How the molecular structure determines the flow of excitation energy in plant light-harvesting complex II

Abstract

Excitation energy transfer in the light-harvesting complex II of higher plants is modeled using excitonic couplings and local transition energies determined from structure-based calculations recently ( Müh et al., 2010). A theory is introduced that implicitly takes into account protein induced dynamic localization effects of the exciton wavefunction between weakly coupled optical and vibronic transitions of different pigments. Linear and non-linear optical spectra are calculated and compared with experimental data reaching qualitative agreement. High-frequency intramolecular vibrational degrees of freedom are found important for ultrafast subpicosecond excitation energy transfer between chlorophyll (Chl) b and Chl a, since they allow for fast dissipation of the excess energy. The slower ps component of this transfer is due to the monomeric excited state of Chl b 605. The majority of exciton relaxation in the Chl a spectral region is characterized by slow ps exciton equilibration between the Chl a domains within one layer and between the lumenal and stromal layers in the 10–20 ps time range. Subpicosecond exciton relaxation in the Chl a region is only found within the terminal emitter domain (Chls a 610/611/612) and within the Chl a 613/614 dimer. Deviations between measured and calculated exciton state life times are obtained for the intermediate spectral region between the main absorbance bands of Chl a and Chl b that indicate that besides Chl b 608 another pigment should absorb there. Possible candidates, so far not identified by structure-based calculations, but by fitting of optical spectra and mutagenesis studies, are discussed. Additional mutagenesis studies are suggested to resolve this issue.