Publications

Abstract Electrochemical properties of Au electrodes sequentially modified by self-assembled 1,6 hexanedithiol (1,6HDT) and gold nanorods (AuNRs) are investigated by cyclic voltammetry, square-wave voltammetry, and electrochemical impedance spectroscopy, using [Fe(CN)6](3-/4-) as redox probes. The nanorods stabilized by cetyltrimetylammonium bromide (CTAB) with aspect ratios of 2.20, 2.80, and 3.77 were grown by a seed-mediated procedure and chemically bonded to the 1,6HDT-coated electrodes by a place exchange reaction at a 27 degrees C solution temperature. Topographic tapping mode atomic force microscopy measurements revealed an end bonding of the 2.20 aspect ratio rods and a side surface bonding of the 2.80 and 2.77 aspect ratio rods. Analysis of the electrochemical responses as a function of the sizes and surface orientations of the rods revealed that the electron transfer is faster at electrodes modified with smaller and vertically aligned nanorods than those modified with larger and randomly attached nanorods (side surface bonding). A progressive increase in the charge transfer resistance RCT from bilayers composed of 1,6HDT and 2.20 aspect ratio rods to bilayers composed of 1,6HDT and rods of 2.80 or 3.77 aspect ratios was described by a tunneling parameter of beta = 1.07 per thiol chain unit, This behavior suggests that the electron transfer kinetics is controlled by coherent electron tunneling across the 1,6HDT monolayer. In addition, the several orders of magnitude changes of the apparent charge transfer resistance upon nanorod adsorption suggest a charging of the rods by. the redox probes in solution and electron transfer across them. It is concluded that the electron transfer proceeds via a three-step process: charging of the rods by the redox probes in solution, electron transport across the rods, and electron tunneling across the 1,6HDT-SAM toward the underlying An substrates.

Abstract Luciferase is a general term for enzymes catalyzing visible light emission by living organisms (bioluminescence). The studies carried out with Photinus pyralis (firefly) luciferase allowed the discovery of the reaction leading to light production. It can be regarded as a two-step process: the first corresponds to the reaction of luciferase's substrate, luciferin (LH(2)), with ATP-Mg(2+) generating inorganic pyrophosphate and an intermediate luciferyl-adenylate (LH(2)-AMP); the second is the oxidation and decarboxylation of LH(2)-AMP to oxyluciferin, the light emitter, producing CO(2), AMP, and photons of yellow-green light (550-570 nm). In a dark reaction LH(2)-AMP is oxidized to dehydroluciferyl-adenylate (L-AMP). Luciferase also shows acyl-coenzyme A synthetase activity, which leads to the formation of dehydroluciferyl-coenzyme A (L-CoA), luciferyl-coenzyme A (LH(2)-CoA), and fatty acyl-CoAs. Moreover luciferase catalyzes the synthesis of dinucleoside polyphosphates from nucleosides with at least a 3'-phosphate chain plus an intact terminal pyrophosphate moiety. The LH(2) stereospecificity is a particular feature of the bioluminescent reaction where each isomer, D-LH(2) or L-LH(2), has a specific function. Practical applications of the luciferase system, either in its native form or with engineered proteins, encloses the analytical assay of metabolites like ATP and molecular biology studies with luc as a reporter gene, including the most recent and increasing field of bioimaging. (C) 2008 IUBMB

Abstract The standard (p(o) = 0.1 MPa) molar energies of combustion in oxygen. at T = 298.15 K, of 5, 6- and 7-methoxy-alpha-tetralone were measured by static bomb calorimetry. The values of the standard molar enthalpies of sublimation were obtained by Calvet microcalorimetry and corrected to T = 298.15 K. Combining these results, the standard molar enthalpies of formation of the compounds, in the gas phase, at T = 298.15 K, have been calculated, 5-methoxy-alpha-tetralone-(244.8 +/- 1.9) kJ . mol(-1), 6- methoxy-alpha-tetralone -(243.0 +/- 2.8) kJ . mol(-1) and 7-methoxy-alpha-tetralone -(242.3 +/- 2.6) kJ . mol(-1). Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional with extended basis sets and more accurate correlated computational techniques of the MCCM/3 suite have been performed for the compounds. The agreement between experiment and theory gives confidence to estimate the enthalpy of formation of 8-methoxy-alpha-tetralone. Similar calculations were done for the 5, 6-, 7- and 8-methoxy-beta-tetralone, for which experimental work was not done. (C) 2008 Published by Elsevier Ltd.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation in the condensed state of chromanone, dihydrocoumarin, and 3-isochromanone were derived from the standard molar energies of combustion in oxygen at T = 298.15 K, measured by combustion calorimetry. Calvet microcalorimetry was used to derive the standard molar enthalpies of sublimation and vaporization. [GRAPHICS] From these values the standard molar enthalpies in the gaseous phase, at T = 298.15 K. were derived. Additionally estimates were performed of the enthalpies of formation of all the studied compounds in gas-phase, using DFT and other more accurate correlated calculations, together with appropriate isodesmic or homodesmic reactions. There is a reasonable agreement between computational and experimental results.

Abstract In this work, we have determined the experimental standard (p degrees = 0.1 MPa) molar enthalpies of formation, in gas phase, of flavone and flavanone. These results were obtained by combining the standard molar enthalpies of formation in the condensed phase with the standard molar enthalpies of sublimation. The former values were derived from combustion experiments in oxygen, at T = 298.15 K, in a static bomb calorimeter. The values of the standard molar enthalpies of sublimation were obtained by Calvet microcalorimetry and corrected to T = 298.15 K. High-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional with extended basis sets and more accurate correlated computational techniques of the MCCM/3 suite have been performed for the compounds. The obtained results, experimental and computational, for flavone and flavanone were compared with those obtained for chromone and chromanone, respectively.

Abstract The electrochemical oxidation of CO has been studied on Pt(S)[(n - 1)(1 0 0) x (1 1 0)] electrodes to investigate the effect of the step density in the reaction. This series shows two different trends for long (n >= 7) and short terraces. For long terraces, the voltammetric peak shifts towards higher potential as the step density increases, unlike the behaviour observed for other stepped surfaces, which exhibit the opposite behaviour in agreement with the Smoluchowski effect. For short terraces, the "normal" behaviour is observed, that is, as the step density increases the peak shifts towards lower potentials. Chronoamperometric measurements were used to determine rate constants and Tafel slopes using the mean field Langmuir-Hinselwood kinetics. Rate constants follow the same trends as the peak potentials in voltammetry. A Tafel slope of 75 +/- 4 mV has been obtained for the surfaces with long terraces whereas a value of the surfaces with short terraces showed a value of 100-120 mV is obtained. This change of slopes is interpreted as a change in the electrochemical behaviour of the species involved in the mechanism, probably, a change in the adsorption isotherm of adsorbed OH. Pt(5 10) electrode exhibits an intermediate behaviour between those of long and short terraces with two different peaks that can be associated with both behaviours previously described.