Melanins represent an important class of natural pigments present in plants and animals and have been considered as a promising material for semiconducting applications. However, despite their interesting opto-electronic properties, until now there is no satisfactory understanding of some basic properties of these materials, such as the macromolecular structure, origin of intrinsic paramagnetism and absorption and luminescence features. In particular, the presence of stable paramagnetic centers evidenced by persistent ESR signals is frequently reported in the literature.
These centers are sensitive to several external factors such as pH, temperature, illumination and the presence of oxidizing/reducing agents, in such way that a wide range of distinct spectroscopic parameters have been reported. In order to better understand the origin of these paramagnetic centers, in this work we have employed electronic structure calculations to evaluate Spin Hamiltonian (SH) parameters.
The ground-state geometries were fully optimized in a DFT approach with Becke’s LYP (B3LYP) exchange-correlation functional and 6-31G basis set. The SH parameters were obtained in a DFT/B3LYP approach, employing 6-31G** and EPRII basis sets. The calculations were carried out with GAMESS (optimization)  and ORCA (SH parameters)  computer packages. The results confirm the presence of at least two groups with different values for g-factor identified in the literature as centered carbon radicals (CCRs) and semiquinone free radicals (SFRs) . In particular, CCRs could be associated with anionic hydroquinone structures (HQ) and N+ hydroquinone defects (Ndef); SFRs were compatible with the semiquinone (SQ), anionic indolquinone (IQ) and anionic quinine-imine (QI) structures. These trends were confirmed in dimmers structures, suggesting a strong localization of the paramagnetic centers.
 P. Meredith, T. Sarna. Pigment Cell Res, Vol. 19(2006) 572-594.
 M. W. Schmidt et al. J. Comput. Chem. Vol. 14 (1993) 1347-1363.
 F. Neese. Wiley Interdiscip. Rev. Comput. Mol. Sci. Vol. 2 (2012) 73-78. A.B. Mostert, et al. J. Phys. Chem. B, 117 (2013) 4965-4972.