Electrogenic reduction of the secondary quinone acceptor in chromatophores of Rhodospirillum rubrum: Rapid kinetics measurements

Electron transfer Q_A → Q_B has been reconstituted with added Q-10 in Rhodospirillum rubrum chromatophores associated with a phospholipid-impregnated collodion film. Rapid kinetics measurements of laser flash-induced ΔΨ generated in the chromatophores show that whereas electron transfer from Q_a⁻ to Q_B upon the first flash is not electrogenic in dark-adapted chromatophores, reduction of Q⁻_B to Q_bh₂ induced by the second flash gives rise to an electrogenic phase with τ = 250 μs at pH 7.5 which contributes about 10% to the total ΔΨ generated upon the flash. The electrogenic phase is ascribed to vectorial protonation of Q²⁻_B.     

Photoelectric effects in bacterial chromatophores. Comparison of spectral and direct electrometric methods

Generation of photoelectric potential in chromatophores of Rhodopseudomonas sphaeroides has been measured (i) spectrophotometrically, using electrochromic shift of carotenoid absorption band or (ii) electrometrically, by means of two electrodes separated by a collodion film covered on one side with chromatophores. A 15 ns laser flash was used to induce a single turnover of photosynthetic reaction centers. It was found that results obtained by both methods are similar in (i) direction of electric vector (the chromatophore interior positive) and (ii) redox titration curves (E_m = 10mV). The magnitudes of the photopotential were about 60 and 25 mV, when monitored with spectral and electrometric techniques, respectively. In both cases, the rise times of the photopotentials were faster than time resolution of the techniques used. Decay of the response of carotenoids was found to be slower than that in the collodion film system. The addition of ubiquinone Q₁₀ into the decane solution of asolectin used to impregnate the collodion film led to slowing down of the decay. The carotenoid response decay could be accelerated by FCCP or o-phenanthroline. In the latter case, the shape of the decay curve coincides with decay of the photopotential measured in the collodion film system. It is suggested that decane extracts secondary ubiquinone from chromatophores attached to the collodion film. Such an unfavorable effect can be strongly decreased by added ubiquinone     

Pressure effects on the photochemistry of bacterial photosynthetic reaction centers from Rhodobacter sphaeroides R-26 and Rhodopseudomonas viridis

Electron-transfer reactions and triplet decay rates have been studied at pressures up to 300 MPa. In reaction centers from Rhodobacter sphaeroides R-26, high pressure hastened the electron transfers from both the primary and secondary quinones (Q_A and Q_B) to the primary electron donor bacteriochlorophyll, P. Motion of Q_A between two sites, one nearer to P and the other nearer to Q_B, could account for these pressure effects. In reaction centers from Rhodopseudomonas viridis, charge recombination was slowed by high pressure. Decay rates were also studied for the triplet state, P^R. In Rb. sphaeroides R-26 with Q_A reduced with Na₂S₂O₄, the decay was hastened by pressure. This could be explained if P^R decays through a charge-transfer triplet state, or if the decay kinetics of P^R are sensitive to the distance between P and Q_A⁻. In Rps. viridis reaction centers, and in Rb. sphaeroides reaction centers that were depleted of Q_A, the lifetime of P^R was not altered by pressure.     

Influence of magnetic fields on the P-870 triplet state in Rps. sphaeroides reaction centers

Magnetic fields influence two properties of the P-870 triplet state observed in Rps. sphaeroides reaction centers: the yield of formation and the kinetics of decay. These effects have been studied in reaction centers which were prepared in three different states: state Q_A ^–, state Q_A ^2– and state (– Q_A) (Q_A depleted). The triplet yields decrease with increasing magnetic fields, with B1/2's of about 140, 41 and 57 Gauss, respectively. The half-time of ³P-870 decay is not influenced by the field in state Q_A ^–; it increases at increasing fields, in state Q_A ^2– and state (– Q_A), with the same B1/2 as the triplet yield. These results are discussed in the framework of current theories of the radical-pair dynamics and of the mechanism of triplet decay.Abbreviations I primary electron acceptor - LDAO lauryldimethylamine oxide - P-870 primary electron donor - Q_A first quinone acceptor - SDS sodium dodecylsulfate - YAG Yttrium Aluminum Garnet     

Kinetics and thermodynamics of the P870+Q−A → P870+Q−B reaction in isolated reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides

The temperature dependences of the P870⁺Q^?_A → P870Q_A and P870⁺Q^?_B → P870Q_B recombination reactions were measured in reaction centers from Rhodopseudomonas sphaeroides. The data indicate that the P870⁺Q^?_B state decays by thermal repopulation of the P870⁺Q^?_A state, followed by recombination. ΔG° for the P870⁺Q^?_A → P870⁺Q^?_B reaction is ?6.89 kJ · mol^?1, while ΔH° = ?14.45 kJ · mol^?1 and ?TΔS° = + 7.53 kJ · mol^?1. The activation ethalpy, $\text{H}{}^3$, for the P870⁺Q^?_A Δ P870⁺Q^?_B reaction is +56.9 kJ · mol^?1, while the activation entropy is near zero. The results permit an estimate of the shape of the potential energy curve for the P870⁺Q^?_A → P870⁺Q^?_B electron transfer reaction.     

A study of the kinetic properties of the stable semiquinone of the reaction-center secondary acceptor in chromatophores of non-sulfur purple bacteria

Flash-induced absorption changes at 450 nm were investigated in isolated chromatophores of Rhodopseudomonas sphaeroides and Rhodospirillum rubrum non-sulfur purple bacteria to follow the redox changes of the semiquinone species of the secondary quinone acceptor of the photosynthetic reaction center. Excitation of a dark-adapted chromatophore suspension by a series of successive flashes in the presence of electron donors capable of rapidly reducing the photooxidized reaction-center pigment causes the formation of a stable semiquinone species (Q^⨪_B) with a lifetime which is shown to be proportional to the amount of the oxidized redox mediator in the incubation medium. It is shown that the disappearance of the flash-induced absorption changes at 450 nm on lowering the ambient redox potential (E_h) to 200–300 mV is the result of increasing the lifetime of Q^⨪_B, as the amount of the oxidized mediator diminishes; consequently, in these circumstances, the 2–5 min dark interval between the flash cycles appears insufficient for Q^⨪_B recovery. After the addition of redox mediators with a low midpoint potential, acting as an oxidant for Q^⨪_B, the flash-induced redox changes of Q^⨪_B were observed at low E_h values unless E_h reached a value at which Q_B underwent reduction at equilibrium to form Q_BH₂. The data provide evidence that reaction centers with a fully oxidized secondary acceptor can donate electrons to the cyclic electron-transport chain only after two turnovers, leading to the formation of the doubly reduced ubiquinone species (Q_BH₂) of the secondary acceptor.     

Probing the fluorescence emission kinetics of the photosynthetic apparatus of Rhodopseudomonas sphaeroides, strain 1760-1, on a picosecond pulse fluorometer

Using the pulse picosecond fluorometric technique the fluorescence properties of intact cells, isolated chromatophores and photosynthetic reaction centres were studied in bacteria Rhodopseudomonas sphaeroides, strain 1760-1.The fluorescent emission from reduced reaction centres excited by 694.3 nm light has a biphasic character, the lifetimes of the components being τ₁ = 15±8 ps and τ₂ = 250 ps. The faster component, τ₁, contributes to the integral fluorescence in the long wavelength region. It disappears with oxidation of the reaction centres and is attributed to photoactive bacteriochlorophyll P870. The slow component, τ, is apparently due to both bacteriochlorophyll P800 and bacteriopheophytin. The fluorescence from intact cells exhibits a monophasic pattern and decays with τ = 200 ps.The fluorescence emitted by chromatophores comprises two components with τ₃ = 200 ps and τ₄ = 4200 ps. The duration of fluorescence τ₃ increases to its maximum of 500–550 ps, as P870 is oxidized chemically or photochemically, while τ₄ remains unchanged. The fluorescence with a lifetime of 200 ps was ascribed to the photosystem and the 4200-ps fluorescence to bacteriochlorophyll which had lost its functional links with the photosystem.The rise time of the fluorescence emitted by chromatophores varies from 60 or 70 ps to 350 ps depending on the wavelength of the exciting light and the recorded spectral region. On the basis of our findings the rate for energy migration was estimated to be 10⁹ s^?1.     

Two regimens of electrogenic cyclic redox chain operation in chromatophores of non-sulfur purple bacteria A study using antimycin A

Antimycin A causes a biphasic suppression of the light-induced membrane potential generation in Rhodospirillum rubrum and Rhodopseudomonas sphaeroides chromatophores incubated anaerobically. The first phase is observed at low antibiotic concentrations and is apparently due to its action as a cyclic electron transfer inhibitor. The second phase is manifested at concentrations which are greater than 1–2 μM and is due to uncoupling that may be connected with an antibiotic-induced dissipation of the electrochemical H⁺ gradient across the chromatophore membrane. The inhibitory effect of anti-mycin added at low concentrations under aerobic conditions is removed by succinate to a large extent. It is expected that the electrogenic cyclic redox chain in the bacterial chromatophores incubated under conditions of continuous illumination may function at two regimes: (1) as a complete chain involving all the redox components, and (2) as a shortened chain involving only the P-870 photoreaction center, ubiquinone and cytochrome c₂.     

Orientation of reaction center complexes fromRhodobacter sphaeroides in proteoliposomes and the effect ofo-phenanthroline on electrogenesis during primary photochemical reaction

The orientation ofRhodobacter sphaeroides reaction center complexes (RC complexes) in proteoliposomal membranes was investigated by a direct electrometric method. Conditions were found that allow monitoring of only that RC complex fraction that is oriented with its donor side to the inner part of the proteoliposome. It is shown thato-phenanthroline, an inhibitor of electron transfer between primary (Q_A) and secondary (Q_B) quinone acceptors, can also inhibit the photoinduced Q_A reduction. The efficiency of this inhibition depends on the concentration of added ubiquinone. It is assumed that the laser flash-inducedo-phenanthroline inhibition of primary dipole (P-870⁺ · Q _A ^– ) formation is of a competitive nature.     

Quinone transport in the closed light-harvesting 1 reaction center complex from the thermophilic purple bacterium Thermochromatium tepidum

Redox-active quinones play essential roles in efficient light energy conversion in type-II reaction centers of purple phototrophic bacteria. In the light-harvesting 1 reaction center (LH1-RC) complex of purple bacteria, Q_B is converted to Q_BH₂ upon light-induced reduction and Q_BH₂ is transported to the quinone pool in the membrane through the LH1 ring. In the purple bacterium Rhodobacter sphaeroides, the C-shaped LH1 ring contains a gap for quinone transport. In contrast, the thermophilic purple bacterium Thermochromatium (Tch.) tepidum has a closed O-shaped LH1 ring that lacks a gap, and hence the mechanism of photosynthetic quinone transport is unclear. Here we detected light-induced Fourier transform infrared (FTIR) signals responsible for changes of Q_B and its binding site that accompany photosynthetic quinone reduction in Tch. tepidum and characterized Q_B and Q_BH₂ marker bands based on their ¹⁵N- and ¹³C-isotopic shifts. Quinone exchanges were monitored using reconstituted photosynthetic membranes comprised of solubilized photosynthetic proteins, membrane lipids, and exogenous ubiquinone (UQ) molecules. In combination with ¹³C-labeling of the LH1-RC and replacement of native UQ₈ by ubiquinones of different tail lengths, we demonstrated that quinone exchanges occur efficiently within the hydrophobic environment of the lipid membrane and depend on the side chain length of UQ. These results strongly indicate that unlike the process in Rba. sphaeroides, quinone transport in Tch. tepidum occurs through the size-restricted hydrophobic channels in the closed LH1 ring and are consistent with structural studies that have revealed narrow hydrophobic channels in the Tch. tepidum LH1 transmembrane region.     

Characterization of a semi-stable,charge-separated state in reaction centers from Rhodobacter sphaeroides

In reaction centers from Rhodobacter sphaeroides, subjected to continuous illumination in the presence of an inhibitor of the Q_A to Q_B electron transfer, the oxidation of P870 consisted of several kinetic phases with a fast initial reaction followed by very slow accumulation of P870⁺ with a halftime of several minutes. When the light was turned off, a phase of fast charge recombination was followed by an equally slow reduction of P870⁺. In reaction centers depleted of Q_B, where forward electron transfer from Q_A is also prevented, the slow reactions were also observed but with different kinetic properties. The kinetic traces of accumulation and decay of P870⁺ could be fitted to a simple three-state model where the initial, fast charge separation is followed by a slow reversible conversion to a long-lived, charge-stabilized state. Spectroscopic examination of the charge-separated, semi-stable state, using optical absorbance and EPR spectroscopy, suggests that the unpaired electron on the acceptor side is located in an environment significantly different from normal. The activation parameters and enthalpy and entropy changes, determined from the temperature dependence of the slow conversion reaction, suggest that this might be coupled to changes in the protein structure of the reaction centers, supporting the spectroscopic results. One model that is consistent with the present observations is that reaction centers, after the primary charge separation, undergo a slow, light-induced change in conformation affecting the acceptor side. This revised version was published online in June 2006 with corrections to the Cover Date.     

Affinity and activity of non-native quinones at the QB site of bacterial photosynthetic reaction centers

Purple, photosynthetic reaction centers from Rhodobacter sphaeroides bacteria use ubiquinone (UQ₁₀) as both primary (Q_A) and secondary (Q_B) electron acceptors. Many quinones reconstitute Q_A function, while a few will act as Q_B. Nine quinones were tested for their ability to bind and reconstitute Q_A and Q_B functions. Only ubiquinone (UQ) reconstitutes both functions in the same protein. The affinities of the non-native quinones for the Q_B site were determined by a competitive inhibition assay. The affinities of benzoquinones, naphthoquinone (NQ), and 2-methyl-NQ for the Q_B site are 7 ± 3 times weaker than that at Q_A site. However, di-ortho-substituted NQs and anthraquinone bind tightly to the Q_A site (K _d ≤ 200 nM), and ≥1,000 times more weakly to the Q_B site, perhaps setting a limit on the size of the site. With a low-potential electron donor, 2-methyl, 3-dimethylamino-1,4-NQ, (Me-diMeAm-NQ) at Q_A, Q_B reduction is 260 meV, more favorable than with UQ as Q_A. Electron transfer from Me-diMeAm-NQ at the Q_A site to NQ at the Q_B site can be detected. In the Q_B site, the NQ semiquinone is estimated to be ≈60–100 meV higher in energy than the UQ semiquinone, while in the Q_A site, the semiquinone energy level is similar or lower with NQ than with UQ. Thus, the NQ semiquinone is more stable in the Q_A than in the Q_B site. In contrast, the native UQ semiquinone is ≈60 meV lower in energy in the Q_B than in the Q_A site, stabilizing forward electron transfer from Q_A to Q_B.     

Absorption and structure of an LM unit isolated with sodium dodecyl sulfate from reaction centers of Rhodopseudomonas sphaeroides

Low-temperature absorption, circular dichroism and resonance Raman spectra of the LM units isolated with sodium dodecyl sulfate from wild-type Rhodopseudomonas sphaeroides reaction centers (Agalidis, I. and Reiss-Husson, F. (1983) Biochim. Biophys. Acta 724, 340–351) are described in comparison with those of intact reaction centers. In LM unit, the Q_y absorption band of P-870 at 77 K shifted from 890 nm (in reaction center) to 870 nm and was broadened by about 30%. In contrast, the 800 nm bacteriochlorophyll absorption band including the 810 species remained unmodified. It was concluded that the 810 nm transition is not the higher excitonic component of P-870. The Q_x band of P-870 shifted from 602 nm (in reaction center) to 598 nm in LM, whereas the Q_x band of the other bacteriochlorophylls was the same in reaction center and LM and had two components at about 605 and 598 nm. The Q_xII band of bacteriopheophytin was upshifted to 538 nm and a slight blue shift of the Q_y band of bacteriopheophytin was observed. Resonance Raman spectra of spheroidene in LM showed that its native cis-conformation was preserved. Resonance Raman spectroscopy also demonstrated that in LM the molecular interactions assumed by the conjugated carbonyls of bacteriochlorophyll molecules were altered, but not those assumed by the bacteriopheophytins carbonyls. In particular at least one Keto group of bacteriochlorophyll free in reaction center, becomes intermolecularly bounded in LM (possibly with extraneous water). This group may belong to the primary donor molecules.     

Differential extraction and structural specificity of specialized ubiquinone molecules in secondary electron transfer in chromatophores from Rhodopseudomonas sphaeroides, Ga

In chromatophores from the facultative photosynthetic bacterium, Rhodopseudomonas sphaeroides, Ga, the function of ubiquinone-10 (UQ-10) at two specialized binding sites (Q_B and Q_Z) has been determined by kinetic criteria. These were the rate of rereduction of flash-oxidized [BChl]₂⁺ through the back reaction, or the binary pattern of cytochrome b₅₆₁ (for the Q_b site), and the rapid rate of rereduction of flash-oxidized cytochrome c, or the relative amplitude of the antimycin-sensitive Phase III ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{~ 1.5}\text{ms}$) of the carotenoid spectral shift induced by a single turnover flash at E_h ~ 100 mV (for the Q_Z site). The phenomenon associated with the two binding sites behaved differently on extraction of UQ from lyophilized chromatophores using isooctane. By this selective extraction procedure it has been possible to show that UQ-10 molecules are required at different concentrations in the membrane for specific redox events in secondary electron transfer. The reduction of cytochrome b occurs in particles which no longer show the phenomena associated with Q_Z, but still possess a large proportion of Q_b, while rapid rereduction of flash-oxidized cytochrome c requires an additional complement of UQ-10 (Q_Z). Extracted particles lacking Q_Z and a large amount of Q_B have been reconstituted with different UQ homologs (UQ-1, UQ-3, and UQ-10). Specific redox events have been studied in reconstituted particles. All UQ homologs act as secondary acceptors from the reaction center; UQ-3 and UQ-10, but not UQ-1, are also able to reconstitute the function of Q_Z as electron donor to cytochrome c. Only UQ-10, however, is able to restore normal rates of the overall cyclic electron transfer induced by a train of flashes, and maximal rates of the light-induced ATP synthesis. The results are interpreted in terms of Q-cycle mechanisms in which quinone and quinol at both the Q_Z and Q_b sites are in rapid equilibrium with the quinone pool.     

Modulation of the redox state of quinones by light in Rhodobacter sphaeroides under anaerobic conditions

Illumination of intact cells of Rhodobacter sphaeroides under anaerobic conditions has a dual effect on the redox state of the quinone pool. A large oxidation of the quinone pool is observed during the first seconds following the illumination. This oxidation is suppressed by the addition of an uncoupler in agreement with a light-induced reverse electron transfer at the level of the complex I, present both in the non-invaginated part of the membrane and in the chromatophores. At longer dark times, this illumination increases the reducing power of the cells leading to a significant reduction of the others reaction centers (RCs). From the observation that a significant proportion of RCs could be reduced by the preillumination without affecting the numbers of charge separation for the RCs, we conclude that there is no rapid thermodynamic equilibrium between the quinones present in the non-invaginated part of the membrane and those localized in the chromatophores. Under anaerobic conditions where the chromatophores quinone pool is fully reduced, we deduce, on the basis of flash-induced fluorescence kinetics, that the reduced RCs are exclusively reoxidized by the quinone generated at the Q _o site of the cyt bc ₁ complex. The supramolecular association between a dimeric RC-LHI complex and one cyt bc ₁ complex allows the confinement of a quinone between the RC-LHI directly associated to the cyt bc ₁ complex.     

Free energy change accompanying electron transfer from P870 to quinone in Rhodopseudomonas sphaeroides chromatophores

Delayed fluorescence of chromatophores of Rhodopseudomonas sphaeroides was measured to estimate the standard free energy change accompanying the electron transfer from the bacteriochlorophyll dimer (P) to the primary acceptor quinone (Q_A). The chromatophores emitted delayed fluorescence with a lifetime of about 60 ms in the presence of o-phenanthroline. By comparing the intensity of the delayed fluorescence with that of the prompt fluorescence, the standard free energy of the P⁺Q_A^? radical pair was evaluated. It was about 0.87 eV below the level of excited singlet state, P1Q_A, or 0.51 eV above the ground state, PQ_A, independent of pH.     

Redox potential of the non-heme iron complex in bacterial photosynthetic reaction center

In bacterial photosynthetic reaction centers (bRC), the electron is transferred from the special pair (P) via accessory bacteriochlorophyll (B_A), bacteriopheopytin (H_A), the primary quinone (Q_A) to the secondary quinone (Q_B). Although the non-heme iron complex (Fe complex) is located between Q_A and Q_B, it was generally supposed not to be redox-active. Involvement of the Fe complex in electron transfer (ET) was proposed in recent FTIR studies [A. Remy and K. Gerwert, Coupling of light-induced electron transfer to proton uptake in photosynthesis, Nat. Struct. Biol. 10 (2003) 637-644]. However, other FTIR studies resulted in opposite results [J. Breton, Steady-state FTIR spectra of the photoreduction of Q_A and Q_B in Rhodobacter sphaeroides reaction centers provide evidence against the presence of a proposed transient electron acceptor X between the two quinones, Biochemistry 46 (2007) 4459-4465]. In this study, we calculated redox potentials of Q_A/B (E_m(Q_A/B)) and the Fe complex (E_m(Fe)) based on crystal structure of the wild-type bRC (WT-bRC), and we investigated the energetics of the system where the Fe complex is assumed to be involved in the ET. E_m(Fe) in WT-bRC is much less pH-dependent than that in PSII. In WT-bRC, we observed significant coupling of ET with Glu-L212 protonation upon oxidation of the Fe complex and a dramatic E_m(Fe) downshift by 230 mV upon formation of Q_A⁻ (but not Q_B⁻) due to the absence of proton uptake of Glu-L212. Changes in net charges of the His ligands of the Fe complex appear to be the nature of the redox event if we assume the involvement of the Fe complex in the ET.     

Effect of inhibitors,redox state and isoprenoid chain length on the affinity of ubiquinone for the secondary acceptor binding site in the reaction centers of photosynthetic bacteria

Quinone and inhibitor binding to Rhodopseudomonas sphaeroides (R-26 and GA) reaction centers were studied using spectroscopic methods and by direct adsorption of reaction centers onto anion exchange filters in the presence of ¹⁴C-labelled quinone or inhibitor. These measurements show that as secondary acceptor, Q_B, ubiquinone (UQ) is tightly bound in the semiquinone form and loosely bound in the quinone and quinol forms. The quinol is probably more loosely bound than the quinone. o-Phenanthroline and terbutryn, a triazine inhibitor, compete with UQ and with each other for binding to the reaction center. Inhibition by o-phenanthroline of electron transfer from the primary to the secondary quinone acceptor (Q_A to Q_B) occurs via displacement of UQ from the Q_B binding site. Displacement of UQ by terbutryn is apparently accessory to the inhibition of electron transfer. Terbutryn binding is lowered by reduction of Q_B to Q^?_B but is practically unaffected by reduction of Q_A to Q^?_A in the absence of Q_B. UQ-9 and UQ-10 have a 5- to 6-fold higher binding affinity to the Q_B site than does UQ-1, indicating that the long isoprenoid chain facilitates the binding to the Q_B site.     

Energy and electron transfer in the photosynthetic reaction center complex of Acidiphilium rubrum containing Zn-bacteriochlorophyll a studied by femtosecond up-conversion spectroscopy

A photosynthetic reaction center (RC) complex was isolated from a purple bacterium, Acidiphilium rubrum. The RC contains bacteriochlorophyll a containing Zn as a central metal (Zn-BChl a) and bacteriopheophytin a (BPhe a) but no Mg-BChl a. The absorption peaks of the Zn-BChl a dimer (P_Zn), the accessory Zn-BChl a (B_Zn), and BPhe a (H) at 4 K in the RC showed peaks at 875, 792, and 753 nm, respectively. These peaks were shorter than the corresponding peaks in Rhodobacter sphaeroides RC that has Mg-BChl a. The kinetics of fluorescence from P_Zn^*, measured by fluorescence up-conversion, showed the rise and the major decay with time constants of 0.16 and 3.3 ps, respectively. The former represents the energy transfer from B_Zn^* to P_Zn, and the latter, the electron transfer from P_Zn to H. The angle between the transition dipoles of B_Zn and P_Zn was estimated to be 36° based on the fluorescence anisotropy. The time constants and the angle are almost equal to those in the Rb. sphaeroides RC. The high efficiency of A. rubrum RC seems to be enabled by the chemical property of Zn-BChl a and by the L168HE modification of the RC protein that modifies P_Zn.     

Coupling of collective motions of the protein matrix to vibrations of the non-heme iron in bacterial photosynthetic reaction centers

Non-heme iron is a conservative component of type II photosynthetic reaction centers of unknown function. We found that in the reaction center from Rba. sphaeroides it exists in two forms, high and low spin ferrous states, whereas in Rsp. rubrum mostly in a low spin state, in line with our earlier finding of its low spin state in the algal photosystem II reaction center (Burda et al., 2003). The temperature dependence of the non-heme iron displacement studied by Mössbauer spectroscopy shows that the surrounding of the high spin iron is more flexible (Debye temperature ~ 165 K) than that of the low spin atom (~ 207 K). Nuclear inelastic scattering measurements of the collective motions in the Rba. sphaeroides reaction center show that the density of vibrational states, originating from non-heme iron, has well-separated modes between lower (4-17 meV) and higher (17-25 meV) energies while in the one from Rsp. rubrum its distribution is more uniform with only little contribution of low energy (~ 6 meV) vibrations. It is the first experimental evidence that the fluctuations of the protein matrix in type II reaction center are correlated to the spin state of non-heme iron. We propose a simple mechanism in which the spin state of non-heme iron directly determines the strength of coupling between the two quinone acceptors (Q_A and Q_B) and fast collective motions of protein matrix that play a crucial role in activation and regulation of the electron and proton transfer between these two quinones. We suggest that hydrogen bond network on the acceptor side of reaction center is responsible for stabilization of non-heme iron in different spin states.