Delocalization and Energy Transfer Dynamics from Two-Dimensional Anisotropy Spectroscopy of LH2 in Rhodobacter sphaeroides

Sara C. Masseya, Po-Chieh Tinga, Peter D. Dahlbergb,c, Marco A. Allodia, Moira L. Flanaganb, Sara K. Hessa, Sarah R. Soltauc, Elizabeth C. Martind, C. Neil Hunterd, Gregory S. Engela,b
aDepartment of Chemistry, James Franck Institute, The University of Chicago, Chicago, IL; bBiophysical Sciences Graduate Program, The University of Chicago, Chicago, IL; cChemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL; dDepartment of Molecular Biology and Biotechnology, University of Sheffield, United Kingdom

Highly efficient energy transfer in photosynthetic organisms is driven by complex electronic structure. We seek to understand the design principles underlying these energy transfer dynamics. Upon initial excitation at 850 nm, light harvesting complex 2 (LH2) in Rhodobacter sphaeroides has a delocalized exciton over a ring of chromophores that quickly localizes to a few dimers. Recent developments in ultrafast two-dimensional electronic spectroscopy have improved sensitivity and allowed us to probe energetic coupling and energy transfer dynamics using polarization sequences at low pulse powers in the native photosynthetic membrane environment. I will present results from third-order ultrafast anisotropy experiments that measure the initial delocalization on the LH2 chromophores, and the delocalization of energy transfer between rings of chromophores in both isolated LH2 samples and intact membranes containing LH2 in Rhodobacter sphaeroides.

Figure 1: (A) Parallel and (B) Perpendicular 2DES spectra of LH2-only membranes at waiting time T = 200 fs. (C) Waiting time traces of the B850 peak of LH2-only membranes for parallel and perpendicular polarization sequences.