Studies on the Photoreduction of Quinone Analogues in Bacterial Photosynthetic Reaction Centers

305 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1986.The kinetics and thermodynamics of photo-induced electron transfer were studied in photosynthetic reaction centers from Rhodopseudomonas sphaeroides, R-26 mutant, which were chemically depleted of ubiquinone (Q-10) and reconstituted with various quinone analogues. Ubiquinone prenylogues were found to reconstitute both the primary and secondary quinone acceptor function of the protein, with shorter-chained prenylouges giving rise to smaller electron transfer equilibrium constant (= Q$\sb{\rm A}$Q$\sb{\rm B}\sp{-}$/Q$\sb{\rm A}\sp{-}$Q$\sb{\rm B}$), faster post-flash association kinetics (P$\sp{+}$Q$\sb{\rm A}\sp-$ + Q $\rightarrow$ P$\sp{+}$Q$\sb{\rm A}\sp{-}$Q$\sb{\rm B},$ and thermodynamically less stable semiquinones (P$\sp{+}$Q$\sb{\rm A}$Q$\sb{\rm B}\sp{-} \rightarrow$ P$\sp{+}$Q$\sb{\rm A}$ + Q$\sp{-}$). Plastoquinone A, menaquinone-8, and other prenylquinone analogues were found to not restore secondary electron acceptor function. 2,3 dimethyl, 5,6 dimethoxy benzoquinone, however, did restore functional Q$\sb{\rm B}$ activity. Synthetic analogues such as duroquinone or anthraquinone did not restore secondary electron transfer, and high redox potential analogues such as benzoquinone were found to transfer charge out of the protein binding sites and reacted through a mechanism involving the quinone pool. The role of the isoprenoid side chain was analysed in terms of the enthalpy-entropy compensation effect, in which the side chain structure gave rise to cooperative interactions of the protein with the quinone ring structure.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD