Propriedades estruturais e espectroscópicas de modelos de vitamina E e de constituintes da eumelanina em água e em mistura de solventes
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2015-10-02
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Universidade Federal de Goiás
Resumo
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.
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OLIVEIRA, L. B. A. Propriedades estruturais e espectroscópicas de modelos de vitamina E e de constituintes da eumelanina em água e em mistura de solventes. 2015. 132 f. Tese (Doutorado em Física) - Universidade Federal de Goiás, Goiânia, 2015.