Ultrafast dynamics of the eumelanin pigment

Ultraviolet (UV) solar radiation can cause many hazardous effects in human body. It is highly carcinogenic; exposure to the sun radiation is leading to the malignant melanoma – the most aggressive and widely spread skin cancer. Most organic (including bioorganic) molecules can absorb the UV light which leads to the DNA photo-damage and the further mutations like tumour formation.

The black-brown pigment eumelanin found in human skin acts as a vital barrier to the UV light. With an ingenious ability to effectively dissipate 99.9% of the incoming UV energy as heat it has shown itself as a natural photo-protectant. The effective photoprotection of the pigment originates in its absorption spectrum. Unlike other organic molecules, eumelanin possesses a broadband featureless spectrum, rising in absorption coefficient toward the UV range, providing the increased photoprotection against the most damaging high energy photons. Eumelanin dissipates the absorbed energy on the picosecond timescale before the damage can occur. Therefore, the complexity of the pigment structure, as well as the ultrafast character of the energy dissipation complicate the characterization of the main energy dissipation pathway.

Over the last years, the pigment was extensively studied using different spectroscopic methods. To overcome the complexity of eumelanin's structure the main building blocks 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) were studied. Ultrafast spectroscopy studies of eumelanin and its constituent building blocks have variously suggested the importance of excited state intramolecular proton transfer in oligomers, proton-coupled electron transfer with water, as well as charge transfer states. Significant inconsistencies between these studies raise questions about the validity of conclusions, and even whether the material measured was in the state assumed.

The bottom-up approach to eumelanin photophysics through its building block DHICA is reflected in Chapter 3 of this thesis. We used the combination of transient absorption and photoluminescence techniques to get a full picture of the excited state behaviour, as well as to track the possible formation of the new species due to degradation. The new way of preparing the samples in oxygen-free conditions revealed a new insight in the building block's photophysics. We then studied both proposed proton transfer mechanisms by placing DHICA monomer in different experimental environments, created by various solvents. We conclude that DHICA intrinsically decays on a 4-ps timescale via an excited-state proton transfer mechanism.

To investigate the role of oligomerization in the excited state decay rate we performed ultrafast spectroscopy studies on DHICA oligomers prepared in two different ways. Chapter 4 of the thesis explains the discrepancy that emerged between our results and the ones previously published. It was revealed that the presence of oxygen in results in oxygenated polymerization of the sample and the elongation of the excited state lifetime. This effect originated in the negative charge formation that created a barrier to proton transfer to occur. The study of the synthesized DHICA dimer demonstrated that its excited state decay is matching that of DHICA monomer. These studies revealed the importance of DHICA monomer, possessing ultrafast excited state proton transfer energy dissipation mechanism.

Chapter 5 of this thesis investigates the different hypotheses about the photophysics of eumelanin. By combining the computational and experimental methods we reveal the dynamic role of the disorder in eumelanin and the role of immobile CT states that was proposed as a possible energy dissipation mechanism. Fluence and excitation dependent transient absorption spectroscopy revealed that the excitations in eumelanin are immobile and confined to a small volume. The comparison of the excited state decays of DHICA monomer and eumelanin revealed that these samples share the same decay rate. This suggests that eumelanin possesses the same excited state proton transfer mechanism as DHICA.

Taken together this thesis provides a deep and valuable insight of eumelanin photophysics using time-resolved ultrafast spectroscopic techniques. The results of the studies of both eumelanin aggregates and DHICA key building block provide enough evidence to propose excited state proton transfer as the main excited state deactivation mechanism.