Publications

Abstract The adsorption of glucose oxidase (GOx) was studied at the interface between two immiscible electrolyte solutions (ITIES) by interfacial capacitance and surface tension measurements and at the air/water (phosphate buffer) interface by surface tension and neutron reflection measurements. The adsorption at both interfaces was found to be time, enzyme concentration, and ionic strength dependent. There was a switch from one interfacial adsorption state to another, as the enzyme concentration was increased. At the ITIES, there was evidence of an interaction between the adsorbed enzyme and the hydrophobic cation in the organic phase (1,2-dichloroethane). The enzyme adsorbed at the air/water interface was found to dissociate into monomers at the lower buffer total concentration of 2 mM while, at the higher buffer concentration of 0.2 M, the adsorbed enzyme retained its dimer structure. The adsorption mostly formed monolayers and the layer thickness varied with bulk concentration, indicating deformation related to the packing of the enzyme at the interface. For enzyme concentrations above 1 muM, in high ionic strength medium, bilayers of enzyme started to form, and the interlayer interactions resulted in a less densely packed second layer forming on the aqueous side of the first one. The switch in properties of the adsorbed layer observed in interfacial tension and capacitance measurements at the ITIES occurred over the same enzyme concentration range as the formation of a more densely packed layer detected from neutron reflection at the air/water interface.

Abstract The reaction between the arginine's guanidino group and alpha-dicarbonyl functionalities was used to develop a novel surface modification chemistry. The first modification step consists of the electrostatic adsorption of a poly-L-arginine layer onto ionizable alkanethiol modified gold surfaces. The strongly basic character of the guanidino group of the arginine residues (pK(a) > 12) guarantees the robust attachment of the polypeptide to negatively charged gold surfaces until very high pH. By varying the pH of the solution from which poly-L-arginine is electrostatically adsorbed, it is possible to control the amount deposited. The availability of the surface guanidino groups of the poly-L-arginine layer for further derivatization with alpha-dicarbonyl reaction probes, yielding stable heterocyclic condensation adducts, is demonstrated. In addition, the reaction with the heterobifunctional reagent p-azidophenyl glyoxal (APG) provides a surface terminated with a photosensitive aryl azide group which was employed for the photochemical immobilization of proteins to the surface. The application of this surface modification chemistry to immobilize antibodies is demonstrated.

Abstract The standard (pdegrees = 0.1 MPa) molar enthalpies of formation for 2-, 3-, and 4-methoxyphenol and 2,3-, 2,6-, and 3,5-dimethoxyphenol in the gaseous phase were derived from the standard molar enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb combustion calorimetry, and the standard molar enthalpies of evaporation at 298.15 K, measured by Calvet microcalorimetry: 2-methoxyphenol, -(246.1 +/- 1.9) kJ mol(-1); 3-methoxyphenol, -(240.4 +/- 2.1) kJ mol(-1); 4-methoxyphenol, -(229.7 +/- 1.8) kJ mol(-1); 2,3-dimethoxyphenol, -(386.0 +/- 2.2) kJ mol(-1); 2,6-dimethoxyphenol, -(381.7 +/- 1.9) kJ mol(-1); 3,5-dimethoxyphenol, -(399.4 +/- 3.0) kJ mol(-1). Density functional theory calculations for all the methoxy- and dimethoxyphenols and respective phenoxyl radicals and phenoxide anions were performed using extended basis sets, which allowed the estimation of the gas-phase enthalpies of formation for all compounds. The good agreement of the calculated and experimental gas-phase enthalpies of formation for the closed-shell systems gives confidence to the estimates concerning the isomers which were not experimentally studied and to the estimates concerning the radicals and the anions. Substituent effects on the homolytic and heterolytic O-H bond dissociation energies have been analyzed, the results being in good agreement with available experimental data. Detailed analysis of these effects suggests that electronic exchange phenomena between the substituents dominate the effect the substituents have on these systems.

Abstract The development of an reverse-phase HPLC-Photodiode Array (HPLC-DAD) to separate, identify and evaluate biological and synthetic catecholamines and their respective aminochromes, was discussed. The aminochromes characterization in HPLC chromatograms was done using collected fractions and tandem mass spectrometry applying a general pattern fragmentation. Spiking serum or whole blood with adrenochrome, followed by perchloric acid addition, sample centrifugation and neutralization allowed adrenochrome recoveries of approximately 100%. It was observed that the comparative analysis of the MS spectra of adrenochrome detected in rat blood and of the adrenochrome standard solution, both on MS and MS/MS modes were very similar.

Abstract The transfer of a phenolsulfonephthalein dye (bromophenol blue, BPB) across the water/1,2-dichloroethane (DCE) interface was studied by means of cyclic voltammetry, two-phase extraction techniques and in situ spectrophotometry. For the first time it was observed the transfer of two differently charged species originated from deprotonation of H2BPB. The values of the acidity constants were obtained (pK(ai)(w) = 3.1 +/- 0.2 and pK(a2)(w) = pK(a1)(w) 4.3 +/- 0.2) and an ionic partition diagram of BPB between water and DCE was constructed. The diagram is the basis for the proposed dye transfer mechanism which was validated using voltabsorptometric measurements. Physicochemical parameters were also obtained and relations between lipophilicity and structure accessed.