The body color is essentially controlled by melanogenesis, which is the production of natural pigments, i.e. melanins. Melanogenesis can be generally defined as a cascade of multiple reactions, which is triggered by oxidation of p-substituted phenols or catechols like tyrosine or dopa. Oxidation of these phenols or catechols results in formation of reactive o-quinones.

Some o-quinones can spontaneously undergo cyclization, which is ring-closure reaction by bonding between benzene ring carbon and p-substituent. In contrast, o-quinones can also react with thiols (R-SH) like cysteine. As the final product, the cyclization results in formation of black to brown eumelanin whereas the binding of cysteine results in formation of yellow to reddish brown pheomelanin. This can be said as the first branching of melanogenesis.

The cyclization is followed by redox exchange between the cyclized molecule and a remaining o-quinone. A molecule resulting from the redox exchange is still unstable so that it undergoes further conversion. This conversion involves intramolecular proton rearrangements. In addition to proton rearrangements, this conversion can also undergo decarboxylation, which is desorption of -COOH, if the melanogenic substrate has a carboxyl group (-COOH) in p-substituent. Two types of monomers of eumelanin are directly generated by the decarboxylative and non-decarboxylative pathways, respectively. Thus, this decarboxylative/non-decarboxylative conversion process can be said as the second branching of melanogenesis.

In physiological melanin synthetic conditions, the first branching is the reaction involving dopaquinone, which is an o-quinone obtained by tyrosine/dopa oxidation, and the second branching is the reaction involving dopachrome, which is a molecule obtained by cyclization dopaquinone and its redox exchange. In our recent work, we have conducted mechanistic investigation for melanogenesis with a focus on dopaquinone and dopachrome reactions from density functional theory based calculation [1-5]. Mechanistic understanding of the branching reactions is a key to predict or design the properties and functions of melanin. We make use a theoretical approach in which quantum mechanics is applied to molecular systems.

Our results show that the rate-limiting step of dopachrome conversion is deprotonation from β-carbon (β-deprotonation) at the first step. Furthermore, the subsequent steps were found to be regulated by the protonation state of the quinonoid oxygens, which are zwitterionic oxygens bound with benzene ring. Our calculations reveal that the quinonoid protonation facilitates decarboxylation. Cu(II) coordinated by quinonoid oxygens was found to protect the oxygens against protonation, leading to suppression of decarboxylation.

For dopaquinone, we investigate the first elementary step of the cyclization and the thiol binding. To evaluate the effects of substituent bound with the benzene ring, we calculate some dopaquinone-like systems, viz. dopaminequinone, dopaquinone, N-methyldopaminequinone, N-formyldopaminequinone, and the corresponding methylene (-CH₂-)-inserted analogues. Our results show that quinones presenting their highest occupied molecular orbital (HOMO) at a higher energy level are reactive for cyclization, and that an anti-bonding shape of lowest unoccupied molecular orbital (LUMO) at a higher energy level in the benzene-substituent region is a signature for low reactivity toward thiol compounds.

This view obtained here reasonably explains previously reported experimental results and would provide a physico-chemical basis of melanin synthesis. Our results will be clues for designing new materials using melanin.

References

[1] R. Kishida et al., Pigment Cell Melanoma Res. 27 (2014) 734-743.

[2] R. Kishida et al., Biochim. Biophys. Acta 1850 (2015) 281-286.

[3] R. Kishida et al., J. Electron. Mater. 46 (2017) 3784-1-5.

[4] R. Kishida et al., J. Phys. Soc. Jpn. 26 (2017) 024804-1-5.

[5] R. Kishida et al., Int. J. Quant. Chem. 117 (2017) e25445-1-9.