Photophysics of eumelanin: ab initio studies on the electronic spectroscopy and photochemistry of 5,6-dihydroxyindole.
Excited-state reaction paths and energy profiles of 5,6-dihydroxyindole (DHI), one of the elementary building blocks of eumelanin, have been determined with the approximated singles-and-doubles coupled-cluster (CC2) method. 6-Hydroxy-4-dihydro-indol-5-one (HHI) is identified as a photochromic species, which is formed via nonadiabatic hydrogen migration from the dangling OH group of DHI to the neighboring carbon atom of the six-membered ring. It is shown that HHI is a typical excited-state hydrogen-transfer (ESIHT) system. HHI absorbs strongly in the visible range of the spectrum. A barrierless hydrogen transfer in the (1)pipi* excited state, followed by barrierless torsion of the hydroxyl group, lead to a low-lying S(1)-S(0) conical intersection and thus to ultrafast internal conversion. This very efficient mechanism of excited-state deactivation provides HHI with a high degree of intrinsic photostability. It is suggested that the metastable photochemical product HHI plays an essential role for the photoprotective biological function of eumelanin.
Sobolewski
Andrzej L
AL
Institute of Physics, Polish Academy of Sciences, 02668 Warsaw, Poland. sobola@ifpan.edu.pl
Domcke
Wolfgang
W
D006859:Hydrogen / Q000737:chemistry*; D007211:Indoles / Q000737:chemistry*; D008543:Melanins / Q000737:chemistry*; D008968:Molecular Conformation; D010777:Photochemistry; D013057:Spectrum Analysis
D007211:Indoles; D008543:Melanins; C041877:eumelanin; D006859:Hydrogen; C033871:5,6-dihydroxyindole