Fundamental building blocks of eumelanins: electronic properties of indolequinone-dimers

Abstract

We present results from the theoretical INDO calculations of the electronic structure for stacked eumelanins' monomers. As basic indolic components of the eumelanin structure 5,6-dihydroxyindole (DHI or HQ) and its oxidized forms (SQ and IQ) were chosen. The results reveal dependency of electronic properties of such aggregates on monomers' redox states. They point out also a tendency to localize an extra charge on one of dimer's subunits that could be suggestive of an electron hopping as a model mechanism for the electron transfer in eumelanins.

5,6-Dihydroxyindole-2-carboxylic acid: a first principles density functional study

“…Comparing our results for DHICA with previously published results for DHI we found that that the addition of the carboxylic acid group has a significant effect on the physical properties of the molecules and in particular on the relative stability of the various redox forms and the HOMO-LUMO gap. Based on the calculated relative stabilities of the various redox forms in their various charge states we find predict that there will be an extremely low unpaired electron density in samples of pure DHICA (this is not the case for DHI [14,15]) and thus a negligible signal should be seen in ESR experiments. This indicates that experimental results cannot be straightforwardly extrapolated from DHICA to DHI or vice versa.…”

“…Recently the role of disorder in determining the physical properties of the melanins has been emphasised [10,14,15]. In particular it has been proposed that the broad band monotonic optical absorption of eumelanin arises from the existence of a large number of different monomers and hence a wide variety of macromolecules.…”

“…Despite the widespread agreement that natural eumelanin contains both DHI and DHICA, previous quantum chemical studies [14,15,17,18,19,20,21,22,23] have only considered DHI. This is in part due to the greater chemical complexity of DHICA, but also in part due to assumption that the carboxylation of DHI does not play a significantly role in determining the properties eumelanin.…”

“…This is in part due to the greater chemical complexity of DHICA, but also in part due to assumption that the carboxylation of DHI does not play a significantly role in determining the properties eumelanin. However, given the potential importance of the role of disorder in the physical properties of eumelanin [10,14,15] a careful study of the similarities and differences between DHI and DHICA is clearly required. Our study is also motivated by recent work on synthetic DHICA [24] and the need for high quality first principles calculations to compare with future experiments on synthetic DHICA.…”

The physical and chemical properties of eumelanin

“…twice the stacking height suggested by Cheng et al (1994a,b). Notable and important theoretical studies aimed at predicting the optical properties of eumelanin based upon the oligomeric model include Stark et al (2003aStark et al ( ,b, 2005 and Bochenek and Gudowska-Nowak (2003). In further support of the oligomeric model Pezzella et al (1997b) have published a number of MALDI mass spectroscopic studies -one of which identifies several oligomeric Raper-Mason species from natural Sepia Officianalis melanin.…”