Long-lived charge-separated states in bacterial reaction centers isolated from Rhodobacter sphaeroides

Mn-porphyrins in a four-helix bundle participate in photo-induced electron transfer with a bacterial reaction center

Hybrid complexes incorporating synthetic Mn-porphyrins into an artificial four-helix bundle domain of bacterial reaction centers created a system to investigate new electron transfer pathways. The reactions were initiated by illumination of the bacterial reaction centers, whose primary photochemistry involves electron transfer from the bacteriochlorophyll dimer through a series of electron acceptors to the quinone electron acceptors. Porphyrins with diphenyl, dimesityl, or fluorinated substituents were synthesized containing either Mn or Zn. Electrochemical measurements revealed potentials for Mn(III)/Mn(II) transitions that are ~ 0.4 V higher for the fluorinated Mn-porphyrins than the diphenyl and dimesityl Mn-porphyrins. The synthetic porphyrins were introduced into the proteins by binding to a four-helix bundle domain that was genetically fused to the reaction center. Light excitation of the bacteriochlorophyll dimer of the reaction center resulted in new derivative signals, in the 400 to 450 nm region of light-minus-dark spectra, that are consistent with oxidation of the fluorinated Mn(II) porphyrins and reduction of the diphenyl and dimesityl Mn(III) porphyrins. These features recovered in the dark and were not observed in the Zn(II) porphyrins. The amplitudes of the signals were dependent upon the oxidation/reduction midpoint potentials of the bacteriochlorophyll dimer. These results are interpreted as photo-induced charge-separation processes resulting in redox changes of the Mn-porphyrins, demonstrating the utility of the hybrid artificial reaction center system to establish design guidelines for novel electron transfer reactions.

Photoactivation and conformational gating for manganese binding and oxidation in bacterial reaction centers

The influence of illumination history of native bacterial reaction centers (BRCs) on the ability of binding and photo-induced oxidation of manganous ions was investigated in the pH range between 8.0 and 9.4. Binding of manganous ions to a buried site required 6 to 11-fold longer incubation periods, depending on the pH, in dark-adapted BRCs than in BRCs that were previously illuminated prior to manganese binding. The intrinsic electron transfer from the bound manganese ion to the photo-oxidized primary electron donor was found to be limited by a 2 to 5-fold slower precursor conformational step in the dark-adapted samples for the same pH range. The conformational gating could be eliminated by photoactivation, namely if the BRCs were illuminated prior to binding. Unlike in Photosystem II, photoactivation in BRCs did not involve cluster assembly. Photoactivation with manganese already bound was only possible at elevated detergent concentration. In addition, also exclusively in dark-adapted BRCs, a marked breaking point in the Arrhenius-plot was discovered around 15 °C at pH 9.4 indicating a change in the reaction mechanism, most likely caused by the change of orientation of the 2-acetyl group of the inactive bacteriochlorophyll monomer located near the manganese binding site.

Identification of amino acid residues in a proton release pathway near the bacteriochlorophyll dimer in reaction centers from Rhodobacter sphaeroides

Insight into control of proton transfer, a crucial attribute of cellular functions, can be gained from investigations of bacterial reaction centers. While the uptake of protons associated with the reduction of the quinone is well characterized, the release of protons associated with the oxidized bacteriochlorophyll dimer has been poorly understood. Optical spectroscopy and proton release/uptake measurements were used to examine the proton release characteristics of twelve mutant reaction centers, each containing a change in an amino acid residue near the bacteriochlorophyll dimer. The mutant reaction centers had optical spectra similar to wild-type and were capable of transferring electrons to the quinones after light excitation of the bacteriochlorophyll dimer. They exhibited a large range in the extent of proton release and in the slow recovery of the optical signal for the oxidized dimer upon continuous illumination. Key roles were indicated for six amino acid residues, Thr L130, Asp L155, Ser L244, Arg M164, Ser M190, and His M193. Analysis of the results points to a hydrogen-bond network that contains these residues, with several additional residues and bound water molecules, forming a proton transfer pathway. In addition to proton transfer, the properties of the pathway are proposed to be responsible for the very slow charge recombination kinetics observed after continuous illumination. The characteristics of this pathway are compared to proton transfer pathways near the secondary quinone as well as those found in photosystem II and cytochrome c oxidase.

Tuning the redox potential of the primary electron donor in bacterial reaction centers by manganese binding and light-induced structural changes

The influence of transition metal binding on the charge storage ability of native bacterial reaction centers (BRCs) was investigated. Binding of manganous ions uniquely prevented the light-induced conformational changes that would yield to long lifetimes of the charge separated state and the drop of the redox potential of the primary electron donor (P). The lifetimes of the stable charge pair in the terminal conformations were shortened by 50-fold and 7-fold upon manganous and cupric ion binding, respectively. Nickel and zinc binding had only marginal effects. Binding of manganese not only prevented the drop of the potential of P/P⁺ but also elevated it by up to 117 mV depending on where the metal was binding. With variable conditions, facilitating either manganese binding or light-induced structural changes a controlled tuning of the potential of P/P⁺ in multiple steps was demonstrated in a range of ~200 mV without the need of a mutation or synthesis. Under the selected conditions, manganese binding was achieved without its photochemical oxidation thus, the energized but still native BRCs can be utilized in photochemistry that is not reachable with regular BRCs. A 42 Å long hydrophobic tunnel was identified that became obstructed upon manganese binding and its likely role is to deliver protons from the hydrophobic core to the surface during conformational changes.

Bound detergent molecules in bacterial reaction centers facilitate detection of tetryl explosive

Bacterial reaction centers (BRC) from Rhodobacter sphaeroides were found to accelerate, about 100-fold, the reaction between tetryl (2,4,6-trinitrophenylmethylnitramine) explosive and n-lauryl-N-N-dimethylamine-N-oxide (LDAO) that results in the formation of picric acid-like product with characteristic UV-VIS absorption spectrum with peaks at 345 and 415 nm. Moreover, this product also affects the spectra of BRC cofactors in the NIR spectral region and stabilizes the conformational changes associated with slow charge recombination. The evolution of the NIR absorption changes correlated with the kinetics of the product formation. Comparison between the wild-type and the R26 carotenoid-less strain indicates that tetryl-LDAO reaction is roughly five times faster for R26, which allows for identifying the carotenoid binding site as the optimal reaction site. Another, less-defined reaction site is located in the BRC's hydrophobic cavity. These effects are highly selective for tetryl and not observed for several other widespread nitric explosives; slowed-down charge recombination allows for distinguishing between tetryl and Q_B-site herbicides. The current limit of detection is in the ppb range or ~ 100 nM. Details of the molecular mechanisms of the reactions and perspectives of using these effects in bioassays or biosensors for explosives detection are also discussed.

Characterization of mercury(II)-induced inhibition of photochemistry in the reaction center of photosynthetic bacteria

Mercuric contamination of aqueous cultures results in impairment of viability of photosynthetic bacteria primarily by inhibition of the photochemistry of the reaction center (RC) protein. Isolated reaction centers (RCs) from Rhodobacter sphaeroides were exposed to Hg²⁺ ions up to saturation concentration (~ 10³ [Hg²⁺]/[RC]) and the gradual time- and concentration-dependent loss of the photochemical activity was monitored. The vast majority of Hg²⁺ ions (about 500 [Hg²⁺]/[RC]) had low affinity for the RC [binding constant K_b ~ 5 mM^-1] and only a few (~ 1 [Hg²⁺]/[RC]) exhibited strong binding (K_b ~ 50 μM^-1). Neither type of binding site had specific and harmful effects on the photochemistry of the RC. The primary charge separation was preserved even at saturation mercury(II) concentration, but essential further steps of stabilization and utilization were blocked already in the 5 < [Hg²⁺]/[RC] < 50 range whose locations were revealed. (1) The proton gate at the cytoplasmic site had the highest affinity for Hg²⁺ binding (K_b ~ 0.2 μM^-1) and blocked the proton uptake. (2) Reduced affinity (K_b ~ 0.05 μM^-1) was measured for the mercury(II)-binding site close to the secondary quinone that resulted in inhibition of the interquinone electron transfer. (3) A similar affinity was observed close to the bacteriochlorophyll dimer causing slight energetic changes as evidenced by a ~ 30 nm blue shift of the red absorption band, a 47 meV increase in the redox midpoint potential, and a ~ 20 meV drop in free energy gap of the primary charge pair. The primary quinone was not perturbed upon mercury(II) treatment. Although the Hg²⁺ ions attack the RC in large number, the exertion of the harmful effect on photochemistry is not through mass action but rather a couple of well-defined targets. Bound to these sites, the Hg²⁺ ions can destroy H-bond structures, inhibit protein dynamics, block conformational gating mechanisms, and modify electrostatic profiles essential for electron and proton transfer.

Dehydration affects the electronic structure of the primary electron donor in bacterial photosynthetic reaction centers: evidence from visible-NIR and light-induced difference FTIR spectroscopy

The photosynthetic reaction center (RC) is a membrane pigment-protein complex that catalyzes the initial charge separation reactions of photosynthesis. Following photoexcitation, the RC undergoes conformational relaxations which stabilize the charge-separated state. Dehydration of the complex inhibits its conformational dynamics, providing a useful tool to gain insights into the relaxational processes. We analyzed the effects of dehydration on the electronic structure of the primary electron donor P, as probed by visible-NIR and light-induced FTIR difference spectroscopy, in RC films equilibrated at different relative humidities r. Previous FTIR and ENDOR spectroscopic studies revealed that P, an excitonically coupled dimer of bacteriochlorophylls, can be switched between two conformations, P866 and P850, which differ in the extent of delocalization of the unpaired electron between the two bacteriochlorophyll moieties (PL and PM) of the photo-oxidized radical P(+). We found that dehydration (at r = 11%) shifts the optical Qy band of P from 866 to 850-845 nm, a large part of the effect occurring already at r = 76%. Such a dehydration weakens light-induced difference FTIR marker bands, which probe the delocalization of charge distribution within the P(+) dimer (the electronic band of P(+) at 2700 cm(-1), and the associated phase-phonon vibrational modes at around 1300, 1480, and 1550 cm(-1)). From the analysis of the P(+) keto C[double bond, length as m-dash]O bands at 1703 and 1713-15 cm(-1), we inferred that dehydration induces a stronger localization of the unpaired electron on PL(+). The observed charge redistribution is discussed in relation to the dielectric relaxation of the photoexcited RC on a long (10(2) s) time scale.

Early bacteriopheophytin reduction in charge separation in reaction centers of Rhodobacter sphaeroides

A question at the forefront of biophysical sciences is, to what extent do quantum effects and protein conformational changes play a role in processes such as biological sensing and energy conversion? At the heart of photosynthetic energy transduction lie processes involving ultrafast energy and electron transfers among a small number of tetrapyrrole pigments embedded in the interior of a protein. In the purple bacterial reaction center (RC), a highly efficient ultrafast charge separation takes place between a pair of bacteriochlorophylls: an accessory bacteriochlorophyll (B) and bacteriopheophytin (H). In this work, we applied ultrafast spectroscopy in the visible and near-infrared spectral region to Rhodobacter sphaeroides RCs to accurately track the timing of the electron on BA and HA via the appearance of the BA and HA anion bands. We observed an unexpectedly early rise of the HA⁻ band that challenges the accepted simple picture of stepwise electron transfer with 3 ps and 1 ps time constants. The implications for the mechanism of initial charge separation in bacterial RCs are discussed in terms of a possible adiabatic electron transfer step between BA and HA, and the effect of protein conformation on the electron transfer rate.

Effects of dehydration on light-induced conformational changes in bacterial photosynthetic reaction centers probed by optical and differential FTIR spectroscopy

Following light-induced electron transfer between the primary donor (P) and quinone acceptor (Q(A)) the bacterial photosynthetic reaction center (RC) undergoes conformational relaxations which stabilize the primary charge separated state P(+)Q(A)(-). Dehydration of RCs from Rhodobacter sphaeroides hinders these conformational dynamics, leading to acceleration of P(+)Q(A)(-) recombination kinetics [Malferrari et al., J. Phys. Chem. B 115 (2011) 14732-14750]. To clarify the structural basis of the conformational relaxations and the involvement of bound water molecules, we analyzed light-induced P(+)Q(A)(-)/PQ(A) difference FTIR spectra of RC films at two hydration levels (relative humidity r=76% and r=11%). Dehydration reduced the amplitude of bands in the 3700-3550cm(-1) region, attributed to water molecules hydrogen bonded to the RC, previously proposed to stabilize the charge separation by dielectric screening [Iwata et al., Biochemistry 48 (2009) 1220-1229]. Other features of the FTIR difference spectrum were affected by partial depletion of the hydration shell (r=11%), including contributions from modes of P (9-keto groups), and from NH or OH stretching modes of amino acidic residues, absorbing in the 3550-3150cm(-1) range, a region so far not examined in detail for bacterial RCs. To probe in parallel the effects of dehydration on the RC conformational relaxations, we analyzed by optical absorption spectroscopy the kinetics of P(+)Q(A)(-) recombination following the same photoexcitation used in FTIR measurements (20s continuous illumination). The results suggest a correlation between the observed FTIR spectral changes and the conformational rearrangements which, in the hydrated system, strongly stabilize the P(+)Q(A)(-) charge separated state over the second time scale.

Investigation of the redox interaction between Mn-bicarbonate complexes and reaction centers from Rhodobacter sphaeroides R-26, Chromatium minutissimum, and Chloroflexus aurantiacus

The change in the dark reduction rate of photooxidized reaction centers (RC) of type II from three anoxygenic bacteria (Rhodobacter sphaeroides R-26, Chromatium minutissimum, and Chloroflexus aurantiacus) having different redox potentials of the P(+)/P pair and availability of RC for exogenous electron donors was investigated upon the addition of Mn(2+) and HCO(3)(-). It was found that the dark reduction of P(870)(+) from Rb. sphaeroides R-26 is considerably accelerated upon the combined addition of 0.5 mM MnCl(2) and 30-75 mM NaHCO(3) (as a result of formation of "low-potential" complexes [Mn(HCO(3))(2)]), while MnCl(2) and NaHCO(3) added separately had no such effect. The effect is not observed either in RC from Cf. aurantiacus (probably due to the low oxidation potential of the primary electron donor, P(865), which results in thermodynamic difficulties of the redox interaction between P(865)(+) and Mn(2+)) or in RC from Ch. minutissimum (apparently due to the presence of the RC-bound cytochrome preventing the direct interaction between P(870)(+) and Mn(2+)). The absence of acceleration of the dark reduction of P(870)(+) in the RC of Rb. sphaeroides R-26 when Mn(2+) and HCO(3)(-) were replaced by Mg(2+) or Ca(2+) and by formate, oxalate, or acetate, respectively, reveals the specificity of the Mn2+-bicarbonate complexes for the redox interaction with P(+). The results of this work might be considered as experimental evidence for the hypothesis of the participation of Mn(2+) complexes in the evolutionary origin of the inorganic core of the water oxidizing complex of photosystem II.

Lipid binding to the carotenoid binding site in photosynthetic reaction centers

Lipid binding to the carotenoid binding site near the inactive bacteriochlorophyll monomer was probed in the reaction centers of carotenoid-less mutant, R-26 from Rhodobacter sphaeroides. Recently, a marked light-induced change of the local dielectric constant in the vicinity of the inactive bacteriochlorophyll monomer was reported in wild type that was attributed to structural changes that ultimately lengthened the lifetime of the charge-separated state by 3 orders of magnitude (Deshmukh, S. S.; Williams, J. C.; Allen, J. P.; Kalman, L. Biochemistry 2011, 50, 340). Here in the R-26 reaction centers, the combination of light-induced structural changes and lipid binding resulted in a 5 orders of magnitude increase in the lifetime of the charge-separated state involving the oxidized dimer and the reduced primary quinone in proteoliposomes. Only saturated phospholipids with fatty acid chains of 12 and 14 carbon atoms long were bound successfully at 8 °C by cooling the reaction center protein slowly from room temperature. In addition to reporting a dramatic increase of the lifetime of the charge-separated state at physiologically relevant temperatures, this study reveals a novel lipid binding site in photosynthetic reaction center. These results shed light on a new potential application of the reaction center in energy storage as a light-driven biocapacitor since the charges separated by ∼30 Å in a low-dielectric medium can be prevented from recombination for hours.

EPR, ENDOR, and special TRIPLE measurements of P(*+) in wild type and modified reaction centers from Rb. sphaeroides

The influence of the protein environment on the primary electron donor, P, a bacteriochlorophyll a dimer, of reaction centers from Rhodobacter sphaeroides, has been investigated using electron paramagnetic resonance and electron nuclear double resonance spectroscopy. These techniques were used to probe the effects on P that are due to alteration of three amino acid residues, His L168, Asn L170, and Asn M199. The introduction of Glu at L168, Asp at L170, or Asp at M199 changes the oxidation/reduction midpoint potential of P in a pH-dependent manner (Williams et al. (2001) Biochemistry 40, 15403-15407). For the double mutant His L168 to Glu and Asn at L170 to Asp, excitation results in electron transfer along the A-side branch of cofactors at pH 7.2, but at pH 9.5, a long-lived state involving B-side cofactors is produced (Haffa et al. (2004) J Phys Chem B 108, 4-7). Using electron paramagnetic resonance spectroscopy, the mutants with alterations of each of the three individual residues and a double mutant, with changes at L168 and L170, were found to have increased linewidths of 10.1-11.0 G compared to the linewidth of 9.6 G for wild type. The Special TRIPLE spectra were pH dependent, and at pH 8, the introduction of aspartate at L170 increased the spin density ratio, rho (L)/rho (M), to 6.1 while an aspartate at the symmetry related position, M199, decreased the ratio to 0.7 compared to the value of 2.1 for wild type. These results indicate that the energy of the two halves of P changes by about 100 meV due to the mutations and are consistent with the interpretation that electrostatic interactions involving these amino acid residues contribute to the switch in pathway of electron transfer.

13C Chemical Shift Map of the Active Cofactors in Photosynthetic Reaction Centers ofRhodobacter sphaeroidesRevealed by Photo-CIDNP MAS NMR

13C photo-CIDNP MAS NMR studies have been performed on reaction centers (RCs) of Rhodobacter sphaeroides wild type (WT) that have been selectively labeled with an isotope using [5-13C]-delta-aminolevulinic acid.HCl in all the BChl and BPhe cofactors at positions C-4, C-5, C-9, C-10, C-14, C-15, C-16, and C-20. 13C CP/MAS NMR and 13C-13C dipolar correlation photo-CIDNP MAS NMR provide a chemical shift map of the cofactors involved in the electron transfer process in the RC at the atomic scale. The 13C-13C dipolar correlation photo-CIDNP spectra reveal three strong components, originating from two BChl cofactors, called P1 and P2 and assigned to the special pair, as well as one BPhe, PhiA. In addition, there is a weak component observed that arises from a third BChl cofactor, denoted P3, which appears to originate from the accessory BChl BA. An almost complete set of assignments of all the aromatic carbon atoms in the macrocycles of BChl and BPhe is achieved in combination with previous photo-CIDNP studies on site-directed BChl/BPhe-labeled RCs [Schulten, E. A. M., Matysik, J., Alia, Kiihne, S., Raap, J., Lugtenburg, J., Gast, P., Hoff, A. J., and de Groot, H. J. M. (2002) Biochemistry 41, 8708-8717], allowing a comprehensive map of the ground-state electronic structure of the photochemically active cofactors to be constructed for the first time. The reasons for the anomaly of P2 and the origin of the polarization on P3 are discussed.

Conformational regulation of charge recombination reactions in a photosynthetic bacterial reaction center

In bright light the photosynthetic reaction center (RC) of Rhodobacter sphaeroides stabilizes the P(+)(870).Q(-)(A) charge-separated state and thereby minimizes the potentially harmful effects of light saturation. Using X-ray diffraction we report a conformational change that occurs within the cytoplasmic domain of this RC in response to prolonged illumination with bright light. Our observations suggest a novel structural mechanism for the regulation of electron transfer reactions in photosynthesis.

Trapping of a long-living charge separated state of photosynthetic reaction centers in proteoliposomes of negatively charged phospholipids

Reaction centers from the purple bacterium Rhodobacter sphaeroides strain R-26.1 were purified and reconstituted in proteoliposomes formed by the anionic phospholipids phosphatidylglycerol, phosphatidylserine and phosphatidylinositol and by the zwitterionic phospholipid phosphatidylcholine by size-exclusion chromatography in the dark and under illumination. We report the large stabilizing effect induced by anionic phospholipids on the protein charge separated state which results trapped in a long-living (up to tens of minutes) state with a yield up to 80%. This fully reversible state is formed in oxygenic conditions regardless the presence of the secondary quinone QB and its lifetime and relative yield increase at low pH. In proteoliposomes formed with QA-depleted reaction centers (RCs) the resulting protein is very light-sensitive and the long living charge separated state is not observed. The data collected in negatively charged proteoliposomes are discussed in terms of the electrostatic effect on the primary quinone acceptor and compared with similar long living species reported in literature and obtained in anionic, zwitterionic, and non-ionic detergents.

Residual water modulates QA- -to-QB electron transfer in bacterial reaction centers embedded in trehalose amorphous matrices

The role of protein dynamics in the electron transfer from the reduced primary quinone, Q(A)(-), to the secondary quinone, Q(B), was studied at room temperature in isolated reaction centers (RC) from the photosynthetic bacterium Rhodobacter sphaeroides by incorporating the protein in trehalose water systems of different trehalose/water ratios. The effects of dehydration on the reaction kinetics were examined by analyzing charge recombination after different regimes of RC photoexcitation (single laser pulse, double flash, and continuous light) as well as by monitoring flash-induced electrochromic effects in the near infrared spectral region. Independent approaches show that dehydration of RC-containing matrices causes reversible, inhomogeneous inhibition of Q(A)(-)-to-Q(B) electron transfer, involving two subpopulations of RCs. In one of these populations (i.e., active), the electron transfer to Q(B) is slowed but still successfully competing with P(+)Q(A)(-) recombination, even in the driest samples; in the other (i.e., inactive), electron transfer to Q(B) after a laser pulse is hindered, inasmuch as only recombination of the P(+)Q(A)(-) state is observed. Small residual water variations ( approximately 7 wt %) modulate fully the relative fraction of the two populations, with the active one decreasing to zero in the driest samples. Analysis of charge recombination after continuous illumination indicates that, in the inactive subpopulation, the conformational changes that rate-limit electron transfer can be slowed by >4 orders of magnitude. The reported effects are consistent with conformational gating of the reaction and demonstrate that the conformational dynamics controlling electron transfer to Q(B) is strongly enslaved to the structure and dynamics of the surrounding medium. Comparing the effects of dehydration on P(+)Q(A)(-)-->PQ(A) recombination and Q(A)(-)Q(B)-->Q(A)Q(B)(-) electron transfer suggests that conformational changes gating the latter process are distinct from those stabilizing the primary charge-separated state.

Spectroscopic characterization of a semi-stable, charge-separated state in Cu2+-substituted reaction centers from Rhodobacter sphaeroides

In reaction centers from Rhodobacter sphaeroides exposed to continuous illumination in the presence of an inhibitor of the Q(A)(-) to Q(B) electron transfer, a semi-stable, charge-separated state was formed with halftimes of formation and decay of several minutes. When the non-heme iron was replaced by Cu(2+), the decay of the semi-stable, charge-separated state became much slower than in centers with bound Fe(2+) with about the same rate constant for formation. In Cu(2+)-substituted reaction centers, the semi-stable state was associated with an EPR signal, significantly different from that observed after chemical reduction of the acceptor-side quinone or after illumination at low temperature, but similar to that of an isolated Cu(2+) in the absence of magnetic interaction. The EPR results, obtained with Cu(2+)-substituted reaction centers, suggest that the slow kinetics of formation and decay of the charge-separated, semi-stable state is associated with a structural rearrangement of the acceptor side and the immediate environment of the metal-binding site.

Energetics and mechanisms of high efficiency of charge separation and electron transfer processes in Rhodobacter sphaeroides reaction centers

Effects of environmental changes due to D(2)O/H(2)O substitution and cryosolvent addition on the energetics of the special pair and the rate constants of forward and back electron transfer reactions in the picosecond-nanosecond time domain have been studied in isolated reaction centers (RC) of the anaxogenic purple bacterium Rhodobacter sphaeroides. The following results were obtained: (i). replacement of H(2)O by D(2)O or addition of either 70% (v/v) glycerol or 35% (v/v) DMSO do not influence the absorption spectra; (ii). in marked contrast to this invariance of absorption, the maxima of fluorescence spectra are red shifted relative to control by 3.5, 6.8 and 14.5 nm for RCs suspended in glycerol, D(2)O or DMSO, respectively; (iii). D(2)O/H(2)O substitution or DMSO addition give rise to an increase of the time constants of charge separation (tau(e)), and Q(A)(-) formation (tau(Q)) by a factors of 2.5-3.1 and 1.7-2.5, respectively; (iv). addition of 70% glycerol is virtually without effect on the values of tau(e) and tau(Q); (v). the midpoint potential E(m) of P/P(+) is shifted by about 30 and 45 mV towards higher values by addition of 70% glycerol and 35% DMSO, respectively, but remains invariant to D(2)O/H(2)O exchange; and (vi). an additional fast component with tau(1)=0.5-0.8 ns in the kinetics of charge recombination P(+)H(A)(-)-->P*(P)H(A) emerges in RC suspensions modified either by D(2)O/H(2)O substitution or by DMSO treatment. The results have been analysed with special emphasis on the role of deformations of hydrogen bonds for the solvation mechanism of nonequilibrium states of cofactors. Reorientation of hydrogen bonds provides the major contribution of the very fast environmental response to excitation of the special pair P. The Gibbs standard free energy gap between 1P* and P(+)B(A)(-) due to solvation is estimated to be approximately 70, 59 and 48 meV for control, D(2)O- and DMSO-treated RC samples, respectively.

Self-regulation phenomena applied to bacterial reaction centers: 2. Nonequilibrium adiabatic potential: dark and light conformations revisited

Experimental and theoretical results in support of nonlinear dynamic behavior of photosynthetic reaction centers under light-activated conditions are presented. Different conditions of light adaptation allow for preparation of reaction centers in either of two different conformational states. These states were detected both by short actinic flashes and by the switching of the actinic illumination level between different stationary state values. In the second method, the equilibration kinetics of reaction centers isolated from Rhodobacter sphaeroides were shown to be inherently biphasic. The fast and slow equilibration kinetics are shown to correspond to electron transfer (charge separation) at a fixed structure and to combined electron-conformational transitions governed by the bounded diffusion along the potential surface, respectively. The primary donor recovery kinetics after an actinic flash revealed a pronounced dependence on the time interval (deltat) between cessation of a lengthy preillumination of a sample and the actinic flash. A pronounced slow relaxation component with a decay half time of more than 50 s was measured for deltat > 10 s. This component corresponds to charge recombination in reaction centers for which light-induced structural changes have not relaxed completely before the flash. The amplitude of this component depended on the conditions of the sample preparation, specifically on the type of detergent used in the preparation. The redox potential parameters as well as the structural diffusion constants were estimated for samples prepared in different ways.