THE ORIENTATION OF MEMBRANE BOUND RADICALS: AN EPR INVESTIGATION OF MAGNETICALLY ORDERED SPINACH CHLOROPLASTS

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The orientation of membrane-bound radicals in spinach chloroplasts is examined by electron paramagnetic resonance (EPR) spectroscopy of chloroplasts oriented by magnetic fields. Several of the membrane-bound radicals which possess g-tensor anisotropy display EPR signals with a marked dependence on the orientation of the membranes relative to the applied EPR field. The fraction of oxidized and reduced plastocyanin, P-700, iron-sulfur proteins A and B, and the X center, an early acceptor of Photosystem I, can be controlled by the light intensity during steady-state illumination and can be trapped by cooling. The X center can be photoreduced and trapped in the absence ... continued below

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39 p.

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Dismukes, G. Charles & Sauer, Kenneth March 1, 1978.

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The orientation of membrane-bound radicals in spinach chloroplasts is examined by electron paramagnetic resonance (EPR) spectroscopy of chloroplasts oriented by magnetic fields. Several of the membrane-bound radicals which possess g-tensor anisotropy display EPR signals with a marked dependence on the orientation of the membranes relative to the applied EPR field. The fraction of oxidized and reduced plastocyanin, P-700, iron-sulfur proteins A and B, and the X center, an early acceptor of Photosystem I, can be controlled by the light intensity during steady-state illumination and can be trapped by cooling. The X center can be photoreduced and trapped in the absence of strong reductants and high pH, conditions previously found necessary for its detection. These results confirm its role as an early electron acceptor in P-700 photo-oxidation. X is oriented with its smallest principal g-tensor axis (g{sub x}) predominantly parallel to the normal to the thylakoid membrane, the same orientation as was found for an early electron acceptor based on time-resolved electron spin polarization studies. We propose that the X center is the first example of a high potential iron-sulfur protein which functions in electron transfer in its 'superreduced' state. We present evidence which suggests that iron-sulfur proteins A and B are 4Fe-4S clusters in an 8Fe-8S protein. Center B is oriented with g{sub y} predominantly normal to the membrane plane. The spectra of center A and plastocyanin do not show significant changes with sample orientation. In the case of plastocyanin, this may indicate a lack of molecular orientation. The absence of an orientation effect for reduced center A is reconcilable with a 4Fe-4S geometry, provided that the electron obtained upon reduction can be shared between any pair of Fe atoms in the center. Orientation of the 'Rieske' iron-sulfur protein is also observed. It has axial symmetry with g{parallel} close to the plane of the membrane. A model is proposed for the organization of these proteins in the thylakoid membrane. A new EPR signal was observed in oriented chloroplasts. This broad unresolved resonance displays a g value of 3.2 when the membrane normal is parallel to the field. It shifts to g = 1.9 when the membrane normal is perpendicular to the field. The signal is sensitive to illumination and to washing of the thylakoid membranes of broken chloroplasts. We suggest that there is a relation between this signal and the water-oxidizing enzyme system.

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39 p.

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  • Journal Name: Biochimica et Biophysica Acta; Journal Volume: 504; Journal Issue: 3

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  • Report No.: LBL-7532
  • Grant Number: DE-AC02-05CH11231
  • DOI: 10.1016/0005-2728(78)90065-8 | External Link
  • Office of Scientific & Technical Information Report Number: 1008318
  • Archival Resource Key: ark:/67531/metadc846254

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  • March 1, 1978

Added to The UNT Digital Library

  • May 19, 2016, 3:16 p.m.

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  • July 26, 2016, 5:52 p.m.

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Dismukes, G. Charles & Sauer, Kenneth. THE ORIENTATION OF MEMBRANE BOUND RADICALS: AN EPR INVESTIGATION OF MAGNETICALLY ORDERED SPINACH CHLOROPLASTS, article, March 1, 1978; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc846254/: accessed July 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.