Synthesis and structure determination of racemic (Δ/Λ)-tris(ethylenediamine)cobalt(III) trichloride hemi(hexaaquasodium chloride)

The synthesis and crystal structure of the title racemic compound, [Co(C2H8N2)3]Cl3.{[Na(H2O)6]Cl}0.5, are reported. The trivalent cobalt atom, which resides on a crystallographic threefold axis, is chelated by a single ethylene diamine (en) ligand and yields the tris-chelate [Co(en)3]3+ cation with distorted octahedral geometry after the application of crystal symmetry. The sodium cation (site symmetry \overline{3}), has a single water molecule bound to it in the asymmetric unit and yields a distorted, octahedrally coordinated hydrated [Na(H2O)6]+ cation after the application of symmetry. One of the chloride ions lies on a general position and the other has \overline{3} site symmetry. An extensive array of C—H...O, N—H...Cl and O—H...Cl hydrogen bonds exists between the ethylene diamine ligands, the water molecules of hydration, and the anions present, thereby furnishing solid-state stability.

An Activatable T1-Weighted MR Contrast Agent: A Noninvasive Tool for Tracking the Vicinal Thiol Motif of Thioredoxin in Live Cells

We have synthesized new magnetic resonance imaging (MRI) T1 contrast agents (CA1 and CA2) that permit the activatable recognition of the cellular vicinal thiol motifs of the protein thioredoxin. The contrast agents showed MR relaxivities typical of gadolinium complexes with a single water molecule coordinated to a Gd3+ center (i.e., ~4.54 mM−1s−1) for both CA1 and CA2 at 60 MHz. The contrast agent CA1 showed a ~140% relaxivity enhancement in the presence of thioredoxin, a finding attributed to a reduction in the flexibility of the molecule after binding to thioredoxin. Support for this rationale, as opposed to one based on preferential binding, came from 1H-15N-HSQC NMR spectral studies; these revealed that the binding affinities toward thioredoxin were almost the same for both CA1 and CA2. In the case of CA1, T1-weighted phantom images of cancer cells (MCF-7, A549) could be generated based on the expression of thioredoxin. We further confirmed thioredoxin expression-dependent changes in the T1-weighted contrast via knockdown of the expression of the thioredoxin using siRNA-transfected MCF-7 cells. The nontoxic nature of CA1, coupled with its relaxivity features, leads us to suggest that it constitutes a first-in-class MRI T1 contrast agent that allows for the facile and noninvasive monitoring of vicinal thiol protein motif expression in live cells.

Structural origins of protein conformational entropy

The thermodynamics of molecular recognition by proteins is a central determinant of complex biochemistry. For over a half-century detailed cryogenic structures have provided deep insight into the energetic contributions to ligand binding by proteins1. More recently, a dynamical proxy based on NMR-relaxation methods has revealed an unexpected richness in the contributions of conformational entropy to the thermodynamics of ligand binding2,3,4,5. There remains, however, a discomforting absence of an understanding of the structural origins of fast internal motion and the conformational entropy that this motion represents. Here we report the pressure-dependence of fast internal motion within the ribonuclease barnase and its complex with the protein barstar. Distinctive clustering of the pressure sensitivity correlates with the presence of small packing defects or voids surrounding affected side chains. Prompted by this observation, we performed an analysis of the voids surrounding over 2,500 methyl-bearing side chains having experimentally determined order parameters. We find that changes in unoccupied volume as small as a single water molecule surrounding buried side chains greatly affects motion on the subnanosecond timescale. The discovered relationship begins to permit construction of a united view of the relationship between changes in the internal energy, as exposed by detailed structural analysis, and the conformational entropy, as represented by fast internal motion, in the thermodynamics of protein function.

Effect of a Single Water Molecule on •CH2OH + 3O2 Reaction Under Atmospheric and Combustion Conditions

The Hydroxymethyl (•CH2OH) radical is an important intermediate species in both atmosphere and combustion reaction systems. The rate coefficients for •CH2OH + 3O2 and (CH2OH + 3O2 (+H2O) reactions were...

Unveiling the Coexistence of Cis and Trans Isomers in the Hydrolysis of ZrO2: A Coupled DFT and High-Resolution Photoelectron Spectroscopy Study

High-resolution anion photoelectron spectroscopy of the ZrO₃H₂^- and ZrO₃D₂^- anions and complementary electronic structure calculations are used to investigate the reaction between zirconium dioxide and a single water molecule, ZrO₂^0/- + H₂O. Experimental spectra of ZrO₃H₂^- and ZrO₃D₂^- were obtained using slow photoelectron velocity-map imaging (cryo-SEVI), revealing the presence of two dissociative adduct conformers and yielding insight into the vibronic structure of the corresponding neutral species. Franck-Condon simulations for both the \textit{cis}-- and \textit{trans}--dihydroxide structures are required to fully reproduce the experimental spectrum. Additionally, it was found that water-splitting is stabilized more by ZrO₂ than TiO₂, suggesting Zr-based catalysts are more reactive toward hydrolysis.

Unveiling the Coexistence of Cis and Trans Isomers in the Hydrolysis of ZrO2: A Coupled DFT and High-Resolution Photoelectron Spectroscopy Study

High-resolution anion photoelectron spectroscopy of the ZrO₃H₂^- and ZrO₃D₂^- anions and complementary electronic structure calculations are used to investigate the reaction between zirconium dioxide and a single water molecule, ZrO₂^0/- + H₂O. Experimental spectra of ZrO₃H₂^- and ZrO₃D₂^- were obtained using slow photoelectron velocity-map imaging (cryo-SEVI), revealing the presence of two dissociative adduct conformers and yielding insight into the vibronic structure of the corresponding neutral species. Franck-Condon simulations for both the \textit{cis}-- and \textit{trans}--dihydroxide structures are required to fully reproduce the experimental spectrum. Additionally, it was found that water-splitting is stabilized more by ZrO₂ than TiO₂, suggesting Zr-based catalysts are more reactive toward hydrolysis.

Unveiling the Coexistence of Structural Isomers in the Hydrolysis of ZrO2: A Combined Computational and Photoelectron Spectroscopy Study

High-resolution anion photoelectron spectroscopy of the ZrO₃H₂^- and ZrO₃D₂^- anions and complementary electronic structure calculations are used to investigate the reaction between zirconium dioxide and a single water molecule, ZrO₂^0/- + H₂O. Experimental spectra of ZrO₃H₂^- and ZrO₃D₂^- were obtained using slow photoelectron velocity-map imaging (cryo-SEVI), revealing the presence of two dissociative adduct conformers and yielding insight into the vibronic structure of the corresponding neutral species. Franck-Condon simulations for both the \textit{cis}-- and \textit{trans}--dihydroxide structures are required to fully reproduce the experimental spectrum. Additionally, it was found that water-splitting is stabilized more by ZrO₂ than TiO₂, suggesting Zr-based catalysts are more reactive toward hydrolysis.

Hydrogen release from a single water molecule on Vn+ (3 ≤ n ≤ 30)

Water and its interactions with metals are closely bound up with human life, and the reactivity of metal clusters with water is of fundamental importance for the understanding of hydrogen generation. Here a prominent hydrogen evolution reaction (HER) of single water molecule on vanadium clusters Vn+ (3 ≤ n ≤ 30) is observed in the reaction of cationic vanadium clusters with water at room temperature. The combined experimental and theoretical studies reveal that the wagging vibrations of a V-OH group give rise to readily formed V-O-V intermediate states on Vn+ (n ≥ 3) clusters and allow the terminal hydrogen to interact with an adsorbed hydrogen atom, enabling hydrogen release. The presence of three metal atoms reduces the energy barrier of the rate-determining step, giving rise to an effective production of hydrogen from single water molecules. This mechanism differs from dissociative chemisorption of multiple water molecules on aluminium cluster anions, which usually proceeds by dissociative chemisorption of at least two water molecules at multiple surface sites followed by a recombination of the adsorbed hydrogen atoms.

4,15-Dimethyl-7,12-diazoniatricyclo[10.4.0.02,7]hexadeca-1(12),2,4,6,13,15-hexaene dibromide monohydrate

The title compound, C16H20N2 2+·2Br−·H2O (1) is a member of the class of compounds called viologens. Viologens are quaternary salts of dipyridyls and are especially useful as redox indicators as a result of their large negative one-electron reduction potentials. Compound 1 consists of a dication composed of a pair of 4-methylpyridine rings mutually joined at the 2-position, with a dihedral angle between the pyridine rings of 62.35 (4)°. In addition, the rings are tethered via the pyridine nitrogen atoms by a tetramethylene bridge. Charge balance is provided by a pair of bromide anions, which are hydrogen bonded to a single water molecule [D O...Br = 3.3670 (15) and 3.3856 (15) Å]. The crystal structure of 1, details of an improved synthesis, and a full analysis of its NMR spectra are presented.