Propriedades estruturais e espectroscópicas de modelos de vitamina E e de constituintes da eumelanina em água e em mistura de solventes

• Autor(a) principal: Oliveira, Leonardo Bruno Assis
• Data de Publicação: 2015
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Repositório Institucional da UFG
• Texto Completo: http://repositorio.bc.ufg.br/tede/handle/tede/5229
• topic: Ressonância magnética nuclear; Efeito de solvente; Mistura de solventes; Nuclear magnetic resonance; Effect of solvent; Solvents mixtures; CIENCIAS EXATAS E DA TERRA::FISICA

We present in this work a theoretical study of structural and spectroscopic properties of vitamin E models in chloroform and water solutions, of eumelanin oligomers in water and also of eumelanin constituents in mixtures of methanol and water. The in uence of solvent e ects in molecular properties was included using an iterative process based on sequential methodology Monte Carlo/Quantum Mechanics. Optical absorption spectra were obtained from TD-DFT calculations with B3LYP, PBE1PBE, BHandHLYP and CAMB3LYP functionals whereas the magnetic shielding constants were calculated employing the gauge-including atomic orbital (GIAO) approach through B3LYP functional. We analyze four homologous forms of vitamin E models, which di er in the number and position of methyl groups on the chromanol ring. The inclusion of solvent e ects in the rst electronic transition indicates solvatochromic shifts ranging between 1 and 3 nm in chloroform, and between 7 and 11 nm in water. The results also show that these e ects on the magnetic shielding constants (13C) and (17O) are small, but an appropriate description of the solvent shift for (17O) of the hydroxyl group in water requires the inclusion of explicit molecules that make hydrogen bonds. Our results for the chemical shifts (13C) of the carbon atoms of the chromanol ring in chloroform solution are in good agreement with the experimental results, although overestimated between 3 and 9 ppm. The analysis of the con gurations obtained from Monte Carlo simulations shows that for eumelanin oligomers of increasing size, monomers, dimers, tetramers and a layer, there is an increase in the average total number of hydrogen bonds. Solvent e ects are particularly relevant for the NMR spectroscopy of nitrogen and oxygen atoms. The shielding constant (17O) may change up to 98% for the monomers, when we consider the presence of explicit water molecules, and up to 41% for the tetramers from calculations with ASEC (Average Solvent Eletrostatic Con guration). In general, the oligomerization of eumelanin constituents extends the region of the electromagnetic spectrum which occurs the optical absorption and also shows a shift of the absorption peak of the HOMO!LUMO transitions for longer wavelength regions. We also present a superposition of the absorption spectra of eumelanin constituents that shows the trend of experimental results. We select some eumelanin constituents to analyze the e ects of mixtures of methanol and water on the spectroscopic properties. The results indicate that the concentration of water in the mixture can in uence the magnetic shielding constants of more electronegative atoms, as well as the position of the absorption spectrum peaks. The results show that the values of can present variations up to 34% while the location of the absorption peak can change up to 73 nm in going from pure methanol to pure water. We also analyze the distribution of the methanol and water molecules around the eumelanin constituents studied in order to determine the preferential solvation.