9723661 Golbeck. The aim of this work is to understand the molecular mechanisms which lead to the high quantum yield and high thermodynamic efficiency of light-to-chemical free energy conversion in Type I (iron-sulfur) photosynthetic reaction centers. To achieve this level of understanding about the intermediate electron acceptor A1, the ligands which bind the phylloquinone whether one or both of the phylloquinone pathways is utilized in electron transfer, and if it is possible to manipulate the amino acids so that quinones with altered redox potentials can be exchanged into the site, must be determined. The issues regarding the mutants of FA and FB include using substituted aryl thiolates to modify the midpoint potentials of the iron-sulfur clusters, using thiolated viologen derivatives to function as kinetic probes of electron transfer, and using viologen derivatives with different tether lengths to investigate the effect of distance on electron transfer rate. The work on the low molecular mass polypeptides PsaE through PsaM includes measurements of a possible electron transfer pathway through PsaE, and whether the PsaF, PsaI, PsaJ, PsaK or PsaM provide links for energy spillover from light absorbed by phycobilisomes. The overall goal of this work is to study the structure, function and organization of the photosystem I reaction center in cyanobacteria and plants. A combination of organic chemical synthesis, site-directed mutagenesis and spectroscopic characterization including time-resolved optical and EPR, will be employed to probe these issues. ***
