Effect of magnetic fields on the triplet state lifetime in photosynthetic reaction centers: Evidence for thermal repopulation of the initial radical pair

Chidsey, Christopher E. D.; Takiff, Larry; Goldstein, Richard A.; Boxer, Steven G.

doi:10.1073/pnas.82.20.6850

Cited by 80 publications

(55 citation statements)

References 20 publications

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“…This time constant is similar to those measured for the decay of pR in Q-depleted and Q--containing RCs (18,19). Debus et al (13) have reported a similar 28-ps component in their flash experiments, and they also have detected PR in EPR studies.…”

Section: Resultssupporting

confidence: 66%

Primary photochemistry of iron-depleted and zinc-reconstituted reaction centers from Rhodopseudomonas sphaeroides

Kirmaier

Holten

Debus

et al. 1986

Proc. Natl. Acad. Sci. U.S.A.

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The primary photochemistry of Fe-depleted and Zn-reconstituted reaction centers from Rhodopseudomonas sphaeroides R-26.1 was studied by transient absorption spectroscopy and compared with native, Fe2+-containing reaction centers. Excitation of metal-free reaction centers with 30-ps flashes produced the initial charge-separated state P+I-(P+BPh-, where P is the primary donor and BPh is bacteriopheophytin) with a yield and visible/near-infrared absorption difference spectrum indistinguishable from that observed in native reaction centers. However, the lifetime of P+I-was found to increase approximately 20-fold to 4.2 ± 0.3 ns (compared to 205 ps in native reaction centers), and the yield of formation of the subsequent state P+Q-(QA is the primary quinone acceptor) was reduced to 47 ± 5% (compared to essentially 100% in native reaction centers). The remaining 53% of the metal-free reaction centers were found to undergo charge recombination during the P+I-lifetime to yield both the ground state (28 ± 5%) and the triplet state PR (25 ± 5%). Reconstitution of Fe-depleted reaction centers with Zn2+ restored the "native" photochemistry. Possible mechanisms responsible for the reduced decay rate of P'l-in metal-free reaction centers are discussed.The reaction center (RC) from the photosynthetic bacterium Rhodopseudomonas sphaeroides is a membrane-spanning protein complex consisting of three polypeptides containing four bacteriochlorophyll molecules, two bacteriopheophytin (BPh) molecules, two ubiquinone molecules (QA and QB), and one high-spin Fe2" atom (1). Of the six tetrapyrrole pigments present, only three have been definitively shown to play a role in the photochemistry of the RC. Two bacteriochlorophyll molecules form a dimer (2-4), P (the primary donor), which is rapidly oxidized after absorption of a photon to produce P+ with an electron appearing on a BPh within about 4 ps (5, 6), thus forming the transient state P+BPh-(often denoted P+I-) (7-9). An electron is transferred from BPh-to the primary ubiquinone (QA) with a time constant of about 200 ps at 295 K, yielding the long-lived state P+QA (7)(8)(9)(10) "150-,us electron-transfer kinetics from QA to QB. They further reported that the yield of P+Q-formation after a saturating 400-ns flash was significantly reduced [from 102 ± 4% in native RCs (14)], with a concomitant formation of the triplet state, pR. The 100% yield of P+Q-was also restored by incorporation of any of the above-mentioned divalent metals.In this work we address the question of the role ofthe metal ion in the primary photochemistry, in particular in the formation and decay of state P+I-after a single turnover light flash. Transient optical spectra and kinetics were obtained with picosecond time resolution on native, Fe-depleted, and Zn-reconstituted RCs. From these and additional kinetic measurements made on a longer time scale, the yields of the different decay pathways of P+I-(to pR, to the ground state, and to P+Q-) were determined. A preliminary account of this work has been given (...

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Section: Resultssupporting

confidence: 66%

Primary photochemistry of iron-depleted and zinc-reconstituted reaction centers from Rhodopseudomonas sphaeroides

Kirmaier

Holten

Debus

et al. 1986

Proc. Natl. Acad. Sci. U.S.A.

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“…Because the absorption bands for the monomeric chromophores overlap substantially and one expects vibronic bands from the special pair in this region, it is difficult to quantitate IAL (19,20), and this shoulder has been ascribed by some to the Pt Be chargetransfer state (20). Measurements of delayed fluorescence (21) and the temperature dependence of the 3P decay rate (22) for Rb. (Fig. 3C) (17) and some calculations (9 The value offJIAtI for the Qy transition of the special pair in both species decreases substantially when the temperature is decreased, while that for the other bands decreases considerably less.…”

Section: Resultsmentioning

confidence: 99%

Stark effect spectroscopy of Rhodobacter sphaeroides and Rhodopseudomonas viridis reaction centers

Lockhart

Boxer

1988

Proc. Natl. Acad. Sci. U.S.A.

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134

129

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The nature of the initially excited state of the primary electron donor or special pair has been investigated by Stark effect spectroscopy for reaction centers from the photosynthetic bacteria Rhodopseudomonas viiis and Rhodobacter sphaeroides at 77 K. The data provide values for the magnitude of the difference in permanent dipole moment between the ground and excited state, IAuI and the angle C between ApL and the transition dipole moment for the electronic transition.IAdA and C for the lowest-energy singlet electronic transition associated with the special pair primary electron donor were found to be very similar for the two species. IA|I for this transition is substantially larger than for the Qy transitions of the monomeric pigments in the reaction center or for pure monomeric bacteriochlorophylls, for which Stark data are also reported. We conclude that the excited state of the special pair has substantial charge-transfer character, and we suggest that charge separation in bacterial photosynthesis is initiated immediately upon photoexcitation of the special pair. Data for Rhodobacter sphaeroides between 340 and 1340 nm are presented and discussed in the context of the detection of chargetransfer states by Stark effect spectroscopy.The recent elucidation of the organization of the reactive components in reaction centers (RCs) from photosynthetic bacteria by x-ray diffraction (1)(2)(3)(4) makes possible an analysis of the initial charge-separation mechanism from first principles. The first step in such an analysis is the calculation of the spectroscopic properties of the RC from several species, including the absorption, circular dichroism, and linear dichroism spectra (5-9). In a recent report we demonstrated that a quantitative analysis of the effect of an electric field on the absorption spectrum (the Stark effect spectrum) in Rhodobacter sphaeroides RCs at room temperature provides additional observables for comparison with theory (10). This work followed up on an earlier preliminary report by deLeeuv et al. (11). In the present communication we extend these measurements to a different species, Rhodopseudomonas viridis, and to low temperatures (12).Quantitative analysis of the Stark effect spectrum provides information on the magnitude of the difference in permanent dipole moment between the ground and excited state, JAput, and on the angle, (, between Ap and the transition dipole moment.Since the orientation of the transition dipole moment can be determined relative to the molecular axes from single-crystal polarized absorption measurements, the angle (can be related to the movement of charge associated with excitation of the chromophores in the RC. Stark effect data have also been obtained for monomeric photosynthetic chromophores, and this information should be useful for testing the reliability ofthe wave functions that are used for the calculation of RC spectral properties and electron transfer pathways. EXPERIMENTAL Rps. viridis RCs were prepared as previously described for Rb. sphaeroides (...

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“…Magnetic field modulated ISC has been detected in the photoconductivity [71][72][73], photoluminescence [74], excited state lifetime [75,76], and chemical reactivity [68] of many molecular systems, and even in the dynamics of photosynthesis in plants [77]. Magnetic field effects on the reaction outcome are already known for dozens of photochemical processes.…”

Section: Nanomagnetic Catalysismentioning

confidence: 99%

Spectroscopy in sculpted fields

2009

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Effect of magnetic fields on the triplet state lifetime in photosynthetic reaction centers: Evidence for thermal repopulation of the initial radical pair

Cited by 80 publications

References 20 publications

Primary photochemistry of iron-depleted and zinc-reconstituted reaction centers from Rhodopseudomonas sphaeroides

Primary photochemistry of iron-depleted and zinc-reconstituted reaction centers from Rhodopseudomonas sphaeroides

Stark effect spectroscopy of Rhodobacter sphaeroides and Rhodopseudomonas viridis reaction centers

Spectroscopy in sculpted fields

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