Experimental steady-state absorption, emission, and nonresonant hole burning (HB) spectra (at T = 5 K) of the intact CP47 antenna complex of Photosystem II (PSII) from spinach are compared with Redfield simulations using Monte-Carlo disorder averaging and Nelder-Mead Simplex algorithm for parameter optimization. Excitonic calculations are based on the 1.9 Å resolution structure of the PSII core from cyanobacteria [Umena et al. Nature 2011, 473, 55] with coupling matrix elements calculated by the TrEsp method [Madjet et al. J. Phys. Chem. B 2006, 110, 17268]. Exciton relaxation and energy transfer is described by the Redfield and Generalized Fӧrster theories. A refined set of site energies is obtained by computerized optimization cycles simultaneously fitting absorption, emission, and HB spectra. The resulting excitonic structure is consistent with the experimentally observed red shifted (~695 nm) emission maximum at both T = 5 and 77 K and properly describes the shape of the nonresonant hole, whose shape (in particular the shape of the anti-hole) is very sensitive to the chlorophyll (Chl) site energies. Importantly our simulated spectra, as well as assignment of site energies, differ significantly from those reported recently by Shibata et al. [J. Am. Chem. Soc. 2013, 135, 6903] for CP47, primarily due to different spectral densities used and differences in the experimental data simulated. In particular, we argue that a lowest state localized on Chl 526, as reported in [Shibata et al. J. Am. Chem. Soc. 2013, 135, 6903], possesses too strong oscillator strength to fit low-temperature experimental data of intact CP47 antenna complexes. Instead, we confirm our earlier findings [Reppert et al., J. Phys. Chem. B 2010, 114, 11884] that Chl 523 strongly contributes to the lowest excitonic state, with Chl 526 mostly contributing to the second excitonic state. Since the fits of nonresonant holes are more restrictive (in terms of possible site energies) than those of absorption and emission spectra, we suggest that fits of linear optical spectra along with HB spectra provide more realistic site energies. We also show that recently reported emission of PSII core complex from T. vulcanus (simulated recently in [Shibata et al. J. Am. Chem. Soc. 2013, 135, 6903]) does not represent intact protein complexes. Possible implications of our findings on structure-function relationships are briefly discussed.

Critical assessment of the low-temperature optical spectra of the CP47 antenna protein complex of Photosystem II: Revised structural assignments

249th National Meeting (2015)

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