Energy transfer events in photosynthesis observed in vivo using nonlinear ultrafast spectroscopie

Ultrafast energy transfer through a dense network of antenna complexes is a crucial process for all photosynthetic organisms. This energy transfer process can be highly efficient and is tightly regulated by organisms both to optimize their light harvesting capabilities and to protect themselves from photodamage. Steps in this energy transfer process have been studied extensively using isolated complexes, but an understanding of the relationship between complexes depends on the observation of the energy transfer pathways in living organisms at physiological temperatures. Here, we present two-dimensional electronic spectroscopy data collected in living cells of Rhodobacter sphaeroides that follows the energy transfer process through the antenna network and into the reaction center. The dynamics present in this data allow for the recovery of energy transfer time between the antenna complexes LH2 and LH1 as well as the trapping time associated with the reaction center. Further singlet-singlet annihilation studies performed on mutants of Rba. Sphaeroides containing only LH2 or LH1 are used to determine the domain size and energy transfer times between isoenergetic complexes. The combination of all these timescales is then used to map the functional photosynthetic unit in the living bacteria.