Excitation energy transfer in the LHC-II trimer: a model based on the new 2.72 Å structure
Energy transfer of the light harvesting complex LHC-II trimer, extracted from spinach, was studied in the Qy region at room temperature by femtosecond transient absorption spectroscopy. Configuration interaction exciton method [Linnanto et al. (1999) J Phys Chem B 103: 8739–8750] and 2.72 Å structural information reported by Liu et al. was used to calculate spectroscopic properties and excitation energy transfer rates of the complex. Site energies of the pigments and coupling constants of pigment pairs in close contact were calculated by using a quantum chemical configuration interaction method. Gaussian random variation of the diagonal and off-diagonal exciton matrix elements was used to account for inhomogeneous broadening. Rate calculations included only the excitonic states initially excited and probed in the experiments. A kinetic model was used to simulate time and wavelength dependent absorption changes after excitation on the blue side of the Qy transition and compared to experimentally recorded rates. Analysis of excitonic wavefunctions allowed identification of pigments initially excited and probed into later. It was shown that excitation of the blue side of the Qy band of a single LHC-II complex results in energy transfer from chlorophyll b’s of the lumenal side to chlorophyll a’s located primarly on one of the monomers of the stromal side.
Keywordsenergy transfer exciton femtosecond LHC-II light harvesting
The authors gratefully acknowledge the Chinese group of Liu et al., who sent us their accurate co-ordinates of the LHC-II trimer prior to publication in the Brookhaven Data Bank. M.Sc. Elina Wük is acknowledged for helping in the literature search (42). JL acknowledges the scholarship form the Finnish Cultural Foundation, Financial support (JM, RK) from Academy of Finland is acknowledged (Contracts No. 74003, 204557 and 205475).
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