Estudo computacional do óxido nítrico em solução: dinâmica molecular, potenciais de redução e fotoliberação por complexos de ru-nitrosil

Abstract

During the last three decades, much eort has been devoted to understanding biological processes in which Nitric Oxide (NO) is involved. This interest is justied by the various biological functions attributed to this molecule, known as the smallest biological messenger. Among the challenges associated with the study of NO in solution, we may emphasize it's non-innocent character, as ligand, and theinterconversion of NO to its protonated form HNO in aqueous solution. NO and HNO has overlapping biological properties and, in some cases, the biological effects associated with NO can indeed be due to the interconversion of NO in HNO and vice-versa. In this Thesis electronic structure calculations, statistical thermodynamics and ab initio molecular dynamics, in the ground and excited states, were employed tostudy processes related to the dynamics of nitric oxide in solution, that allowed us to contribute to the understanding of some aspects of the NO chemistry in biological medium. Nitric Oxide (NO) was investigated in three dierent aspects: (i) Dynamics of NO, in its dierent formal oxidation states, in solution (ii) Determination of the Gibbs Free Energy of solvation of nitroxyl (HNO) and the protoncoupled electron transfer reduction potential of 2NO. to 1HNO (2NO.,H+/1HNO) in solution and (iii) the dynamics of photorelease of NO from the transition metalcomplex, using the complex [RuCl5(NO]2- as model. Ab Initio Molecular Dynamics calculations revealed a rapidly consumption of nitrosonium (NO+) cation, yielding the nitrite ion (NO2-). Ab Initio Molecular Dynamics of NO shows an equilibrium between Nitric Oxide and Hidroxyl ion, generated through the water self-ionization, generating the radical anion specie 2HONO.-. Nitroxil Ion are not involved in any reaction during the simulated time. This is directly related to its triplet ground state and the singlet ground state of the protonated form, HNO. The methods used in this works allows only events in a single potential energy surface to occurs. The Gibbs free energy of solvation of the nitroxyl species, HNO, was obtained through three dierent methods, two of them based on Linear Response Theory and the last one based on the continuum model COSMO(SMD). Our results show values at least five times smaller than the estimated and reference value in literature. The proton-coupled electron transfer reduction of 2NO. to 1HNO (2NO.,H+/1HNO) in aqueous solution was obtained and indicates a favorable process, in both physical-chemical and physiological conditions. The dierence between the values calculated here and the accepted in literature is strictly related to the description of the solvation free Energy and indicates a favorable Proton-Coupled Electron Transfer. Furthermore, a new route for the reduction process of NO to HNO was proposed, based on the intermediate radical anion 2HONO.-,observed in NO Ab Initio Molecular Dynamics. Our results also indicates a spontaneous process for the reduction when the radical anion is involved. The dynamics of photorelease of the NO ligand from an excited state of the [RuCl5(NO]2- in the excited state was investigated. Two models were employed: (i) [RuCl5(NO]2- complex treated at the B3LYP/SBKJC level surrounded by 50 water molecules treated in EFP formalism and (ii) [RuCl5(NO]2- and the 10 nearest water molecules at B3LYP/SBKJC surrounded by 40 water molecules at EFP level. The First model revealed a rapid liberation of NO in its radical form NO0, which occurs around 30 fs. The second model employed showed only a sequence of breaking and formation of Ru-Cl bond. These results show how the explicit treatment of the solvent may inuence the electronic transitions and also the dynamics of photorelease.