Andreas Kamlowski

Time-resolved EPR of the radical pair P865+.QA−. in bacterial reaction centers. Observations of transient nutations, quantum beats and envelope modulation effects

Abstract The time evolution of the spin-polarized transient EPR signals of the radical pair state P 865 +. Q A −. is studied in perdeuterated reaction centers of Rhodobacter sphaeroides R-26 in which the non-heme iron has been replaced by zinc. The transients show modulations of the signal amplitude on several time scales. Fast quantum beat oscillations with frequencies around 15 MHz occur within the first 100 ns after the light excitation because the radical pair is generated in a coherent superposition of its eigenstates. Transient nutations also occur as a result of the precession of the magnetization about the applied microwave field B 1 . At the maximum B 1 field available (0.125 mT) a nutation frequency of ≈ 3.5 MHz is obtained. Small variations in the nutation frequency throughout the spectrum occur as a result of the dipolar coupling in the radical pair. Additional oscillations in the frequency ranges 1.5 to 2.0 MHz and 2.5 to 3.0 MHz are also observed which are attributed to the interaction between the nuclear spins and the unpaired electron spins. Analogies and differences to well-known nuclear modulation phenomena observed in pulsed EPR experiments are discussed.

LOW TEMPERATURE EPR ON PHOTOSYSTEM I SINGLE CRYSTALS : ORIENTATION OF THE IRON-SULFUR CENTERS FA AND FB

Abstract Low temperature EPR results from Photosystem I (PS I) single crystals of Synechococcus elongatus are presented. Illumination at 150 K and pH 6.4 is used to photoreduce the two terminal 4Fe–4S centers to the noninteracting state (F−A)+(F−B). From the EPR data and the analysis of the rotation pattern for both F−A and F−B the following information is obtained: (i) the principal values of the g tensors of F−A and F−B, (ii) the orientations of their principal g tensor axes with respect to the crystal axes for each of the six PS I centers per unit cell, and (iii) their orientation with respect to one another. In addition, significant differences between F−A and F−B are noted with respect to the orientational dependence of the linewidth and the saturation behavior of their EPR signals. In principle, six relative arrangements of the g tensors of F−A and F−B are consistent with the EPR data. Only two out of these six are compatible with the known structure of the bacterial ferredoxin from P. aerogenes (PaFd) (Adman, E.T., Siecker, L.C. and Jensen, L.H. (1976) J. Biol. Chem. 251, 3801) which has been used as a model for the core of the PsaC protein carrying FA and FB in PS I. It is concluded that the PaFd and the PsaC protein are analogous with respect to the central part of their structures. The results and conclusions are compared to those obtained from studies on oriented membranes

The structural organization of the PsaC protein in Photosystem I from single crystal EPR and X-ray crystallographic studies

In Photosystem I (PS I) the terminal electron acceptors, FA and FB, are iron-sulfur (4Fe-4S) centers, which are bound to the stromal subunit PsaC. The orientation of PsaC is determined relative to the whole PS I complex (see Schubert, W.-D. et al. (1995) in From Light to Biosphere (Mathis, P. ed.), Vol. II, pp. 3-10, Kluwer) from which a molecular model for the structure of PsaC within PS I is derived. Two strategies are followed: (i) PS I single crystal EPR data on the orientation of the g tensors of both FA- and FB- relative to each other and relative to the crystal axes (see preceding paper) are used in conjunction with the central structural part of the bacterial 2 [Fe4S4] ferredoxins, the cysteine binding motifs of which are known to be homologous to those of PsaC; (ii) the same core structure is fitted into the intermediate resolution electron density map of PS I. The PsaC orientation obtained both ways agree well. The local twofold symmetry axis inherent to the ferredoxin model leaves a twofold ambiguity in the structural conclusion. Deviations from this C2-symmetry in the amino acid sequence of PsaC are analyzed with respect to observable properties which would resolve the remaining structural ambiguity. Arguments both for and against FA being the distal iron-sulfur center (to FX) are discussed.

A Comparison of the Quinone Binding Sites in Photosystem I and Purple Bacteria

All known photosynthetic reaction centres (RC) can be grouped into just two classes referred to as type 1 (FeS type) and type 2 (Pheo-Q type). The two RC types show close similarity in the arrangement, structure and function of the primary donor P, the intermediate electron acceptors and the first quinone acceptor Q (usually termed Al in type 1 and QA in type 2 RC). In contrast, they differ strongly with respect to the terminal acceptors. In type 1, a series of iron-sulfur (FeS) centres accept the electron from Al on a timescale of 100 ns or less, whereas in type 2, the electron is transferred from QA to a secondary quinone QB within roughly 100 µs. Thus, the primary quinone acceptor serves as an interface between a similar RC core and different terminal acceptor channels. To allow chemically similar quinones to function as donors in these two different environments, their properties have been adapted through specific interactions with the surrounding protein. In this contribution we will investigate these interactions more closely.