Temperature dependence of electron transfer in photosynthetic reaction centers from Rhodobacter sphaeroides: Trapping and characterization of conformational substates.
Item
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Title
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Temperature dependence of electron transfer in photosynthetic reaction centers from Rhodobacter sphaeroides: Trapping and characterization of conformational substates.
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Identifier
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AAI3024846
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identifier
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3024846
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Creator
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Xu, Qiang.
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Contributor
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Adviser: Marilyn R. Gunner
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Date
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2001
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Language
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English
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Publisher
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City University of New York.
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Subject
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Biophysics, General
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Abstract
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The photosynthetic reaction center protein (RC) provides an excellent model system for study of electron transfer process. The influence of conformational changes on the electron transfer energetics and kinetics was investigated in this study.;By measuring the temperature dependence of P+QA -charge recombination for RCs with low potential quinones as QA, we established that at room temperature, the entropy change between the P+QA- state and P+H- state is quite small on the time scale of the measurement. When temperature is lowered, a break in the temperature dependence was observed at ∼210 K. Analysis of the data suggests that at low temperature, the product is trapped in an unrelaxed state which is higher in energy. Such relaxation could help explain why different values of DeltaS are found in previous measurements done on different time scale.;It was proposed previously that a conformational change is the rate limiting step in the QA- to QB electron transfer. We confirmed that by freezing the sample under illumination, RCs can be trapped in the active, light adapted conformation. QA - to QB electron transfer proceeds with high quantum efficiency at low temperature in light adapted RCs, while it stops in dark adapted proteins. Direct electron tunneling from QB- to P+ was measured in the light adapted sample and found to be temperature independent. The relaxation of RCs in the active conformation to a new inactive conformation was observed in the temperature range of 120--200 K.;The freeze out of the QA- to QB electron transfer can be influenced by many factors such as pH, substrate and residues near the QB site. By measuring how these factors change the temperature dependence of the quantum efficiency of the electron transfer, we conclude that the proper protonation state for residues near the Q B site is required for the electron transfer. The energy barriers between the protonated and unprotonated reactant substate and product state is characterized. L209 Proline mutation and change of the quinone hydrocarbon tail length indicate that a simple shift of the quinone position is unlikely to be the only conformational change required for the electron transfer.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
