Convergent proton-transfer photocycles violate mirror-image symmetry in a key melanin monomer

TitleConvergent proton-transfer photocycles violate mirror-image symmetry in a key melanin monomer
Publication TypeJournal Article
Year of Publication2007
AuthorsOlsen, Seth, Riesz J. J., Mahadevan I. B., Coutts A., Bothma J. P., Powell Ben J., McKenzie Ross H., Smith S. C., and Meredith Paul
JournalJournal Of The American Chemical Society
Volume129
Issue21
Pagination6672–+
AbstractWe present absorption and emission spectra of 5,6-dihydroxyindole-2-carboxylic acid (DHICA), a key melanin monomer, which violate mirror symmetry, and propose that this violation is due to convergent excited-state intramolecular proton-transfer photocycles. Dual features in the absorption spectra arise from excitation into the S-1 and S-2 states of a catecholate anion form of DHICA. Emission arises from the S-1 state of its proton-transfer conjugate following conversion via dual adiabatic and nonadiabatic reaction paths. In support of our postulated mechanism, we offer results from ab initio equation-of-motion coupled cluster (EOM-CCSD) and multistate multireference second-order perturbation theory (MS-MRPT2) calculations. Melanin pigments display an extremely broad, monotonically decreasing absorbance indicative of a dense, coupled manifold of excited states with varying localization character. Our results raise the possibility that intramonomer proton transfer may function as an energy dissipation mechanism from high-lying photoexcited states of the macromolecule.
DOI10.1021/ja069280u