title: Structure of the P700(+ )A1(-) radical pair intermediate in photosystem I by high time resolution multifrequency electron paramagnetic resonance: analysis of quantum beat oscillations.
abstract: The geometry of the secondary radical pair P700(+)A1(-), in photosystem I (PSI) from the deuterated and 15N-substituted cyanobacterium Synechococcus lividus, has been determined by high time resolution electron paramagnetic resonance (EPR), performed at three different microwave frequencies. Structural information is extracted from light-induced quantum beats observed in the transverse magnetization of P700(+)A1(-) at early times after laser excitation. A computer analysis of the two-dimensional Q-band experiment provides the orientation of the various magnetic tensors of with respect to a magnetic reference frame. The orientation of the cofactors of the primary donor in the g-tensor system of is then evaluated by analyzing time-dependent X-band EPR spectra, extracted from a two-dimensional data set. Finally, the cofactor arrangement of P700(+)A1(-) in the photosynthetic membrane is deduced from angular-dependent W-band spectra, observed for a magnetically aligned sample. Thus, the orientation of the g-tensor of P700(+) with respect to a chlorophyll based reference system could be determined. The angle between the g1(z) axis and the chlorophyll plane normal is found to be 29 +/- 7 degrees, while the g1(y) axis lies in the chlorophyll plane. In addition, a complete structural model for the reduced quinone acceptor, A1(-), is evaluated. In this model, the quinone plane of is found to be inclined by 68 +/- 7 degrees relative to the membrane plane, while the P700(+)-A1(-) axis makes an angle of 35 +/- 6 degrees with the membrane normal. All of these values refer to the charge separated state, observed at low temperatures, where forward electron transfer to the iron-sulfur centers is partially blocked. Preliminary room temperature studies of P700(+)A1(-), employing X-band quantum beat oscillations, indicate a different orientation of A1(-) in its binding pocket. A comparison with crystallographic data provides information on the electron-transfer pathway in PSI. It appears that quantum beats represent excellent structural probes for the short-lived intermediates in the primary energy conversion steps of photosynthesis.
authors.affiliation: Department of Physical Chemistry, University of Freiburg, Albertstrasse 21, D-79104 Freiburg, Germany.
authors.forename: G
authors.initials: G
authors.lastname: Link
authors.forename: T
authors.initials: T
authors.lastname: Berthold
authors.forename: M
authors.initials: M
authors.lastname: Bechtold
authors.forename: J U
authors.initials: JU
authors.lastname: Weidner
authors.forename: E
authors.initials: E
authors.lastname: Ohmes
authors.forename: J
authors.initials: J
authors.lastname: Tang
authors.forename: O
authors.initials: O
authors.lastname: Poluektov
authors.forename: L
authors.initials: L
authors.lastname: Utschig
authors.forename: S L
authors.initials: SL
authors.lastname: Schlesselman
authors.forename: M C
authors.initials: MC
authors.lastname: Thurnauer
authors.forename: G
authors.initials: G
authors.lastname: Kothe
mesh_terms: D002462:Cell Membrane / Q000737:chemistry; D000458:Cyanobacteria / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D045331:Photosystem I Protein Complex; D011487:Protein Conformation
chemical_list: D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex