New insights on the nature of the chemical species involved during the process of dopamine deprotonation in aqueous solution: theoretical and experimental study.

Due to dopamine's chemical structure and the fact that it has three pKa values, its deprotonation process, in aqueous solution, may involve different chemical species. For instance, the first deprotonation step, from the fully protonated dopamine molecule (H3DA+) to the neutral one (H2DA), will result in zwitterionic species if a proton from one of the OH groups in the catechol ring is lost or into a neutral species if the proton is lost from the amino group. Given that the interaction of such a product with its environment will be quite different depending on its nature, it is very important, therefore, to have an accurate knowledge of which is the dopamine chemical species that results after each deprotonation step. In order to gain a better understanding of dopamine chemistry and to establish a plausible dopamine deprotonation pathway, the optimized geometries of the aforementioned species were calculated in this work by means of the density functionals theory (B3LYP/6-311+G(d,p)) in both cases: in vacuo and with solvent effect, to assess, among other theoretical criteria, the proton affinities of the different dopamine species. This permitted us to propose the following reaction pathway: [reaction in text]. Moreover, the calculations of the chemical shift (NMR-GIAO) modeling the effect of the solvent with a continuum method (PCM) was in agreement with the 13C NMR experimental spectra, which confirmed even further the proposed deprotonation pathway.

Determination of beta-D-glucose using flow injection analysis and composite-type amperometric tubular biosensors.

An amperometric tubular cell involving composite biosensors for the determination of beta-d-glucose in a flow injection analysis (FIA) system is proposed. Diverse configurations and parameters are evaluated to improve the system's response. The configuration producing less noise resulted when the biosensor was located closer to the auxiliary electrode, which also required coupling both electrodes within the system under a continuous flow regime. Further, we report on the influence of the active area of the biosensor and of the flow rate used. Statistical analyses of the data revealed two regions with a linear response range for the determination of beta-d-glucose, with a detection limit of 4.7 x 10(-4) M and in the low concentration region a sensitivity of 17.46 +/- 0.12 microAM(-1). At the beta-d-glucose concentrations studied there was no evidence of enzymatic saturation. An increment on the ionic strength of the sample and carrier passing through the analysis system decreases its sensitivity. The reproducibility of the analytical system in terms of its standard deviation was 2.9% with a 95% confidence level, having a lifetime that lasted at least 100 days. beta-d-Glucose was determined in different commercial medical glucose-containing solutions, the experimental results are in good agreement with those reported by the manufacturer.