UV-dissipation mechanisms in the eumelanin building block DHICA.
The UV-dissipative mechanisms of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and the 4,7-dideutero derivative (DHICA-d(2)) in buffered H(2)O or D(2)O have been characterized by using ultrafast time-resolved fluorescence spectroscopy. Excitation of the carboxylate anion form, the dominating state at neutral pH, leads to dual fluorescence. The band peaking at lambda=378 nm is caused by emission from the excited initial geometry. The second band around lambda=450 nm is owed to a complex formed between the mono-anion and specific buffer components. In the absence of complex formation, the mono-anion solely decays non-radiatively or by emission with a lifetime of about 2.1 ns. Excitation of the neutral carboxylic acid state, which dominates at acidic pH, leads to a weak emission around lambda=427 nm with a short lifetime of 240 ps. This emission originates from the zwitterionic state, formed upon excitation of the neutral state by sub-ps excited-state intramolecular proton transfer (ESIPT) between the carboxylic acid group and the indole nitrogen. Future studies will unravel whether this also occurs in larger building blocks and ESIPT is a built-in photoprotective mechanism in epidermal eumelanin.
Huijser
Annemarie
A
Department of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden. Annemarie.Huijser@chemphys.lu.se
Pezzella
Alessandro
A
Hannestad
Jonas K
JK
Panzella
Lucia
L
Napolitano
Alessandra
A
d'Ischia
Marco
M
Sundström
Villy
V
D017666:Deuterium Oxide / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007211:Indoles / Q000737:chemistry* / Q000528:radiation effects; D008543:Melanins / Q000737:chemistry*; D011522:Protons; D013050:Spectrometry, Fluorescence; D014466:Ultraviolet Rays; D014867:Water / Q000737:chemistry
D007211:Indoles; D008543:Melanins; D011522:Protons; D014867:Water; C041877:eumelanin; C030692:5,6-dihydroxy-2-indolylcarboxylic acid; D017666:Deuterium Oxide