SAMs de Moléculas Sulfuradas: Estudo Termodinâmico e Cinético de Adsorção e Aplicação em Reações de Transferência de Elétrons de Metaloproteinas.

Abstract

Electrochemical techniques, quartz crystal microbalance (QCM) and surface plásmons resonance (SPR) were used to study the formation of self-assembled monolayers (SAMs) of 1,4-ditiano (1,4-dt), 4-mercaptopyridine (pyS), 5-(4-piridinyl)-1,3,4-oxadiazole-2-thiol (Hpyt), thionicotinamide (TNA) and thioisonicotinamide (iTNA) as a result of the immersion of gold substrates into the respective solutions. STM (Scanning Tunneling Microscopy) images indicate the sulfur atom as the adsorption site of these molecules and a hexagonal conformation on surface. For the iTNA molecule, the images were not conclusive. In addition, the images indicated the existence of defects even after longer immersion times (24 h). Thermodynamic and kinetic aspects of the adsorption process were evaluated by reductive desorption in alkaline media and QCM. The reductive desorption potentials, Edr, were observed at −0.9, −0.8, and −0.5 V vs. Ag/AgCl for the desorption of iTNA, TNA and Hpyt, respectively. In comparison to 1,4-dt (−0.8 V) and pyS (−0.5 V) species, the Edr value of Hpyt indicates a sigma interaction whereas those of iTNA and TNA indicate an additional pi contribution. The values of the concentration of adsorbed material, gama, and fractional coverage (teta ~ 0.9), determined, respectively, by reductive desorption and impedance are consistent with the STM images. In comparison to the desorption data, the higher values of gama calculated by QCM were assigned to the presence of water molecules since a linear relation was observed between the dipole moment and the mass change calculated by QCM. The desorption curves acquired for the pyS SAM indicated the decomposition of the monolayer thus not allowing the determination of the thermodynamic and kinetic parameters of adsorption. In comparison with the results obtained for the electrode modified after immersion in Na2S solution, it was suggested that this process is associated to the cleavage of the C─S bond which results in the formation of an adlayer composed of atomic and/or oligomeric sulfur species. The adsorption isotherms for Hpyt, TNA and iTNA fitted the Langmuir model of adsorption allowing the determination of the free energy of adsorption, ΔGads, as −35.9, −38.5 and −34,9 kJ/mol, respectively. These values are indicative of strong interaction being typical of chemisorption. For the Frumkin model, the best correlation was found when the interaction parameter, g, was established as −0.45, −0.20 and −0.10 for Hpyt, TNA and iTNA, respectively, indicating repulsive interactions between the adjacent molecules. Cyclic voltammetry was used to determinate the pKa of the SAMs of Hpyt (4.2), TNA (5.0 and 8.5) and iTNA (4.5 and 7.9) by using [Fe(CN)6]3− as a probe molecule. For this study, some changes were suggested since in the method proposed in the literature the existence of more than one protonation site was not considered thus not allowing the determination of more than one pKa value as was observed for TNA and iTNA molecules. Electron transfer reactions (TE) of cytochrome c (cyt c) and myoglobin (Mb) metalloproteins were studied by using the SAMs of Hpyt, TNA, iTNA, pyS and 1,4-dt. For the SAMs formed with TNA and iTNA, the positive shift of 0.2V on the half-wave potential of cyt c in relation to that of the native protein, was assigned to the positive charge density on surface in consequence of the protonation of NH2 groups (pKa~8.0) since these measurements were carried out in physiological medium. QCM and SPR data indicated the formation of a monolayer of cyt c on the studied SAMs. This monolayer, although not being electroactive, allows the study of the TE reaction of the cyt c molecules in solution suggesting that this process involves the orbitals of the modifier molecules. For the Mb metalloprotein, a SAM of L-cysteine amino acid was used since none of the studied sulfur molecules was able to access the TE reaction. The redox process was observed at 0.086 V suggesting the native form of Mb. QCM and SPR data indicated, also, the formation of a monolayer of Mb on the cys SAM (Au/cys/Mb). Values of –49.67 kJ mol−1 and −0.15 for ΔGads and g, respectively, were calculated for the formation of the monolayer of Mb on the cys SAM. The electrode Au/cys/Mb presented catalytic activity toward the oxidation reaction of ascorbic acid presenting a decrease of 400 mV in the overpotential and a kinetic controlled with a rate constant, kf, of 2.0 x 104 L mol-1 s-1.