New Insights on the Nature of the Chemical Species Involved during the Process of Dopamine Deprotonation in Aqueous Solution: Theoretical and Experimental Study

Abstract

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.

On the electrochemistry of dopamine in aqueous solution. Part I: The role of [SDS] on the voltammetric behavior of dopamine on a carbon paste electrode

“…It becomes plain for all previous reactions that the pH can certainly play a very important role to establish the most probable DA oxidation mechanism. Recently, our group has shown by an experimental and theoretical approach, which is the most probable DA deprotonation pathway [41]. …”

Catechol derivatives-coated Fe3O4 and γ-Fe2O3 nanoparticles as potential MRI contrast agents

“…2. In fresh water dopamine is positively charged [34] (pH $ 4) and then the electrostatic repulsion between dopamine functionalized particles is sufficient to prevent their aggregation.…”

Electrochemical and Spectrophotometric Evaluation of the Formation Constants of the AA-βCD and DA-βCD Inclusion Complexes

“…Following the same method, and considering the predominance zones diagram shown in Figure 3, it can be established that the inclusion reaction between the DA and the βCD is as shown in Reaction R2, where the DA is fully protonated, H 3 …”

“…Given the DA's enormous significance for human kind, a large diversity of studies have been published with the aim of gaining deeper knowledge of its chemical behavior (2)(3)(4). In particular, previous results from our research group have contributed to determine successfully its acidity constants (2) and to clarify the deprotonation mechanism, refer to figure 3, thereby giving rise to the species lasting each of the equilibria (3). Further, electrochemical studies have been carried out, with which it became possible to determine the DA's oxidation kinetics and mechanism with and without the anionic surfactant sodium dodecylsulphate (4).…”

“…Predominance zones diagram of the dopamine's species and its minimum energy structures at different pH (3). …”