Vibrational Mechanism for Primary Charge Separation in the Reaction Center of <i>Rhodobacter Sphaeroides</i>

Ivashin, N. V.; Larsson, Sven

doi:10.1021/jp013431s

Cited by 22 publications

(22 citation statements)

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“…The nature of the oscillations is uncertain. Motions in the protein, 21,22 in the cofactors and their side groups 10 have been suggested. [23][24][25][26] Coupled motion between the two monomers in the special pair (SP), producing a rich dimer resonance Raman spectrum 27,28 and introducing oscillations in the energy of the P* state relative to the charge separated P + B A -state, is another very likely source.…”

Section: Introductionmentioning

confidence: 99%

“…[23][24][25][26] Coupled motion between the two monomers in the special pair (SP), producing a rich dimer resonance Raman spectrum 27,28 and introducing oscillations in the energy of the P* state relative to the charge separated P + B A -state, is another very likely source. 10,[22][23][24][25][26][27][28][29] Streltsov et al found a distinct oscillation at 12 ( 2 cm -1 and another at 30 ( 3 cm -1 . [16][17][18] More recently a 32 cm -1 oscillation was found in the work of Yakovlev et al and connected to ET.…”

Section: Introductionmentioning

confidence: 99%

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Trapped Water Molecule in the Charge Separation of a Bacterial Reaction Center

Ivashin

Larsson

2008

J. Phys. Chem. B

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Low-frequency oscillations in the absorption spectrum at 1020 nm, connected to the primary charge separation process in Rhodobacter sphaeroides, have been shown by Yakovlev et al. to be caused by rotational motion of an interstitial water molecule called "water-A". The same water molecule was shown by Potter et al. to increase the rate of charge separation by a factor of 8. We have carried out geometry optimization of water-A and its nearest atoms in the protein pocket, using density functional theory (DFT). There are strong hydrogen bonds to the axial imidazol group of the B part of the special pair (P=PAPB) and to the keto carbonyl group of ring V of the accessory chlorophyll (BA). Rotation of water-A is thus impossible in the electronic ground state. We have tried to support our speculations on other possible mechanisms by calculations. The P(+)BA(-) charge transfer state is stabilized by proton transfer from water-A and simultaneous proton transfer from the axial group of PB to water-A. After double proton transfer the hydrogen bond to the keto group disappears whereby a possibility opens up for almost free water rotation. The results therefore would explain the 32 cm(-1) oscillation of Yakovlev et al. The proposed mechanism assumes, however, that the general assumption that the activation energy disappears in the primary charge separation of bacterial photosynthesis, holds also for this special case.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Trapped Water Molecule in the Charge Separation of a Bacterial Reaction Center

Ivashin

Larsson

2008

J. Phys. Chem. B

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“…It has been confirmed that there is indeed an increase of electron density on B A during some time after excitation of P (see Refs. [20]). Recenty more accurate quantum chemical calculations (see Refs.…”

Section: Discussion Of Et In the Prc And Concluding Remarksmentioning

confidence: 99%

A Nonadiabatic Theory for Ultrafast Catalytic Electron Transfer: A Model for the Photosynthetic Reaction Center

Aubry

Kopidakis

2005

J Biol Phys

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A non-adiabatic theory of Electron Transfer (ET), which improves the standard theory near the inversion point and becomes equivalent to it far from the inversion point, is presented. The complex amplitudes of the electronic wavefunctions at different sites are used as Kramers variables for describing the quantum tunneling of the electron in the deformable potential generated by its environment (nonadiabaticity) which is modeled as a harmonic classical thermal bath. After exact elimination of the bath, the effective electron dynamics is described by a discrete nonlinear Schrödinger equation with norm preserving dissipative terms and a Langevin random force, with a frequency cut-off, due to the thermalized phonons.This theory reveals the existence of a specially interesting marginal case when the linear and nonlinear coefficients of a two electronic states system are appropriately tuned for forming a Coherent Electron-Phonon Oscillator (CEPO). An electron injected on one of the electronic states of a CEPO generates large amplitude charge oscillations (even at zero temperature) associated with coherent phonon oscillations and electronic level oscillations. This fluctuating electronic level may resonate with a third site which captures the electron so that Ultrafast Electron Transfer (UFET) becomes possible. Numerical results are shown where two weakly interacting sites, a donor and a catalyst, form a CEPO that triggers an UFET to an acceptor. Without a catalytic site, a very large energy barrier prevents any direct ET. This UFET is shown to have many qualitative features similar to those observed in the primary charge separation in photosynthetic reaction centers. We suggest that more generally, CEPO could be a paradigm for understanding many selective chemical reactions involving electron transfer in biosystems.

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“…It was previously suggested [12] that modes with frequencies 80-150 cm -1 are due to vibrations of the protein environment. Now the prevailing opinion is that the corresponding vibration is localized on the P dimer (see, for example, [17,44,54,76,[78][79][80]). The authors of [80] believe that the initiator of electron transfer is out-of-plane vibration of the acetyl side group of ring I.…”

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“…The temperature dependence of the halfwidth of the P870 and P960 bands are not described by the relation Γ(T) = Γ 0 + γ 0 coth 2 (θ/T) characteristic for h µ modes (see the examples of their appearance in [17]). Therefore the explanation in [80] seems quite dubious.…”

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Model for primary electron transfer and coupling of electronic states at reaction centers of purple bacteria

Pavlovich

2006

J Appl Spectrosc

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A detailed derivation is presented for relations making it possible to describe the effect of temperature on the halfwidth of the P960 and P870 absorption bands and also on the electron transfer (ET) rate at reaction centers (RCs) of the purple bacteria Rps. viridis and Rb. sphaeroides. Primary electron transfer is considered as a resonant nonradiative transition between P * and P + B L − states (where P is a special pair, B L is an additional bacteriochlorophyll in the L branch of the reaction center). It has been shown that the vibrational h α mode with frequency 130-150 cm -1 controls primary electron transfer. It has been found that the matrix element of the electronic transition between the states P * and P + B L − is equal to 12.7 ± 0.9 and 12.0 ± 1.2 cm -1 for Rps.viridis and Rb. sphaeroides respectively. The mechanism is discussed for electron transport from P * and B L and then to bacteriopheophytin H L .Introduction. In bacterial photosynthesis, we can isolate two fundamentally important steps in conversion of solar energy (see, for example, [1][2][3][4][5][6][7][8]). The first step involves absorption of light by light-harvesting antennas LH2 and LH1 followed by transfer of excitation energy to the reaction center. The second step involves electron transport processes in the reaction center. In turn, it includes primary electron transfer from a special pair (P), which is a dimer of bacteriochlorophyll a or b (BChl) to bacteriopheophytin (H) with participation of additional BChl (B), and also secondary electron transfer from bacteriopheophytin to ubiquinone. As a result, rather stable charge separation occurs in the membrane, which controls the sequence of dark electrochemical processes.It is important that in the native forms of the reaction centers, the efficiency of charge separation reaches 100% [9]. For comparison, we note that the best types of semiconductor solar cells provide an efficiency <20% [10]. So persistent efforts have been applied to designing "molecular devices" which under artificial conditions would reproduce the highly efficient processes of conversion of excitation energy to electrical energy, like what occurs in bacterial photosynthesis. Specialists working in the field of storage and conversion of solar energy are especially interested in photosynthesis problems [6,8,11].The task of this work is narrower. It involves refining the theory of the effect of temperature on the spectra and rate of electron transfer (ET), and also developing a method for determining the matrix element for an electronic transition with electron transfer to the reaction center of purple bacteria.Theory of activationless electron transfer. The well-known theory of Bixon and Jortner [12] is based on ideas concerning the effect of polar modes in complex biological systems on the electron transfer rate k ET . According to [12]:

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Vibrational Mechanism for Primary Charge Separation in the Reaction Center of Rhodobacter Sphaeroides

Cited by 22 publications

References 70 publications

Trapped Water Molecule in the Charge Separation of a Bacterial Reaction Center

Trapped Water Molecule in the Charge Separation of a Bacterial Reaction Center

A Nonadiabatic Theory for Ultrafast Catalytic Electron Transfer: A Model for the Photosynthetic Reaction Center

Model for primary electron transfer and coupling of electronic states at reaction centers of purple bacteria

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