The CP43 Proximal Antenna Complex of Higher Plant Photosystem II Revisited: Modeling and Hole Burning Study. I

The CP43 core antenna complex of photosystem II is known to possess two quasi-degenerate “red”-trap states ( Jankowiak R. et al. J. Phys. Chem. B 2000, 104, 11805 ). It has been suggested recently ( Zazubovich V. ; Jankowiak R. J. Lumin. 2007, 127, 245 ) that the site distribution functions of the red states (A and B) are uncorrelated and that narrow holes are burned in the subpopulations of chlorophylls (Chls) from states A and B that are the lowest-energy Chl in their complex and previously thought not to transfer energy. This model of uncorrelated excitation energy transfer (EET) between the quasidegenerate bands is expanded by taking into account both electron−phonon and vibrational coupling. The model is applied to fit simultaneously absorption, emission, zero-phonon action, and transient hole burned (HB) spectra obtained for the CP43 complex with minimized contribution from aggregation. It is demonstrated that the above listed spectra can be well-fitted using the uncorrelated EET model, providing strong evidence for the existence of efficient energy transfer between the two lowest energy states, A and B (either from A to B or from B to A), in CP43. Possible candidate Chls for the low-energy A and B states are discussed, providing a link between CP43 structure and spectroscopy. Finally, we propose that persistent holes originate from regular NPHB accompanied by the redistribution of oscillator strength due to excitonic interactions, rather than photoconversion involving Chl−protein hydrogen bonding, as suggested before ( Hughes J. L. et al. Biochemistry 2006, 45, 12345 ). In the accompanying paper ( Reppert M. Zazubovich V. ; Dang N. C. Seibert M. Jankowiak R. J. Phys. Chem. B 2008, 9934 ), it is demonstrated that the model discussed in this manuscript is consistent with excitonic calculations, which also provide very good fits to both transient and persistent HB spectra obtained under non-line-narrowing conditions.