Ultrafast Energy Transfer of Biohybrid Photosynthetic Antenna Complexes in Molecular Assembly Systems

Photosynthesis in natural systems is one of the typical examples where the interaction between multiple photons and multiple chromophores leads to the synergetic responses in three-dimensionally arranged molecular aggregates. Accordingly, the investigation of primary events in the excited state could provide important information on the progress in the artificial photosynergetic systems. Along with this line, we have prepared biohybrid light-harvesting complexes to study ultrafast dynamics of excitation energy transfer in the present project. Light-harvesting 2 complex (LH2) from photosynthetic purple bacteria is a molecular assembly, in which multiple photosynthetic chromophores are well organized. To expand high-harvesting activity of LH2, we attached a fluorophore, Alexa647, to lysine residues on the LH2 polypeptide so as to cover broad solar spectrum. Ultrafast energy transfer from Alexa 647 to the intrinsic chromophores B800 and B850 in LH2 was observed using femtosecond transient absorption spectroscopy. Detailed analysis revealed two possible energy transfer pathways. In addition, we made a LH2 mutant bearing reactive Cys residue on the N-terminal region of LH2β polypeptide. The engineered LH2 allows us to provide the defined position of Alexa647. Together with the system in which LH2 was dissolved in micellar solution, a lipid bilayer system in which the bioengineered LH2 was incorporated was applied to control the location of the external fluorophore.