The quest for energy traps in the CP43 antenna of photosystem II

To identify energy traps in CP43, a subcomplex of the photosystem II antenna system, site energies and excitonic couplings of the Q. transitions of chlorophyll (Chl) a pigments bound to CP43 are computed using electrostatic models of pigment-protein and pigment-pigment interactions. The computations are based on recent crystal structures of the photosystem II core complex with resolutions of 1.9 and 2.1 angstrom and compared to earlier results obtained at 2.9 angstrom resolution. Linear optical spectra (i.e., absorption, linear dichroism, circular dichroism, and fluorescence) are simulated using the computed excitonic couplings, a refinement fit for the site energies, and a dynamical theory of optical lineshapes. A comparison of the obtained root mean square deviation of about 100 cm(-1) between directly calculated and refined site energies with the maximum range of about 350 cm(-1) of directly calculated site energies shows that the combined quantum chemical/electrostatic approach provides a semi-quantitative agreement with experiment. Possible reasons for the deviations are discussed, including limits of the electrostatic models and the lineshape theory as well as structural alterations of CP43 upon detachment from the core complex. Based on the simulations, an assignment of the two low-energy exciton states A and B of CP43, that where observed earlier in hole burning studies, is suggested. State A is assigned to a localized exciton state on Chl 37 in the lumenal layer of pigments. State B is assigned to an exciton state that is delocalized over several pigments in the cytoplasmic layer. The delocalization explains the smaller inhomogeneous width of state B compared to state A observed in hole burning spectra, which is proposed to be due to exchange narrowing. The assignment of states A and B largely confirms our earlier suggestion that was based on a fit of linear optical spectra and electrostatic calculations using the 2.9