Formation of P{sup +}Q{sub B}{sup -} via B-branch electron transfer in mutant reaction centers.

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The crystallographic observation of two symmetry-related branches of electron transfer cofactors in the structure of the bacterial reaction center (RC) 13 years ago [1] remains an enigma in light of experimental observations that show that only the A branch is active in the initial electron transfer steps in wild-type RCs. Unidirectional electron flow has been attributed to localized asymmetries between the A and B branches that lead to differences in: (1) the electronic couplings of the cofactors [2]; (2) the relative electrostatic environments of the cofactors, caused by amino acid differences which modulate the free energies of their charge-separated states ... continued below

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6 p.

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Laible, P. D. August 14, 1998.

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The crystallographic observation of two symmetry-related branches of electron transfer cofactors in the structure of the bacterial reaction center (RC) 13 years ago [1] remains an enigma in light of experimental observations that show that only the A branch is active in the initial electron transfer steps in wild-type RCs. Unidirectional electron flow has been attributed to localized asymmetries between the A and B branches that lead to differences in: (1) the electronic couplings of the cofactors [2]; (2) the relative electrostatic environments of the cofactors, caused by amino acid differences which modulate the free energies of their charge-separated states [3] and/or create a higher dielectric constant on the active side, resulting in a stronger static field for stabilizing A-branch charge transfer states [4,5]. Some photo-induced bleaching of H{sub B} has been observed, in wild-type RCs following trapping of HA{sub A}{sup {minus}}[6], and in ''hybrid'' RCs where the redox potentials of cofactors were manipulated by pigment exchange [7] or mutagenesis [8]. Transient bleaching of the 530-nm band of H{sub B} was more easily observed in the hybrid RCs because the H{sub A} transition at 545 nm was shifted to {approximately}600 nm due to incorporation of a bacteriochlorophyll, designated ''{beta}'', at the H{sub A} site. No experiments to detect further electron transfer to Q{sub B} were done with either type of modified RCs. Many site-specific mutagenesis experiments have given us insight into the nature and magnitude of the effects that amino acid side chains can exert in tuning the relative energy levels of the cofactors to optimize the balance between forward and reverse reactions, and the large distances through which some of these effects are manifested. In this paper, we show that in mutant RCs of Rhodobacter capsulatus, P{sup +}Q{sub B}{sup {minus}} can be formed in the absence of prior formation of P{sup +}Q{sub A}{sup {minus}}, solely through activity of B-branch cofactors.

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6 p.

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OSTI as DE00010858

Medium: P; Size: 6 pages

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  • 11th International Congress on Photosynthesis, Budapest (HU), 08/17/1998--08/22/1998

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  • Report No.: ANL/CMB/CP-96703
  • Grant Number: W-31109-ENG-38
  • Office of Scientific & Technical Information Report Number: 10858
  • Archival Resource Key: ark:/67531/metadc620207

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  • August 14, 1998

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  • June 16, 2015, 7:43 a.m.

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  • April 11, 2017, 2:32 p.m.

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Laible, P. D. Formation of P{sup +}Q{sub B}{sup -} via B-branch electron transfer in mutant reaction centers., article, August 14, 1998; Illinois. (digital.library.unt.edu/ark:/67531/metadc620207/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.