Excitation energy transfer in the LHC-II trimer: From carotenoids to chlorophylls in space and time

Exciton model for description of experimentally determined excitation energy transfer from carotenoids to chlorophylls in the LHC-II trimer of spinach is presented. Such an approach allows connecting the excitonic states to the spatial structure of the complex and hence descriptions of advancements of the initially created excitations in space and time. Carotenoids were excited at 490 nm and at 500 nm and induced absorbance changes probed in the Chl Q_y region to provide kinetic data that were interpreted by using the results from exciton calculations. Calculations included the 42 chlorophylls and the 12 carotenoids of the complex, Soret, Q_x and Q_y states of the chlorophylls, and the main absorbing S₂ state of the carotenoids. According to the calculations excitation at 500 nm populates mostly a mixed Lut S₂ Chl a Soret state, from where excitation is transferred to the Q_x and Q_y states of the Chl a's on the stromal side. Internal conversion of the mixed state to a mixed Lut S₁ and Chl a Q_y state provides a channel for Lut S₁ to Chl a Q _y energy transfer. The results from the calculations support a picture where excitation at 490 nm populates primarily a mixed neoxanthin S ₂ Chl b Soret state. From this state excitation from neoxanthin is transferred to iso-energetic Chl b Soret states or via internal conversion to S₁ Chl b Q_y states. From the Soret states excitation proceeds via internal conversion to Q_y states of Chl b's mostly on the lumenal side. A rapid Chl b to Chl a transfer and subsequent transfer to the stromal side Chl a's and to the final state completes the process after 490 nm excitation. The interpretation is further supported by the fact that excitation energy transfer kinetics after excitation of neoxanthin at 490 nm and the Chl b Q_y band at 647 nm (Linnanto et al., Photosynth Res 87:267-279, 2006) are very similar.