,p) level of theory. Relative energies of various conformers and isomers are provided by single point energy calculations carried out at the B3LYP, B3P86, M06, and MP2(full) levels using the 6-311+G(2p,2d) basis set. The comparisons between the IRMPD and theoretical IR spectra indicate that 18C6 binds to Gly and His via the protonated backbone amino group, whereas protonated Lys prefers binding via the protonated side-chain amino group. Results for Arg are less definitive with strong evidence for binding to the protonated guanidino side chain (the calculated ground conformer at most levels of theory), but contributions from backbone binding to a zwitterionic structure are likely.
3. Atomic and Electronic Structure Determinants Distinguish between Ethylene Formation and L‑Arginine Hydroxylation Reaction Mechanisms in the Ethylene-Forming Enzyme

   https://doi.org/https://dx.doi.org/10.1021/acscatal.0c03349  

   Chaturvedi, Shobhit S. ; Ramanan, Rajeev ; Hu, Jian ; Hausinger, Robert P. ; Christov, Christo Z. ( January 2021 , ACS catalysis)

   The ethylene-forming enzyme (EFE) is a non-heme Fe(II), 2-oxoglutarate (2OG), and l-arginine (l-Arg)-dependent oxygenase that catalyzes dual reactions: the generation of ethylene from 2OG and the C5 hydroxylation of l-Arg. Using an integrated molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) approach that references previous experimental studies, we tested the hypothesis that synergy between the conformation of l-Arg and the coordination mode of 2OG directs the reaction toward ethylene formation or l-Arg hydroxylation. The dynamics of EFE·Fe(III)·OO•–·2OG·l-Arg show that l-Arg can exist in conformation A (productive for hydroxylation) and conformation B (unproductive for hydroxylation). QM/MM calculations show that when 2OG is bound in an off-line mode and l-Arg is present in conformation A, the Fe(III)-OO•– intermediate undergoes the standard O2 activation mechanism involving ferryl-dependent hydroxylation. With the same off-line 2OG coordination, but with conformation B of l-Arg, a unique pathway produces a half-bond ferric-bicarbonate intermediate that decomposes to ethylene, two CO2, and a ferrous-bicarbonate species. The results demonstrate that when 2OG is coordinated in off-line mode to the Fe center, the l-Arg conformation acts as a switch that directs the reaction toward ethylene formation or hydroxylation. Analysis of the electronic structure shows that the l-Arg conformation defines the precise locationmore »of an unpaired β electron in the Fe(III)-OO– complex, either in a π*∥ orbital that triggers ethylene formation or a π*⊥ orbital that cascades to l-Arg hydroxylation. A change in 2OG coordination from off-line to in-line reduces stabilization of the 2OG C1 carboxylate such that neither conformation of l-Arg produces the ethylene-forming half-bond ferric-bicarbonate intermediate. Thus, l-Arg conformation-dependent changes in the electronic structure of the Fe(III)-OO•– orbitals, together with the 2OG binding mode-associated stabilization of the C1-carboxylate, distinguish whether the EFE reaction proceeds via the ethylene-forming pathway or catalyzes a hydroxylation mechanism.« less
4. Synthesis and characterization of a tert- butyl ester-substituted titanocene dichloride: ^t-BuOOC Cp ₂ TiCl ₂

   https://doi.org/10.1107/S2056989020011834  

   McCarthy, Jackson S. ; McMillen, Colin D. ; Pienkos, Jared A. ; Wagenknecht, Paul S. ( October 2020 , Acta Crystallographica Section E Crystallographic Communications)

   Bis[η 5 -( tert -butoxycarbonyl)cyclopentadienyl]dichloridotitanium(IV), [Ti(C 10 H 13 O 2 ) 2 Cl 2 ], was synthesized from LiCp COO t -Bu using TiCl 4 , and was characterized by single-crystal X-ray diffraction and 1 H NMR spectroscopy. The distorted tetrahedral geometry about the central titanium atom is relatively unchanged compared to Cp 2 TiCl 2 . The complex exhibits elongation of the titanium–cyclopentadienyl centroid distances [2.074 (3) and 2.070 (3) Å] and a contraction of the titanium–chlorine bond lengths [2.3222 (10) Å and 2.3423 (10) Å] relative to Cp 2 TiCl 2 . The dihedral angle formed by the planes of the Cp rings [52.56 (13)°] is smaller than seen in Cp 2 TiCl 2 . Both ester groups extend from the same side of the Cp rings, and occur on the same side of the complex as the chlorido ligands. The complex may serve as a convenient synthon for titanocene complexes with carboxylate anchoring groups for binding to metal oxide substrates.
5. Spectroscopic and Theoretical Studies of Hg(II) Complexation with Some Dicysteinyl Tetrapeptides

   https://doi.org/10.1155/2021/9911474  

   Springfield, Elliot ; Willis, Alana ; Merle, John ; Mazlo, Johanna ; Ngu-Schwemlein, Maria ( July 2021 , Bioinorganic Chemistry and Applications)

   Osella, Domenico (Ed.)

   Tetrapeptides containing a Cys-Gly-Cys motif and a propensity to adopt a reverse-turn structure were synthesized to evaluate how O-, N-, H-, and aromatic π donor groups might contribute to mercury(II) complex formation. Tetrapeptides Xaa-Cys-Gly-Cys, where Xaa is glycine, glutamate, histidine, or tryptophan, were prepared and reacted with mercury(II) chloride. Their complexation with mercury(II) was studied by spectroscopic methods and computational modeling. UV-vis studies confirmed that mercury(II) binds to the cysteinyl thiolates as indicated by characteristic ligand-to-metal-charge-transfer transitions for bisthiolated S-Hg-S complexes, which correspond to 1 : 1 mercury-peptide complex formation. ESI-MS data also showed dominant 1 : 1 mercury-peptide adducts that are consistent with double deprotonations from the cysteinyl thiols to form thiolates. These complexes exhibited a strong positive circular dichroism band at 210 nm and a negative band at 193 nm, indicating that these peptides adopted a β-turn structure after binding mercury(II). Theoretical studies confirmed that optimized 1 : 1 mercury-peptide complexes adopt β-turns stabilized by intramolecular hydrogen bonds. These optimized structures also illustrate how specific N-terminal side-chain donor groups can assume intramolecular interactions and contribute to complex stability. Fluorescence quenching results provided supporting data that the indole donor group could interact with the coordinated mercury. The results from this study indicate that N-terminal side-chain residues containingmore »carboxylate, imidazole, or indole groups can participate in stabilizing dithiolated mercury(II) complexes. These structural insights on peripheral mercury-peptide interactions provide additional understanding of the chemistry of mercury(II) with side-chain donor groups in peptides.« less