Photochemistry of Eumelanin Precursors Role of Excited State Proton Transfer for UV Photoprotection

Melanin is an epidermal pigment commonly known to give darker skin coloration under sun exposure. It is also present in the hair, eyes, inner ear and brain. The first function of epidermis melanin is believed to be photoprotective against harmful ultraviolet (UV) light, but the recent increase of skin cancer correlated to an increase of sun exposure questions the properties of melanin. Its presence in different body parts suggests that its function is not solely protective against UV-light. Melanin in epidermis is divided in two categories eumelanin responsible of the dark coloration and pheomelanin, which does not have great influence on the skin shade, but gives the red coloration of hair. The amount of skin cancer has been observed to be greater in patients presenting a fair type of skin. The mechanism after melanin UV absorption is poorly understood. Two main problems arise in the study of melanin photochemistry. First the pigment is believed to be an oligomer assembly of different sizes, resulting in a broad heterogeneity of a studied sample, which makes the distinction of active species difficult. On the other hand, this is probably a key property of melanin, to ensure a photoprotective barrier against especially UV-light. The second main difficulty in the study of melanin is the solubility. The larger the pigment the less soluble in aqueous solution. An additional issue in the study of melanin is the reproducibility of the sample. The work presented here focuses on eumelanin and its interaction with UV-light. With help of fluorescence steady state and time-resolved methods we have investigated eumelanin photochemistry. We present here a model of the energy dissipation mechanism of the pigment after UV absorption. Our method is based first on the study of synthetic samples, which allows us to have control over the heterogeneity and thus identify the function of each molecule involved in the whole melanin structure. Secondly, we have performed a bottom up approach, starting with the study of monomer constituents up to the polymer. Moreover, we have developed a method to solubilize the polymer, which does not interfere with the photodynamics of the molecules. We demonstrate that the main dissipation channel of eumelanin after UV absorption in aqueous solution is controlled by Excited State Proton Transfer (ESPT). The surrounding solvent is essential to have a rapid and efficient UV dissipation on the order of hundreds of femtoseconds. We show that the melanin precursor DHICA, in its polymeric form, is much more efficient than the DHI precursor in the dissipation mechanism. Our approach brings new insight to the eumelanin photochemistry and shows that one of the eumelanin components has great photoprotection properties against UV-light, while the other one present longer excited state lifetimes that leave more time to the molecule to produce radicals and reactive species, possibly responsible of melanoma formation. We hope to have brought a better understanding to the property of the black polymer and opened a way to deepen the study of melanin and its interaction with UV-light.