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Chacabuco, Argentina

Vega-Hissi E.G.,University Nac Of San Luis | Estrada M.R.,University Nac Of San Luis | Lavecchia M.J.,National University of La Plata | Pis Diez R.,National University of La Plata
Journal of Chemical Physics | Year: 2013

The pKa, the negative logarithm of the acid dissociation equilibrium constant, of the carboxylic acid groups of unconjugated bilirubin in water is a discussed issue because there are quite different experimental values reported. Using quantum mechanical calculations we have studied the conformational behavior of unconjugated bilirubin species (in gas phase and in solution modeled implicitly and explicitly) to provide evidence that may clarify pKa values because of its pathophysiological relevance. Our results show that rotation of carboxylate group, which is not restricted, settles it in a suitable place to establish stronger interactions that stabilizes the monoanion and the dianion to be properly solvated, demonstrating that the rationalization used to justify the high pKa values of unconjugated bilirubin is inappropriate. Furthermore, low unconjugated bilirubin (UCB) pKa values were estimated from a linear regression analysis. © 2013 American Institute of Physics.

Vega-Hissi E.G.,University Nac Of San Luis | Andrada M.F.,University Nac Of San Luis | Zamarbide G.N.,University Nac Of San Luis | Estrada M.R.,University Nac Of San Luis | Tomas-Vert F.,University of Valencia
Journal of Molecular Modeling | Year: 2011

Quantum chemical methods have been used to study the conformational and electronic properties of sulfanilamide and derivatives with antibacterial activity. Calculations at B3LYP/6-311++G(3df,2p) level of theory predict the existence of four conformers for sulfanilamide depending on the orientation of p-amino and amide groups. Focusing on the sulfonamide moiety, amide NH 2 and SO 2 groups could exist either in an eclipsed or staggered arrangement. Gas-phase results predict the eclipsed conformer to be most stable but opposite to what has been rationalized previously, no stabilizing hydrogen bonds between those groups has been found through NBO analysis. When solvent effect is taken into account through the IEF-PCM method, staggered conformer is preferred; in fact, eclipsed conformation changed when explicit solvent molecules were included. Conformational analysis of all derivatives has shown two global minima which are specular images. Five out of the seven derivatives studied adopted a particular minimum energy conformation with very similar geometries. © 2010 Springer-Verlag.

Haggi E.,Austral University | Blasich N.,Austral University | Montana M.P.,University Nac Of San Luis | Ferrari G.,University Nac Of San Luis | And 6 more authors.
Current Topics in Peptide and Protein Research | Year: 2013

Within the framework of the frequently invoked correlation between antioxidant contents and potential antioxidative protection, the stability of the amino acid (AA), tryptophan (trp), taken as a prototype of relevant photooxidizable biological target, was evaluated in the presence of the known antioxidant ascorbate ion (AsA). The peptide glutathione (GSH) was also included in order to eliminate thermal decomposition of AsA. Riboflavin (Rf, vitamin B2), endogenous in most biological environments, was employed as a photosensitizer. A systematic kinetic and mechanistic study was conducted under aerobic conditions in pH 7 aqueous medium. The evaluation of oxygen uptake rates (OUR) in the photoirradiated mixtures, taken as an overall indicator of the total oxidation degree of the mixture components, constitutes an interesting and useful tool in this case. In the presence of Rf, the isolated AA and AsA undergo photodegradation under conditions currently found in natural media. The photodegradations occur in a non-simple reaction, mediated by reactive oxygen species (ROS) such as singlet molecular oxygen (O2(1AS)), superoxide radical anion,hydrogen peroxide and hydroxyl radical. All ROS are photogenerated from triplet excited Rf after interaction with the ground state of AsA and trp. The photoirradiation also triggers an already described parallel thermally-driven and dark reaction on AsA which is stopped by the addition of GSH. The global outcome can be interpreted as a sort of conjunctive antioxidant protection exerted on the system AsA + trp + GSH upon Rf-photosensitization, a result that cannot be predicted from the individual behavior of the mixture components.

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