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Twenty-two reaction schemes have been tested, within the cluster-continuum model including up to seven explicit water molecules. They have been used in conjunction with nine different methods, within the density functional theory and with second-order Møller-Plesset. The quality of the pKa predictions was found to be strongly dependent on the chosen scheme, while only moderately influenced by the method of calculation. We recommend the E1 reaction scheme [HA + OH(-) (3H2O) ↔ A(-) (H2O) + 3H2O], since it yields mean unsigned errors (MUE) lower than 1 unit of pKa for most of the tested functionals. The best pKa values obtained from this reaction scheme are those involving calculations with PBE0 (MUE = 0.77), TPSS (MUE = 0.82), BHandHLYP (MUE = 0.82), and B3LYP (MUE = 0.86) functionals. This scheme has the additional advantage, compared to the proton exchange method, which also gives very small values of MUE, of being experiment independent. It should be kept in mind, however, that these recommendations are valid within the cluster-continuum model, using the polarizable continuum model in conjunction with the united atom Hartree-Fock cavity and the strategy based on thermodynamic cycles. Changes in any of these aspects of the used methodology may lead to different outcomes.

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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.

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