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Pabellon, Argentina

Kuzmanich G.,University of California at Los Angeles | Simoncelli S.,Analitica y Quimica Fisica | Gard M.N.,University of California at Los Angeles | Spanig F.,The Interdisciplinary Center | And 3 more authors.
Journal of the American Chemical Society

We report an efficient triplet state self-quenching mechanism in crystals of eight benzophenones, which included the parent structure (1), six 4,4′-disubstituted compounds with NH 2 (2), NMe 2 (3), OH (4), OMe (5), COOH (6), and COOMe (7), and benzophenone-3,3′,4, 4′-tetracarboxylic dianhydride (8). Self-quenching effects were determined by measuring their triplet-triplet lifetimes and spectra using femtosecond and nanosecond transient absorption measurements with nanocrystalline suspensions. When possible, triplet lifetimes were confirmed by measuring the phosphorescence lifetimes and with the help of diffusion-limited quenching with iodide ions. We were surprised to discover that the triplet lifetimes of substituted benzophenones in crystals vary over 9 orders of magnitude from ca. 62 ps to 1 ms. In contrast to nanocrystalline suspensions, the lifetimes in solution only vary over 3 orders of magnitude (1-1000 μs). Analysis of the rate constants of quenching show that the more electron-rich benzophenones are the most efficiently deactivated such that there is an excellent correlation, ρ = -2.85, between the triplet quenching rate constants and the Hammet θ + values for the 4,4′ substituents. Several crystal structures indicate the existence of near-neighbor arrangements that deviate from the proposed ideal for "n-type" quenching, suggesting that charge transfer quenching is mediated by a relatively loose arrangement. © 2011 American Chemical Society. Source

Perec M.,Analitica y Quimica Fisica | Baggio R.,Comision Nacional de la Energia Atomica
Acta Crystallographica Section E: Structure Reports Online

The title centrosymmetric bimetallic complex, [Cu2(C 2H3O2)4(C6H 6N2O)4], is composed of two copper(II) cations, four acetate anions and four isonicotinamide (INA) ligands. The asymmetric unit contains one copper cation to which two acetate units bind asymmetrically; one of the Cu - O distances is rather long [2.740 (2) Å], almost at the limit of coordination. These Cu - O bonds define an equatorial plane to which the Cu - N bonds to the INA ligands are almost perpendicular, the Cu - N vectors subtending angles of 2.4 (1) and 2.3 (1)° to the normal to the plane. The metal coordination geometry can be described as a slightly distorted trigonal bipyramid if the extremely weak Cu - O bond is disregarded, or as a highly distorted square bipyramid if it is not. The double acetate bridge between the copper ions is not coplanar with the CuO4 equatorial planes, the dihedral angle between the (O - C - O)2 and O - Cu - O groups being 34.3 (1)°, resulting in a sofa-like conformation for the 8-member bridging loop. In the crystal, N - H⋯O hydrogen bonds occur, some of which generate a head-to tail-linkage between INA units, giving raise to chains along [101]; the remaining ones make inter-chain contacts, defining a three-dimensional network. There are in addition a number of C - H⋯O bonds involving aromatic H atoms. Probably due to steric hindrance, the aromatic rings are not involved in significant π⋯π inter-actions. Source

Rodriguez-Granillo A.,Rice University | Crespo A.,Rice University | Crespo A.,Merck And Co. | Estrin D.A.,Analitica y Quimica Fisica | And 2 more authors.
Journal of Physical Chemistry B

The molecular details of how copper (Cu) is transferred from the human Cu chaperone Atox1 to metalbinding domains (MBDs) of P1B-type ATPases are still unclear. Here, we use a computational approach, employing quantum mechanics/molecular mechanics (QM/MM) methods, to shed light on the reaction mechanism [probable intermediates, Cu(I) coordination geometries, activation barriers, and energetics] of Cu(I) transfer from Atox1 to the fourth MBD of Wilson disease protein (WD4). Both Atox1 and WD4 have solvent-exposed metal-binding motifs with two Cys residues that coordinate Cu(I). After assessing the existence of all possible 2-, 3- and 4-coordinate Cu-intermediate species, one dominant reaction path emerged. First, without activation barrier, WD4's Cys1 binds Cu(I) in Atox1 to form a 3-coordinated intermediate. Next, with an activation barrier of about 9.5 kcal/mol, a second 3-coordinated intermediate forms that involves both of the Cys residues in WD4 and Cys1 of Atox1. This species can then form the product by decoordination of Atox1's Cys1 (barrier of about 8 kcal/mol). Overall, the Cu-transfer reaction from Atox1 to WD4 appears to be kinetically accessible but less energetically favorable (△E = 7.7 kcal/mol). Our results provide unique insights into the molecular mechanism of protein-mediated Cu(I) transfer in the secretory pathway and are in agreement with existing experimental data. © 2010 American Chemical Society. Source

Zeida A.,Analitica y Quimica Fisica | Reyes A.M.,University of the Republic of Uruguay | Lebrero M.C.G.,University of Buenos Aires | Radi R.,University of the Republic of Uruguay | And 2 more authors.
Chemical Communications

Peroxiredoxins (Prxs) catalyze the reduction of peroxides, a process of key relevance in a variety of cellular processes. The first step in the catalytic cycle of all Prxs is the oxidation of a cysteine residue to sulfenic acid, which occurs 103-107 times faster than in free cysteine. We present an experimental kinetics and hybrid QM/MM investigation to explore the reaction of Prxs with H2O2 using alkyl hydroperoxide reductase E from Mycobacterium tuberculosis as a Prx model. We report for the first time the thermodynamic activation parameters of H2O2 reduction using Prx, which show that protein significantly lowers the activation enthalpy, with an unfavourable entropic effect, compared to the uncatalyzed reaction. The QM/MM simulations show that the remarkable catalytic effects responsible for the fast H2O2 reduction in Prxs are mainly due to an active-site arrangement, which establishes a complex hydrogen bond network activating both reactive species. This journal is © the Partner Organisations 2014. Source

De La Llave E.,Analitica y Quimica Fisica | Molinero V.,University of Utah | Scherlis D.A.,Analitica y Quimica Fisica
Journal of Physical Chemistry C

The liquid-vapor transition in cylindrical pores is studied as a function of pore size and hydrophilicity through molecular dynamics simulations with the mW coarse-grained model of water. We identify two distinct filling mechanisms, depending on whether the water-pore interaction is smaller or larger than the water-water interaction. In the former case (that we term hydrophobic pore), the formation of the condensed phase proceeds gradually with filling, through the nucleation of a water cluster which grows toward the center of the cavity. In hydrophilic pores, instead, the condensed phase develops in conditions of supersaturation, which in principle become more extreme with increasing pore radius and surface affinity. For highly hydrophilic interfaces (those with adsorption energy for water above 10 kcal/mol), the equilibrium and dynamical properties of water in confinement turn out to be practically independent of water affinity. © 2011 American Chemical Society. Source

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