Structure-based identification of energy sinks in plant light-harvesting complex II

The local S(0) -> S(1) transition energies (site energies) and corresponding excitonic couplings of chlorophyll a (Chla) and b (Chlb) pigments bound to trimeric, major light-harvesting complex II (LHCII) of higher plants are calculated on the basis of the two crystal structures (Liu et al. Nature 2004, 428, 287-292; Standfuss et al. EMBO J. 2005, 24, 919-928) by using a combined quantum chemical/electrostatic method (Muh et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 16862-16867) that has been modified to cover membrane proteins and to account more realistically for the behavior of protonatable groups under the conditions of low-temperature optical spectroscopy. The obtained exciton levels are in reasonable agreement with experimental information (including linear absorption, linear dichroism, circular dichroism, fluorescence spectra of native as well as wild-type-minus-mutant difference absorption spectra of recombinant LHCII) and differ from earlier treatments based on fitted site energies (Novoderezhkin et al. J. Phys. Chem. B 2005, 109, 10493-10504) mainly by assigning a lower energy level to Chla 604 (in the nomenclature of Liu et al.) and Chlb 608 and a higher energy level to Chlb 605 and 609. The energy sink at cyrogenic ...