Time Resolved EPR investigation on oxygen and temperature effects on a synthetic eumelanin

Eumelanins are the fundamental components of the mammalian pigmentary system. Their biological functions, in all the different localization, e.g. the skin or the retinal pigment epithelium, are influenced by uncommon physicochemical features [1]. Eumelanin properties include a permanent EPR signal due to a number of intrinsic quinone/semiquinone like radicals and a reversibly generated EPR signal under UV and visible irradiation, that suggests the formation of additional extrinsic radicals. Recently, TR-EPR studies have demonstrated that in the photoproduction of new melanin free radicals, they are generated as Radical Pairs from the triplet manifold [2]. Here, we compare photoreactivity of a synthetic eumelanins as function of the temperature in the range 290-140K. Moreover, we analyze the electron spin polarization of the EPR signal in the presence and in the absence of oxygen. TR-EPR spectra of samples under vacuum exhibit only a net emission after the exciting laser pulse, while for sample in equilibrium with oxygen in the air an additional distinct pattern consisting of an enhanced absorption followed by emission is present at the earliest times (delay<1ms). It is possible that the spin photochemistry of melanin determines its observable photoreactivity.