Energy and Electron Transfer in Photosynthetic Reaction Centers and Biohybrid Antenna Complexes

This dissertation focuses on two areas studied in an effort to increase important fundamental understandings of electron and energy transfer events leading to solar conversion in natural and artificial systems: (1) electron transfer in the bacterial photosynthetic reaction center (RC) and (2) biohybrid photosynthetic light harvesting (LH) complexes containing native-like peptides and synthetic chromophores. The RC in photosynthetic bacteria converts light energy to chemical energy through a series of electron transfer steps to separate charge across a membrane. The native process has a ~100% quantum yield of electron transfer down one cofactor branch (the L-side) whereas the seemingly symmetrical branch (the M-side) with identical pigments is inactive. Understanding M-side inactivity has been the goal of numerous studies, and has been investigated through mutations of the native peptides to favor electron transfer down the M-side or disfavor electron transfer down the L-side. In these mutants, the normal bacteriochlorophyll-bacteriochlorophyll primary electron donor is replaced with a bacteriochlorophyll-bacteriopheophytin dimer (bacteriopheophytin is the Mg-free analog of bacteriochlorophyll). This switch alters the mechanisms, rates, and yields of electron transfer in heterodimer RCs compared to wild-type. We have investigated the effects of three further mutations in which either the L or M macrocycle of the dimer is a bacteriopheophytin, for a total of eight combinations (e.g., addition of a hydrogen bond to the dimer, swap of nearby amino acids). Using ultrafast transient absorption spectroscopy to observe changes in the rate and the yield of L- versus M-side charge separation, the contributions of the electronic and energetic factors to directionality of electron transfer in RCs were assessed. The second area of my studies has been to investigate biohybrid antenna complexes formed by combining native-length bacterial LH peptide analogs with designer chromophores. The native LH systems are highly efficient. When a photon is absorbed, the energy is transferred to the RC with near unity quantum yield. However, the complex doesn\u27t absorb parts of the solar spectrum in the visible and near-infrared regions. To address this deficiency, we performed four major studies on biohybrid antennas: (1) relay energy transfer between, (2) studies on location, density and spectral coverage of conjugated chromophores, (3) studies on bioconjugatable chromophores of hydrophobic, hydrophilic and amphiphilic nature, and (4) incorporation of non-bioconjugatable amphiphilic chromophores. Static emission and transient absorption studies indicate energy transfer efficiencies comparable to those found in native antenna systems