Photosynthetic Design Principles for Ultrafast Energy Transfer Processes

As chemists, we often aim to solve each new problem with a unique molecule. Biology takes the opposite approach. Photosynthetic organisms have evolved to survive in diverse habitats by densely packing many of the same pigment molecules to spatially delocalize excitations and support vibronic coherence between pigments. Pigment-pigment and pigment-environment interactions tune chromophore absorption to capture broad wavelengths of light. We seek to understand the design principles that govern the ultrafast dynamics of light harvesting on a femtosecond timescale and apply these design principles to develop molecular artificial light harvesting systems. In this thesis, I present two-dimensional electronic spectra on living cells of the anoxygenic purple bacterium Rhodobacter sphaeroides, the fused antenna/reaction center photosystems from the oxygenic cyanobacterium Synechocystsis sp. PCC 6803 in their native membrane environment, and DNA-dye constructs that are candidates for artificial light harvesting applications. In Rhodobacter sphaeroides, we collected the first fully absorptive two-dimensional electronic spectra of a living cell, revealing a sub-100 fs intracomplex relaxation in light harvesting complex 1. We present a synthetic system of coupled cyanine dyes that supports vibronic coherence, a photosynthetic design principle implicated in efficient exciton energy transfer in photosynthetic organisms, by tethering the dyes to a DNA scaffold. It is the first report of using DNA to engineer vibronic coherence. We use singlet-singlet annihilation to track inter-complex energy transfer between isoenergetic complexes in intact thylakoids prepared from wild-type and mutant cyanobacterial cells. We observe diffusion-limited trapping behavior between red Chl pools on neighboring Photosystem I monomers in these Synechocystis sp. PCC 6803 membrane fragments. We also present the first two-dimensional electronic spectra of living cyanobacteria. This represents a major advance in in vivo ultrafast spectroscopy, opening the door to studying regulatory and response mechanisms in these more complex oxygenic phototrophes. Future planned experiments include the investigation of the effects of light quantity and quality on the organization of the photosynthetic machinery in cyanobacteria, experiments to elucidate the molecular mechanics and quantum dynamics of photoprotective quenching in green algae, and engineering efficient energy transfer aided by vibronic coherence in a synthetic dye-DNA assembly