Abstract
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 -stacked structure defined by the dimer of a tetrameric building block indicate that unprotonated N atoms are significantly deshielded upon 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.
Highlights
IntroductionMelanins are a class of biopolymers derived from the tyrosine oxidation process, which make up the human pigment system, have antioxidant properties and can act as free radical scavengers (in particular, metal ions from oxidation-reduction processes), preventing cells against the oxidation process [1,2,3]
Melanins are a class of biopolymers derived from the tyrosine oxidation process, which make up the human pigment system, have antioxidant properties and can act as free radical scavengers, preventing cells against the oxidation process [1,2,3]
The number of water molecules in each micro-solvated structure varies according to the size of the eumelanin building blocks and different possibilities of making hydrogen bonds should be taken into account
Summary
Melanins are a class of biopolymers derived from the tyrosine oxidation process, which make up the human pigment system, have antioxidant properties and can act as free radical scavengers (in particular, metal ions from oxidation-reduction processes), preventing cells against the oxidation process [1,2,3]. There are two main classes of melanins: eumelanins, which give brown-black colors and pheomelanins, which favor yellow-red tones. Their main functions are associated with skin, hair and iris pigmentation and photoprotection [4,5]. They can play other important biological functions in degenerative diseases like the Parkinson’s disease (neurodegenerative disorder) [6,7] and age-related macular degeneration [8]. Eumelanins comprise molecular structures derived from the coupling of
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