18
1 Melanin Chemistry
As described above, it has been long considered that eumelanin is a large
extended polymer. However, X-ray diffraction study using synchrotron radiation
rather suggested a stacked oligomer model as the melanin structure, in which about
4–5 monomers are bonded in a plane, and these protomolecules are stacked by
π-π stacking in about four layers. [78, 79]. This model was later supported by
observation using scanning tunneling microscopy (STM) [80]. Thus, the “chemical disorder model”, which regards melanins as amorphous materials consisting of
various oligomers, has become gradually accepted [81]. However, it should also be
noted that the proposed planar oligomers have not yet been isolated in experiments,
and only one-dimensional oligomers have been obtained as described above.
For DHI and IQ, first-principles calculations based on density functional theory
(DFT) were performed [82, 83], and the electronic excitation energies were also
calculated [84, 85]. Result confirmed that IQ has the electronic excitation energy
lower than that of DHI. First-principles calculations, which were performed for
DHICA and its oxidants (IQ and SQ), revealed that the presence of 2-carboxyl group
significantly affects the stability and the HOMO–LUMO gap of the various oxidation
forms [86]. In comparison with DHI-derived eumelanins, which are known to have
considerable amounts of radicals, this study indicated that DHICA-derived eumelanins should contain negligibly small amounts of radicals. The electronic structure
of DHICA dimer was also calculated, showing a red-shifted electronic excitation
energy with respect to that of monomer [87]. These studies suggest that the HOMO–
LUMO gaps of eumelanin monomers and their oligomers are highly dependent on
their structures. This qualitatively explains the broad UV-visible absorption profile
of eumelanin [81, 87].
Okuda et al. calculated the reactivity of DHI and its dimer using an index called the
general-purpose reactivity indicator [88, 89] devised by Anderson and others [90].
This takes into account both effects of the electrostatic interaction and the charge
transferring interaction. This is calculated using the atomic charges and the condensed
Fukui function, the latter of which is the derivative of the atomic charge with respect
to the total electron number with a negative sign [90–92]. The calculated reactivity
results are consistent with the experimentally identified oligomers (dimers, trimers,
and tetramers). For example, 2-carbon in DHI was shown to be highly reactive.
Easier formation of tetramers was also indicated as compared to formation of trimers.
The reactivity indicators showed that these reactions are mainly predominated by
interactions involving charge transfer.
In this section, the major studies on the oxidative polymerization processes and the
structural model of eumelanin were reviewed. Eumelanogenesis is a very complicated
process, and its analysis is also difficult. There are still points that have not been
clarified yet. However, by continuous efforts from both experimental and theoretical
approaches, chemistry of eumelanogenesis is getting established as compared to
pheomelanin production (pheomelanogenesis).
1 Melanin Chemistry
As described above, it has been long considered that eumelanin is a large
extended polymer. However, X-ray diffraction study using synchrotron radiation
rather suggested a stacked oligomer model as the melanin structure, in which about
4–5 monomers are bonded in a plane, and these protomolecules are stacked by
π-π stacking in about four layers. [78, 79]. This model was later supported by
observation using scanning tunneling microscopy (STM) [80]. Thus, the “chemical disorder model”, which regards melanins as amorphous materials consisting of
various oligomers, has become gradually accepted [81]. However, it should also be
noted that the proposed planar oligomers have not yet been isolated in experiments,
and only one-dimensional oligomers have been obtained as described above.
For DHI and IQ, first-principles calculations based on density functional theory
(DFT) were performed [82, 83], and the electronic excitation energies were also
calculated [84, 85]. Result confirmed that IQ has the electronic excitation energy
lower than that of DHI. First-principles calculations, which were performed for
DHICA and its oxidants (IQ and SQ), revealed that the presence of 2-carboxyl group
significantly affects the stability and the HOMO–LUMO gap of the various oxidation
forms [86]. In comparison with DHI-derived eumelanins, which are known to have
considerable amounts of radicals, this study indicated that DHICA-derived eumelanins should contain negligibly small amounts of radicals. The electronic structure
of DHICA dimer was also calculated, showing a red-shifted electronic excitation
energy with respect to that of monomer [87]. These studies suggest that the HOMO–
LUMO gaps of eumelanin monomers and their oligomers are highly dependent on
their structures. This qualitatively explains the broad UV-visible absorption profile
of eumelanin [81, 87].
Okuda et al. calculated the reactivity of DHI and its dimer using an index called the
general-purpose reactivity indicator [88, 89] devised by Anderson and others [90].
This takes into account both effects of the electrostatic interaction and the charge
transferring interaction. This is calculated using the atomic charges and the condensed
Fukui function, the latter of which is the derivative of the atomic charge with respect
to the total electron number with a negative sign [90–92]. The calculated reactivity
results are consistent with the experimentally identified oligomers (dimers, trimers,
and tetramers). For example, 2-carbon in DHI was shown to be highly reactive.
Easier formation of tetramers was also indicated as compared to formation of trimers.
The reactivity indicators showed that these reactions are mainly predominated by
interactions involving charge transfer.
In this section, the major studies on the oxidative polymerization processes and the
structural model of eumelanin were reviewed. Eumelanogenesis is a very complicated
process, and its analysis is also difficult. There are still points that have not been
clarified yet. However, by continuous efforts from both experimental and theoretical
approaches, chemistry of eumelanogenesis is getting established as compared to
pheomelanin production (pheomelanogenesis).