15N NMR shifts of eumelanin building blocks in water: a combined quantum mechanics/statistical mechanics approach
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2020
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Resumo
Theoretical results for the magnetic shielding of protonated and unprotonated nitrogens of
eumelanin building blocks including monomers, dimers, and tetramers in gas phase and water are
presented. The magnetic property in water was determined by carrying out Monte Carlo statistical
mechanics sampling combined with quantum mechanics calculations based on the gauge-including
atomic orbitals approach. The results show that the environment polarization can have a marked
effect on nitrogen magnetic shieldings, especially for the unprotonated nitrogens. Large contrasts
of the oligomerization effect on magnetic shielding show a clear distinction between eumelanin
building blocks in solution, which could be detected in nuclear magnetic resonance experiments.
Calculations for a π-stacked structure defined by the dimer of a tetrameric building block indicate that
unprotonated N atoms are significantly deshielded upon π stacking, whereas protonated N atoms
are slightly shielded. The results stress the interest of NMR experiments for a better understanding of
the eumelanin complex structure.
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Eumelanin, NMR, Photoprotective pigments, Solvent effects, 15N NMR shielding constants, π stacking
Citação
OLIVEIRA, Leonardo Bruno Assis; FONSECA, Tertius L.; CABRAL, Benedito J. C. 15N NMR shifts of eumelanin building blocks in water: a combined quantum mechanics/statistical mechanics approach. Molecules, Basel, v. 25, n. 16, e3616, 2020. DOI: 10.3390/molecules25163616. Disponível em: https://pubmed.ncbi.nlm.nih.gov/32784827/. Acesso em: 29 maio 2023.