Ultraviolet (UV) radiation that reaches the Earth's surface is a major cause of the many hazardous effects on humans’ skin. Those effects include erythema, photo-aging, and photoinduced DNA breakage, leading to mutations, responsible for tumour formation. The black-brown skin pigment eumelanin acts as a natural photoprotectant, effectively dissipating 99.9% of the incoming UV radiation into the heat on a timescale of a few picoseconds. The high degree of the disorder in the system and the ultrafast character of the energy dissipation complicates the study of the photoprotection nature of the pigment and the main dissipation mechanism remains obscure.<br/>To study the minimal functional unit responsible for the effective UV energy deactivation in eumelanin, we focus on one of the main building blocks 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Using the ultrafast spectroscopy methods, we revealed that the DHICA in its monomeric form exhibits an ultrafast ~ 4 ps excited state decay in physiological conditions of the human body. By placing DHICA in environments with different pH we could observe the role of the different proton transfers in the molecule. The pH dependent ultrafast spectroscopy studies revealed that the main dissipation mechanism in DHICA monomer in physiological conditions is the ultrafast excited state proton transfer from the phenol group to the surrounding solvent. This contradicts the previously discovered results stating that in monomeric form in a neutral environment DHICA would exhibit the slow excited state decay. This apparent controversy with the previously discovered DHICA behaviour lies in the way of the sample preparation and the product’s degradation. We demonstrate that the oxidative polymerization of DHICA leads to the elongation of the excited state lifetime compromising the results. These findings serve as the first step in a bottom-up study of the energy dissipation pathways in eumelanin pigment.

Bottom-up study of the ultrafast energy dissipation mechanism in black-brown skin pigment eumelanin

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