Electron spin polarization in the donor triplet state of bacterial photosynthetic reaction centres. II. Anisotropic inversion of electron spin polarization in a three-spin model reaction centre

Hore, P. J.; Hunter, David; Wijk, F.G.H. van; Schaafsma, T.J.; Hoff, A.J.

doi:10.1016/0005-2728(88)90242-3

Cited by 24 publications

(19 citation statements)

References 21 publications

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“…The interaction of the primary radical pair (P*+I-) with a third spin (QA-), however, does not only create electron spin polarization on QA-but is also expected to affect the spin evolution of P+I'_ (Hore et al, 1988). We, therefore, studied the electron spin polarization of the triplet state of the primary donor, 3P, and its temperature dependence for RCs of Rb.…”

Section: Resultsmentioning

confidence: 99%

“…The temperature-dependent population of the T± sublevels has a different physical origin in Fe2+-containing and in Zn2+substituted RCs, namely, an incoherent relaxation process in the primary radical pair triplet state in the former (Van den Brink et al, 1994) and coherent singlet-triplet mixing in the three-spin radical complex [P,+I*_QA'_] in the latter case (Hore et al, 1988). In Fe2+-containing RCs, the g -1.8 Fe2+QA'~spin system enhances (fast) spin-lattice relaxation in the primary radical pair triplet state 3[P*+P~] via 7iq, thus populating its T± sublevels.…”

Section: Temperature (K)mentioning

confidence: 99%

“…In Zn2+substituted RCs, on the contrary, the (slowly relaxing) g -2.0 electron spin of the semiquinone anion effectively couples with the primary radical-pair states, forming the three-spin radical complex [P,+I,_QA'~]. In this complex, a coherent mixing process populates the T± sublevels of 3P, concomitantly producing spin polarization of QA'" (Cast & Hoff, 1979;Hore et al, 1988Hore et al, , 1993. The relative populations of the triplet sublevels of the primary radical pair in Zn2+substituted RCs are related to the interaction between I*ã nd Qa'" and to the relaxation rate of the quinone spin [see Figure 6 in Hore et al (1988)].…”

Section: Temperature (K)mentioning

confidence: 99%

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QA Binding in Reaction Centers of the Photosynthetic Purple Bacterium Rhodobacter sphaeroides R26 Investigated with Electron Spin Polarization Spectroscopy

Brink¹,

Hulsebosch²,

Gast³

et al. 1994

Biochemistry

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The relation between quinone (QA) binding and electron transport in reaction centers (RCs) of photosynthetic purple bacteria is investigated, using electron spin polarization (ESP) X-band (9 GHz) EPR as a tool to probe for structural changes resulting from charge separation and stabilization and from replacing the native QA molecule with other quinones. We present a study of possible changes in QA-binding that might be responsible for the remarkably prolonged lifetime of the charge-separated state at cryogenic temperatures for RCs of Rhodobacter sphaeroides R26 cooled under illumination [Kleinfeld, D., et al. (1984) Biochemistry 23, 5780-5786]. It is shown that this effect is not caused by a major reorientation of the chromophores. Furthermore, we studied the effects of structurally different quinones functioning as primary electron acceptor in different purple bacteria. With simulations of ESP X-band spectra of the spin-polarized secondary radical pair P.+QA.+- in menaquinone-reconstituted, Zn(2+)-substituted RCs of Rb. sphaeroides R26, we show that quinone reconstitution is highly selective for site and orientation. Furthermore, we find that a very small exchange interaction between P.+ and QA.+- (magnitude of JPQ approximately 1 microT) is needed to account accurately for the observed relative line intensities at X-band, without affecting the accuracy of the simulations of reported ESP K-band spectra [Füchsle, G., et al. (1993) Biochim. Biophys. Acta 1142, 23-35; Van der Est, A., et al. (1993) Chem. Phys. Lett. 212, 561-568]. This pronounced influence of small values for JPQ on the X-band ESP line shape results from cancellation effects of absorptive and emissive contributions to the spectrum, such that small shifts can be observed. The exchange interaction has opposite sign for the native, ubiquinone-containing RC [viz. JP.UQ = (-0.8 +/- 0.2) microT] and the menaquinone-substituted RC [JP.MK = (+0.3 +/- 0.2) microT]. The implications of these observations for electron-transport theory are discussed.

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Section: Resultsmentioning

confidence: 99%

Section: Temperature (K)mentioning

confidence: 99%

Section: Temperature (K)mentioning

confidence: 99%

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QA Binding in Reaction Centers of the Photosynthetic Purple Bacterium Rhodobacter sphaeroides R26 Investigated with Electron Spin Polarization Spectroscopy

Brink¹,

Hulsebosch²,

Gast³

et al. 1994

Biochemistry

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“…Temperature-induced spectral changes of the 3 P EPR signal similar to those of 3 P680 have been reported for Rps . viridis RCs, though manifested at lower temperatures. ,, One explanation involved coherent S−T ± mixing in the radical pair due to a fast relaxing electron spin in the iron−quinone acceptor complex . Direct-detection EPR studies of the temperature dependence of the 3 P spectrum, however, showed that this model is insufficient to explain the temperature dependence. , The decrease of initial polarization was therefore attributed to SLR between the T 0 and T ± levels of the radical pair, induced by the presence of the high-spin Fe 2+ ion …”

Section: Discussionmentioning

confidence: 99%

Time-Resolved EPR Study of the Primary Donor Triplet in D1-D2-cytb559Complexes of Photosystem II: Temperature Dependence of Spin−Lattice Relaxation

et al. 1996

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The temperature dependence of the EPR spectrum and kinetics of the primary donor triplet state 3P680 are measured with direct-detection CW EPR and electron spin echo (ESE) spectroscopy, respectively. The EPR spectrum was recorded up to 230 K; kinetics could be traced up to 70 K. The observed anisotropy of the temperature dependence of the EPR spectrum recorded within 1 μs after a laser flash is explained by anisotropic spin relaxation in the precursor primary radical pair. The ESE-detected decay kinetics of the Z-canonical peak of 3P680 is close to its optical lifetime for T < 20 K but deviates strongly from the lifetime for T > 30 K, due to a rapid acceleration of spin relaxation with temperature. This relaxation is not caused by the presence of oxygen or the paramagnetic heme iron in the b559 cytochrome; it is attributed to slow triplet hopping in the P680 dimer itself. Comparison of the low-temperature kinetics with that of chlorophyll a in solution confirmed that the central Mg ion in the triplet-bearing Chl of 3P680 is pentacoordinated.

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“…As the Zeeman part of the Hamiltonian commutes with the exchange Hamiltonian, this interaction alone is insufficient to produce MFEs (see SI [32]). However, mutual exchange coupling can provide the premise for near level-crossings at certain strengths of an external magnetic field, whereupon hyperfine-driven spin conversion can proceed efficiently [33,34], and may also transmit the effect of a fast-relaxing third radical [35]. A perturbative approach based on a Hubbard-trimer Hamiltonian has been used to show that the additional radical can enhance the intersystem crossing rate [36].…”

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confidence: 99%

Magnetosensitivity in Dipolarly Coupled Three-Spin Systems

2018

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The Radical Pair Mechanism is a canonical model for the magnetosensitivity of chemical reaction processes. The key ingredient of this model is the hyperfine interaction that induces a coherent mixing of singlet and triplet electron spin states in pairs of radicals, thereby facilitating magnetic field effects (MFEs) on reaction yields through spin-selective reaction channels. We show that the hyperfine interaction is not a categorical requirement to realize the sensitivity of radical reactions to weak magnetic fields. We propose that, in systems comprising three instead of two radicals, dipolar interactions provide an alternative pathway for MFEs. By considering the role of symmetries and energy level crossings, we present a model that demonstrates a directional sensitivity to fields weaker than the geomagnetic field and remarkable spikes in the reaction yield as a function of the magnetic field intensity; these effects can moreover be tuned by the exchange interaction. Our results further the current understanding of the effects of weak magnetic fields on chemical reactions, could pave the way to a clearer understanding of the mysteries of magnetoreception and other biological MFEs and motivate the design of quantum sensors. Further still, this phenomenon will affect spin systems used in quantum information processing in the solid state and may also be applicable to spintronics.There is growing excitement about the possibility of quantum coherence and entanglement underpinning the optimal functioning of biological processes [1]. A notable example is the avian inclination compass [2][3][4][5][6][7][8][9][10][11][12], which has recently been realised as a truly quantum-biological process [3]. The leading explanation of this phenomenon utilizes the Radical Pair Mechanism (RPM), which describes the unitary evolution of singlet-triplet (S-T) coherences in systems comprising two radicals, i.e. two electron spins [2,13,14]. The RPM has also been suggested to underpin controversial health-related implications of exposure to weak electromagnetic fields [15][16][17][18]. For these phenomena, the so-called low-field effect (LFE) is crucial to foster sensitivity to magnetic fields of intensity comparable to the geomagnetic field (≈ 50 µT) [6,[19][20][21][22][23][24]. The electron-electron dipolar interaction is often neglected when addressing MFEs within the RPM framework, but preliminary explorations have been conducted: electron-electron dipolar coupling is expected to resemble the exchange coupling, which, as the dominant interaction, suppresses S-T conversion by lifting the neardegeneracy of triplet and singlet states, reducing their susceptibility to mixing by weak hyperfine interactions [25], and quenching the LFE [26]. Efimova et al. proposed that the dipolar interaction could be partly compensated by the exchange interaction, thereby allowing high sensitivity to the geomagnetic field despite sizeable electron-electron dipolar coupling interactions [27].In contrast to the two-spin systems of the classical RPM, spin tr...

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Electron spin polarization in the donor triplet state of bacterial photosynthetic reaction centres. II. Anisotropic inversion of electron spin polarization in a three-spin model reaction centre

Cited by 24 publications

References 21 publications

QA Binding in Reaction Centers of the Photosynthetic Purple Bacterium Rhodobacter sphaeroides R26 Investigated with Electron Spin Polarization Spectroscopy

QA Binding in Reaction Centers of the Photosynthetic Purple Bacterium Rhodobacter sphaeroides R26 Investigated with Electron Spin Polarization Spectroscopy

Time-Resolved EPR Study of the Primary Donor Triplet in D1-D2-cytb559Complexes of Photosystem II: Temperature Dependence of Spin−Lattice Relaxation

Magnetosensitivity in Dipolarly Coupled Three-Spin Systems

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