Extension of Light-Harvesting Ability of LH2 through Ultrafast Energy Transfer from Covalently Attached Fluorophores

Takehisa Dewaa, Yusuke Yonedab, Naoto Mizutania, Daiki Moria, Tomoyasu Nojia,c, Masaharu Kondoa, Hiroshi Miyasakab, Shigeru Itohd, Yutaka Nagasawae,f
aDepartment of Life and Materials Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan; bGraduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan; cThe OCU Advanced Research Institute for Natural Science and Technology, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585 Japan; dCenter for Gene Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan; eDepartment of Applied Chemistry, College of Life Science, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan; fPRESTO, Japan Science and Technology, Kawaguchi, Saitama 332-0012, Japan

Introducing appropriate artificial components into natural biological systems could enrich the original functionality. In order to expand the available wavelength range of photosynthetic bacterial light-harvesting complex 2 (LH2 from Rhodopseudomonas acidophila 10050), we have reported that an artificial fluorescent dye (Alexa Fluor 647: A647), which was covalently attached to N- and C-terminal Lys residues in LH2 α-polypeptides with a molar ratio of A647/LH2 ~= 9/1, can efficiently transfer excitation energy to B800 and B850 bacteriochlorophylls in LH2. [1] Femtosecond transient absorption spectroscopies revealed that intra-complex energy transfer from A647(excited at ~650 nm) to intrinsic chromophores of LH2 (B850) occurs in a multiexponential manner, with time constants varying from 440 fs to 23 ps through direct and B800-mediated indirect pathways. Kinetic analyses suggested that B800 chromophores mediate faster energy transfer, and the mechanism was interpretable in terms of Förster theory.

This study demonstrates that a simple attachment of external chromophores with a flexible linkage can enhance the light harvesting activity of LH2 without affecting inherent functions of energy transfer, and can achieve energy transfer in the subpicosecond range. Addition of external chromophores, thus, represent a useful methodology for construction of advanced hybrid light-harvesting systems that afford solar energy in the broad spectrum. To extend this methodology, in stead of hydrophilic dye A647, we examined a hydrophobic fluorophore ATTO647N (AT647N). Energy transfer rate from AT647 to B850 was comparable to that from A647. Interestingly, when the LH2–AT647N conjugate was assembled in lipid bilayer, the energy transfer rates were significantly accelerated (2.8-12 ps) but not the case of LH2–A647, suggesting that the AT647N is placed to hydrophobic region of LH2 proximal to the acceptor bacteriochlorophylls. Precise mechanisms will be discussed in terms of the pigment alignments in the lipid membrane.

Reference

[1] Y. Yoneda, et al., J. Am. Chem. Soc. 2015, 137, 13121–13129.