Role of Solvent, pH, and Molecular Size in Excited-State Deactivation of Key Eumelanin Building Blocks: Implications for Melanin Pigment Photostability

 

Ultrafast time-resolved fluorescence spectroscopy has been used to investigate the excited state dynamics of the basic eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid  (DHICA) its acetylated, methylated and carboxylic ester derivatives as well as two oligomers, a dimer and a trimer in the O-acetylated forms. The results show that: 1) excited state decays are faster for the trimer relative to the monomer; 2) for parent DHICA, excited state lifetimes are much shorter in aqueous acidic medium (380 ps) as compared to organic solvent (acetonitrile, 2.6 ns); 3) variation of fluorescence spectra and excited state dynamics can be understood as a result of excited state intramolecular proton transfer (ESIPT). The dependence on DHICA oligomer size of excited state deactivation and its ESIPT mechanism provides important insight into the photostability and photoprotective function of eumelanin. Mechanistic analogies with the corresponding processes in DNA and other biomolecules are recognized.

Udgivelsesdato: Dec 08