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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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Since their introduction by E. L. Eliel nearly four decades ago, derivatives of 1,3-dioxane have proved useful in conformational analysis. Examples are discussed, where 5-polar substituents permit the evaluation of fundamental phenomena such as attractive and repulsive gauche effects, electrostatic interactions, and stereoelectronic effects. By the same token, 2-substituted 1,3-dioxanes, 1,3-dithianes, and 1,3-oxathianes are useful frameworks for the study of the anomeric effect and the associated structural and spectroscopic manifestations, such as the so-called Perlin effects. In view of the varied and essential involvement of metal ions and inorganic salts in biological processes, 5-substituted 1,3-dioxanes are presently being examined in the presence of Li + , Na + , K + , Ag + , Mg 2+ , Ca 2+ , Ba 2+ , and other cations, with the aim to increase the understanding of biomolecular properties in vivo. 1,3-DIOXANES AS MODEL SYSTEMS IN CONFORMATIONAL ANALYSISInterestingly, H. Sachse's proposal of a strain-free, chair-shaped cyclohexane [1a] was published in 1890, the same year A. Baeyer advanced the idea that cyclohexane was planar (and thus strained) [1b]. Baeyer's model was favored during several decades, owing to the fact that rapid-ring inversion prevented isolation of the axial and equatorial isomers that should originate in monosubstituted cyclohexane derivatives (Scheme 1).

Quantum chemical calculations were performed to characterize the interaction of the flavonol molecule (FL) with methacrylic acid (MAA) and 4-vinylpyridine (4VPy) in the formation of imprinted polymers. The polarizable continuum model (PCM) was used to gain insight on the type of interaction between the reactant molecules under vacuum conditions and in the presence of different solvents. The effect of solvent on the pre-polymerization complex formation was evaluated through the stability energy, in which chloroform behaves as the best solvent for the synthesis of the imprinted polymers since it facilitates the reaction by lowering its degree of stabilization. The reactivity was analyzed in terms of the electrostatic surface potential (ESP) and Mulliken charge. By means of these results, it has been possible to determine two potential recognition sites for the interaction of the MAA monomer and one for the 4VPy in relation to the strength of interaction with FL. In this concern, the interaction of the system FL-MAA is stronger than FL-4VPy.

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The varied and essential involvement of metal ions and inorganic salts in biological and chemical processes motivated the present study where 5-carboxy- and 5-hydroxy-1,3-dioxanes are used as model frameworks for the evaluation of the conformational behavior of oxygen-containing receptors in the presence of Li(+), Na(+), K(+), Ag(+), Mg(2+), Ca(2+), Ba(2+), and Zn(2+). Thus, the position of equilibria, established by means of BF(3), between diastereomeric cis- and trans-5-substituted-2-phenyl-1,3-dioxanes, in solvent THF and in the presence of 0, 1, and 5 equiv of salt, has been determined. The observed Delta G(o) degrees values for the conformational equilibria of 5-carboxy-1,3-dioxane show that Ag(+), Li(+), and Ca(2+) complexation leads to increased stability of the axial isomer. In the case of the 5-hydroxy-1,3-dioxane, Mg(2+), Ag(+), and Zn(2+) are the metal ions that stabilize the axial conformer of the heterocycle upon association. Interpretation of the experimental observations was based on DFT molecular modeling studies at the Becke3LYP/6-31G* and Becke3LYP/6-31+G** levels of theory. Although gas-phase calculations give Delta E values that are too large when modeling equilibria involving ionic species in polar solution, the computational results confirm the structural and energetic consequences of metal cation coordination to the oxygen atom in carbonyls or ethers. The results derived from the present study contribute to our understanding of the chemical processes involved in molecular recognition and physiological events.