1.4 Biosynthesis of Eumelanin—Oxidative Polymerization to Form Eumelanin
17
several days (100 °C for 18 days and 40 °C for 180 days), and chemical degradation
produced increased amounts of PTeCA, reproducing the aging [72].
1.4.2 Theoretical Study of Monomer Polymerization
and Melanin Structure Model
The above introduced experiments have successfully identified up to tetramers of DHI
or DHICA homo-oligomers, and dimers of DHI-DHICA hetero-oligomers. However,
the extent to which these coupling reactions proceed and how the reactivity changes
by the polymerization remains unclear at present. For this reason, studies have been
made on approaches based on theoretical calculations for the structures and properties
of various oligomers composed of DHI or DHICA.
In early theoretical studies, the eumelanin structure was usually modeled by
macromolecules, which were simply built by means of polymerized DHI and/or
IQ (through coupling with IQ and/or DHI, respectively). Based on this model, H.C.
Longuet-Higgins proposed that a mainly DHI-derived eumelanin should behave like
a p-type semiconductor, while a mainly IQ-derived eumelanin is likely to behave as
an n-type semiconductor [73].
Since both DHI and IQ have closed-shell electron configurations, the band theory
will not find the presence of conduction carriers (electrons or holes) in the lower
excited states, if only DHI and IQ constructed eumelanin in a straightforward manner.
Experiments using electron spin resonance (ESR) have revealed that melanin exhibits
paramagnetism, indicating that some of the monomer units of melanin have unpaired
electrons [74]. Since semiquinone (SQ) obtained by one-electron oxidation of DHI
has an unpaired electron, conduction carriers (electrons) can be generated when
SQs are partially contained in eumelanin to create donor levels. Similarly, partially
SQ-substituted IQ polymers, which arises from one-electron reduction, would have
conducting holes with acceptor levels.
Pullman and Pullman employed Hückel approximation (or tight-binding approximation) to calculate the molecular orbital of the dimer linked at 3–7’ carbons of
two IQs [75]. As a result, the lowest unoccupied molecular orbitals (LUMOs) of IQ
overlapped to each other to create a bonding orbital, yielding a decreased energy gap
between the highest occupied orbital (HOMO) and LUMO (HOMO–LUMO gap).
It was then considered that the band gap of eumelanin could be smaller when IQ
content was increased with respect to the other monomer units (DHI and SQ).
However, a study of the band calculation for a one-dimensional polymer, in which
IQs were periodically linked at the same binding sites, showed that the band gap
became larger than that of the dimer [76]. Furthermore, the band gap of the onedimensional polymer of SQ (at least in the case of 3–7’ bonded structure) is smaller
than that of IQ, and the one-dimensional polymer of DHI has a higher band gap than
that of IQ with smaller band dispersion [77].
17
several days (100 °C for 18 days and 40 °C for 180 days), and chemical degradation
produced increased amounts of PTeCA, reproducing the aging [72].
1.4.2 Theoretical Study of Monomer Polymerization
and Melanin Structure Model
The above introduced experiments have successfully identified up to tetramers of DHI
or DHICA homo-oligomers, and dimers of DHI-DHICA hetero-oligomers. However,
the extent to which these coupling reactions proceed and how the reactivity changes
by the polymerization remains unclear at present. For this reason, studies have been
made on approaches based on theoretical calculations for the structures and properties
of various oligomers composed of DHI or DHICA.
In early theoretical studies, the eumelanin structure was usually modeled by
macromolecules, which were simply built by means of polymerized DHI and/or
IQ (through coupling with IQ and/or DHI, respectively). Based on this model, H.C.
Longuet-Higgins proposed that a mainly DHI-derived eumelanin should behave like
a p-type semiconductor, while a mainly IQ-derived eumelanin is likely to behave as
an n-type semiconductor [73].
Since both DHI and IQ have closed-shell electron configurations, the band theory
will not find the presence of conduction carriers (electrons or holes) in the lower
excited states, if only DHI and IQ constructed eumelanin in a straightforward manner.
Experiments using electron spin resonance (ESR) have revealed that melanin exhibits
paramagnetism, indicating that some of the monomer units of melanin have unpaired
electrons [74]. Since semiquinone (SQ) obtained by one-electron oxidation of DHI
has an unpaired electron, conduction carriers (electrons) can be generated when
SQs are partially contained in eumelanin to create donor levels. Similarly, partially
SQ-substituted IQ polymers, which arises from one-electron reduction, would have
conducting holes with acceptor levels.
Pullman and Pullman employed Hückel approximation (or tight-binding approximation) to calculate the molecular orbital of the dimer linked at 3–7’ carbons of
two IQs [75]. As a result, the lowest unoccupied molecular orbitals (LUMOs) of IQ
overlapped to each other to create a bonding orbital, yielding a decreased energy gap
between the highest occupied orbital (HOMO) and LUMO (HOMO–LUMO gap).
It was then considered that the band gap of eumelanin could be smaller when IQ
content was increased with respect to the other monomer units (DHI and SQ).
However, a study of the band calculation for a one-dimensional polymer, in which
IQs were periodically linked at the same binding sites, showed that the band gap
became larger than that of the dimer [76]. Furthermore, the band gap of the onedimensional polymer of SQ (at least in the case of 3–7’ bonded structure) is smaller
than that of IQ, and the one-dimensional polymer of DHI has a higher band gap than
that of IQ with smaller band dispersion [77].