Homogeneous reduction of CO2 by photogenerated pyridinyl radicals

We report that 1-hydropyridinyl radicals (1-PyH\u2022) photogenerated in solution react with dissolved CO2 en route to its 2e 12 reduction into carboxylic acids. The 254 nm excitation of pyridine (Py) in deaerated 2-PrOH/H2O mixtures saturated with 1 atm of CO2 yields a suite of products, among which we identified Na(HCOO)2 12(m/z 12 = 113), C5H6NCOO 12 (m/z 12 = 124), and C5H10O2NCOO 12 (m/z 12 = 160) species by electrospray ionization mass spectrometry. These products demonstrably contain carboxylate functionalities that split CO2 neutrals via collisionally induced dissociation. We infer that 1-PyH\u2022 [from (1) 3Py* + 2-PrOH \u2192 1-PyH\u2022 + \u2022PrOH] adds to CO2, in competition with radical 12radical reactions, leading to intermediates that are in turn reduced by \u2022PrOH into the observed species. The formation of carboxylates in this system, which is shown to require CO2, Py, 2-PrOH, and actinic radiation, amounts to the homogeneous 2e 12 reduction of CO2 by 2-PrOH initiated by Py*. We evaluate a rate constant (2) k2(1-PyH\u2022 + CO2 \u2192 \u2022Py-1-COOH) 48 O (10) M 121 s 121 and an activation energy E2 65 9 kcal mol 121 that are compatible with thermochemical estimates for this reaction

Homogeneous reduction of CO2 by photogenerated pyridinyl radicals

The impact of fossil fuel combustion emissions on Earth's climate would be alleviated by an ideal process able to capture atmospheric CO2 and convert it into fuels and/or chemical feedstocks via an artificial photosynthetic process driven by sunlight. However, the endoergonic reduction of CO2 into organic matter is still very challenging. Indeed, due to its negative electron affinity in the gas phase, conversion of CO2 into the carbon dioxide radical anion is both very slow and hindered by unfavorable thermodynamics.The chemical stabilization of the carbon dioxide radical anion has recently been achieved with the formation of a carbamyl intermediate from the homogeneous reaction between 1-hydroxypyridinyl radicals formed through th electrochemical reduction of pyridimium with dissolved CO2. However, the mechanism has not yet been fully clarified, quantum mechanical calculations having predicted a pyridinium reduction potential significantly more negative than the experimentally reported value. In our study, CO2 reduction was performed under monochromatic irradiation at 254 nm in dearated homogeneous aqueous solutions containing pyridine and an electron donor (2-Pr-OH). The final reaction products, identified by electrospray ionization mass spectrometry, were shown to contain carboxylate functionalities, Accounting for the homogeneous 2-electrons reduction of CO2 by 2-PrOH initiated by electronically excited pyridine.. The evaluated rate constant and activation energy for 1-pyridinyl addition to CO2 are comparable with thermochemical estimates for this reaction

Low intensity, continuous wave photodoping of ZnO quantum dots - Photon energy and particle size effects

The unique properties of semiconductor quantum dots (QDs) have found application in the conversion of solar to chemical energy. How the relative rates of the redox processes that control QD photon efficiencies depend on the particle radius (r) and photon energy (Eλ), however, is not fully understood. Here, we address these issues and report the quantum yields (Φs) of interfacial charge transfer and electron doping in ZnO QDs capped with ethylene glycol (EG) as a function of r and Eλ in the presence and absence of methyl viologen (MV2+) as an electron acceptor, respectively. We found that Φs for the oxidation of EG are independent of Eλ and photon fluence (φλ), but markedly increase with r. The independence of Φs on φλ ensures that QDs are never populated by more than one electron-hole pair, thereby excluding Auger-type terminations. We show that these findings are consistent with the operation of an interfacial redox process that involves thermalized carriers in the Marcus inverted region. In the absence of MV2+, QDs accumulate electrons up to limiting volumetric densities ρe,∞ that depend sigmoidally on excess photon energy E∗ = Eλ - EBG(r), where EBG(r) is the r-dependent bandgap energy. The maximum electron densities: ρev,∞ ∼ 4 × 1020 cm-3, are reached at E∗ > 0.5 eV, independent of the particle radius.Fil: Aguirre, Matías Ezequiel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Físicas de Mar del Plata. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Físicas de Mar del Plata; ArgentinaFil: Municoy, Sofia. Comisión Nacional de Energía Atómica. Centro Atómico Constituyentes; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Grela, Maria Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Físicas de Mar del Plata. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Físicas de Mar del Plata; ArgentinaFil: Colussi, A.j.. California Institute Of Technology; Estados Unido

Essential Roles for GPI-anchored Proteins in African Trypanosomes Revealed Using Mutants Deficient in GPI8

The survival of Trypanosoma brucei, the causative agent of Sleeping Sickness and Nagana, is facilitated by the expression of a dense surface coat of glycosylphosphatidylinositol (GPI)-anchored proteins in both its mammalian and tsetse fly hosts. We have characterized T. brucei GPI8, the gene encoding the catalytic subunit of the GPI:protein transamidase complex that adds preformed GPI anchors onto nascent polypeptides. Deletion of GPI8 (to give Δgpi8) resulted in the absence of GPI-anchored proteins from the cell surface of procyclic form trypanosomes and accumulation of a pool of non–protein-linked GPI molecules, some of which are surface located. Procyclic Δgpi8, while viable in culture, were unable to establish infections in the tsetse midgut, confirming that GPI-anchored proteins are essential for insect-parasite interactions. Applying specific inducible GPI8 RNAi with bloodstream form parasites resulted in accumulation of unanchored variant surface glycoprotein and cell death with a defined multinuclear, multikinetoplast, and multiflagellar phenotype indicative of a block in cytokinesis. These data show that GPI-anchored proteins are essential for the viability of bloodstream form trypanosomes even in the absence of immune challenge and imply that GPI8 is important for proper cell cycle progression