Abstract
In photosystem I (PS I), phylloquinone (PhQ) acts as a low potential electron acceptor during light-induced electron transfer (ET). The origin of the very low midpoint potential of the quinone is investigated by introducing anthraquinone (AQ) into PS I in the presence and absence of the iron-sulfur clusters. Solvent extraction and reincubation is used to obtain PS I particles containing AQ and the iron-sulfur clusters, whereas incubation of the menB rubA double mutant yields PS I with AQ in the PhQ site but no iron-sulfur clusters. Transient electron paramagnetic resonance spectroscopy is used to investigate the orientation of AQ in the binding site and the ET kinetics. The low temperature spectra suggest that the orientation of AQ in all samples is the same as that of PhQ in native PS I. In PS I containing the iron sulfur clusters, (i) the rate of forward electron transfer from the AQ*- to F(X) is found to be faster than from PhQ*- to F(X), and (ii) the spin polarization patterns provide indirect evidence that the preceding ET step from A0*- to quinone is slower than in the native system. The changes in the kinetics are in accordance with the more negative reduction midpoint potential of AQ. Moreover, a comparison of the spectra in the presence and absence of the iron-sulfur clusters suggests that the midpoint potential of AQ is more negative in the presence of F(X). The electron transfer from the AQ- to F(X) is found to be thermally activated with a lower apparent activation energy than for PhQ in native PS I. The spin polarization patterns show that the triplet character in the initial state of P700)*+AQ*- increases with temperature. This behavior is rationalized in terms of a model involving a distribution of lifetimes/redox potentials for A0 and related competition between charge recombination and forward electron transfer from the radical pair P700*+A0*-.
Highlights
In photosystem I (PS I), phylloquinone (PhQ) acts as a low potential electron acceptor during light-induced electron transfer (ET)
The structures reveal an accessory chlorophyll monomer located between P680 and pheophytin in photosystem II (PS II) and between P700 and A0 in PS I
Efficiency of Incorporation of AQ into the A1 Binding Site— When native PhQ is extracted from PS I, the spin polarization pattern of the 3P700 state, formed by recombination of the primary radical pair state P71⁄7ϩ00A01⁄7Ϫ, is observed
Summary
Quinones—Quinones and the reagents for the preparation of buffers were purchased from Aldrich. The extraction of the phylloquinone was monitored using a modified Bruker ESP 200 X-band spectrometer (described below) by measuring the disappearance of the spin polarized EPR signal due to P71⁄7ϩ00A11⁄7Ϫ and the accompanying appearance of the characteristic polarization pattern of. A rectangular resonator and a liquid nitrogen temperature control unit were used, and the samples were illuminated using a Q-switched, frequency-doubled Continuum Surelite Nd-YAG laser at 532 nm with a repetition rate of 10 Hz. All other X-band transient EPR experiments were carried out using a Bruker ER046 XK-T microwave bridge equipped with a Flexline dielectric resonator [34] and an Oxford liquid helium gas flow cryostat. Q-band (35-GHz) transient EPR spectra of the samples were measured with the same set-up except that a Bruker ER 056 QMV microwave bridge equipped with a home-built cylindrical resonator with access for light irradiation was used. The samples were illuminated using a Spectra Physics Nd-YAG/MOPO laser system operating at 10 Hz at either the second harmonic 532 nm or near the long wavelength absorption edge of PS I at ϳ700 nm
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