Reaction Centers Of Rb Research Articles

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.

Моделирование конформационного перехода в фотосинтетическом реакционном центре бактерии Rb. sphaeroides

Методом квантово-химического моделирования исследован возможный конформационный переход в локальном окружении первичного хинона в фотосинтетическом реакционном центре (РЦ) бактерии Rhodobacter sphaeroides, сопровождающий процесс переноса электрона. Исходя из представления о наличии двух устойчивых конформационных состояний РЦ, предложена кинетическая модель, хорошо описывающая экспериментальные температурные зависимости скорости реакции рекомбинации P+QA- → PQA. Результаты квантово-химического моделирования сайта связывания первичного хинона позволяют предложить на роль указанного конформационного изменения небольшое смещение кольца убихинона, приводящее к разрыву водородной связи, образуемой 4–C=O группой убихинона с гистидином M219, и образованию новой водородной связи с гидроксильной группой треонина M222. Значения параметров модели, полученные с помощью квантово-химических расчетов, качественно согласуются со значениями параметров кинетической модели, используемых для описания реакции рекомбинации.

Femtosecond phase of charge separation in reaction centers of Chloroflexus aurantiacus

Difference absorption spectroscopy with temporal resolution of approximately 20 fsec was used to study the primary phase of charge separation in isolated reaction centers (RCs) of Chloroflexus aurantiacus at 90 K. An ensemble of difference (light-minus-dark) absorption spectra in the 730-795 nm region measured at -0.1 to 4 psec delays relative to the excitation pulse was analyzed. Comparison with analogous data for RCs of HM182L mutant of Rhodobacter sphaeroides having the same pigment composition identified the 785 nm absorption band as the band of bacteriopheophytin Phi(B) in the B-branch. By study the bleaching of this absorption band due to formation of Phi(B)(-), it was found that a coherent electron transfer from P* to the B-branch occurs with a very small delay of 10-20 fsec after excitation of dimer bacteriochlorophyll P. Only at 120 fsec delay electron transfer from P* to the A-branch occurs with the formation of bacteriochlorophyll anion B(A)(-) absorption band at 1028 nm and the appearance of P* stimulated emission at 940 nm, as also occurs in native RCs of Rb. sphaeroides. It is concluded that a nuclear wave packet motion on the potential energy surface of P* after a 20-fsec light pulse excitation leads to the coherent formation of the P(+)Phi(B)(-) and P(+)B(A)(-) states.

WAVE PACKET MOTIONS COUPLED TO ELECTRON TRANSFER IN REACTION CENTERS OF CHLOROFLEXUS AURANTIACUS

Transient absorption difference spectroscopy with approximately 20 femtosecond (fs) resolution was applied to study the time and spectral evolution of low-temperature (90 K) absorbance changes in isolated reaction centers (RCs) of Chloroflexus (C.) aurantiacus. In RCs, the composition of the B-branch chromophores is different with respect to that of purple bacterial RCs by occupying the B(B) binding site of accessory bacteriochlorophyll by bacteriopheophytin molecule (Phi(B)). It was found that the nuclear wave packet motion induced on the potential energy surface of the excited state of the primary electron donor P* by approximately 20 fs excitation leads to a coherent formation of the states P+Phi(B)(-) and P+B(A)(-) (B(A) is a bacteriochlorophyll monomer in the A-branch of cofactors). The processes were studied by measuring coherent oscillations in kinetics of the absorbance changes at 900 nm and 940 nm (P* stimulated emission), at 750 nm and 785 nm (Phi(B) absorption bands), and at 1,020-1028 nm (B(A)(-) absorption band). In RCs, the immediate bleaching of the P band at 880 nm and the appearance of the stimulated wave packet emission at 900 nm were accompanied (with a small delay of 10-20 fs) by electron transfer from P* to the B-branch with bleaching of the Phi(B) absorption band at 785 nm due to Phi(B)(-) formation. These data are consistent with recent measurements for the mutant HM182L Rb. sphaeroides RCs (Yakovlev et al., Biochim Biophys Acta 1757:369-379, 2006). Only at a delay of 120 fs was the electron transfer from P* to the A-branch observed with a development of the B(A)(-) absorption band at 1028 nm. This development was in phase with the appearance of the P* stimulated emission at 940 nm. The data on the A-branch electron transfer in C. aurantiacus RCs are consistent with those observed in native RCs of Rb. sphaeroides. The mechanism of charge separation in RCs with the modified B-branch pigment composition is discussed in terms of coupling between the nuclear wave packet motion and electron transfer from P* to Phi(B) and B(A) primary acceptors in the B-branch and A-branch, respectively.

The Unusually Strong Hydrogen Bond between the Carbonyl of QAand His M219 in theRhodobacter sphaeroidesReaction Center Is Not Essential for Efficient Electron Transfer from QA-to QB

In native reaction centers (RCs) from photosynthetic purple bacteria the primary quinone (QA) and the secondary quinone (QB) are interconnected via a specific His-Fe-His bridge. In Rhodobacter sphaeroides RCs the C4=O carbonyl of QA forms a very strong hydrogen bond with the protonated Npi of His M219, and the Ntau of this residue is in turn coordinated to the non-heme iron atom. The second carbonyl of QA is engaged in a much weaker hydrogen bond with the backbone N-H of Ala M260. In previous work, a Trp side chain was introduced by site-directed mutagenesis at the M260 position in the RC of Rb. sphaeroides, resulting in a complex that is completely devoid of QA and therefore nonfunctional. A photochemically competent derivative of the AM260W mutant was isolated that contains a Cys side chain at the M260 position (denoted AM260(W-->C)). In the present work, the interactions between the carbonyl groups of QA and the protein in the AM260(W-->C) suppressor mutant have been characterized by light-induced FTIR difference spectroscopy of the photoreduction of QA. The QA-/QA difference spectrum demonstrates that the strong interaction between the C4=O carbonyl of QA and His M219 is lost in the mutant, and the coupled CO and CC modes of the QA- semiquinone are also strongly perturbed. In parallel, a band assigned to the perturbation of the C5-Ntau mode of His M219 upon QA- formation in the native RC is lacking in the spectrum of the mutant. Furthermore, a positive band between 2900 and 2400 cm-1 that is related to protons fluctuating within a network of highly polarizable hydrogen bonds in the native RC is reduced in amplitude in the mutant. On the other hand, the QB-/QB FTIR difference spectrum is essentially the same as for the native RC. The kinetics of electron transfer from QA- to QB were measured by the flash-induced absorption changes at 780 nm. Compared to native RCs the absorption transients are slowed by a factor of about 2 for both the slow phase (in the hundreds of microseconds range) and fast phase (microseconds to tens of microseconds range) in AM260(W-->C) RCs. We conclude that the unusually strong hydrogen bond between the carbonyl of QA and His M219 in the Rb. sphaeroides RC is not obligatory for efficient electron transfer from QA- to QB.

Primary charge separation between P* and B A: Electron-transfer pathways in native and mutant GM203L bacterial reaction centers

Coherent components in the dynamics of decay of stimulated emission from the primary electron donor excited state P*, and of population of the product charge-separated states P + B A - and P + H A - , were studied in GM203L mutant reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides by measuring oscillations in the kinetics of absorbance changes at 940 nm (P* stimulated emission region), 1020 nm ( B A - absorption region) and 760 nm (H A bleaching region). Absorbance changes were induced by excitation of P (870 nm) with 18 fs pulses at 90 K. In the GM203L mutant, replacement of Gly M203 by Leu results in exclusion of the crystallographically defined water molecule (HOH55) located close to the oxygen of the 13 1-keto carbonyl group of B A and to His M202, which provides the axial ligand to the Mg of the P B bacteriochlorophyll. The results of femtosecond measurements were compared with those obtained with Rb. sphaeroides R-26 RCs containing an intact water HOH55. The main consequences of the GM203L mutation were found to be as follows: (i) a low-frequency oscillation at 32 cm −1, which is characteristic of the HOH55-containing RCs, disappears from the kinetics of absorbance changes at 1020 and 760 nm in the mutant RC; (ii) electron transfer from P* to B A in the wild type RC was characterized by two time constants of 1.1 ps (80%) and 4.3 ps (20%), but in the GM203L mutant was characterized by a single time constant of 4.3 ps, demonstrating a slowing of primary charge separation. The previously postulated rotation of water HOH55 with a fundamental frequency of 32 cm −1, triggered by electron transfer from P* to B A, was confirmed by observation of an isotopic shift of the 32 cm −1 oscillation in the kinetics of P + B A - population in deuterated, pheophytin-modified RCs of Rb. sphaeroides R-26, by a factor of 1.6. These data are discussed in terms of the influence of water HOH55 on the energetics of the P ∗ → P + B A - reaction, and protein dynamic events that occur on the time scale of this reaction.

Attenuated total reflection Fourier transform infrared spectroscopy of redox transitions in photosynthetic reaction centers: comparison of perfusion- and light-induced difference spectra

Chemically induced Fourier transform infrared difference spectra associated with redox transitions of several primary electron donors and acceptors in photosynthetic reaction centers (RCs) have been compared with the light-induced FTIR difference spectra involving the same cofactors. The RCs are deposited on an attenuated total reflection (ATR) prism and form a film that is enclosed in a flow cell. Redox transitions in the film of RCs can be repetitively induced either by perfusion of buffers poised at different redox potentials or by illumination. The perfusion-induced ATR-FTIR difference spectra for the oxidation of the primary electron donor P in the RCs of the purple bacteria Rb. sphaeroides and Rp. viridis and P700 in the photosystem I of Synechocystis 6803, as well as the Q A −/Q A transition of the quinone acceptor (Q A) in Rb. sphaeroides RCs are reported for the first time. They are compared with the light-induced ATR-FTIR difference spectra P +Q A −/PQ A for the RCs of Rb. sphaeroides and P700 +/P700 for photosystem 1. It is shown that the perfusion-induced and light-induced ATR-FTIR difference spectra recorded on the same RC film display identical signal to noise ratios when they are measured under comparable conditions. The ATR-FTIR difference spectra are very similar to the equivalent FTIR difference spectra previously recorded upon photochemical or electrochemical excitation of these RCs in the more conventional transmission mode. The ATR-FTIR technique requires a smaller amount of sample compared with transmission FTIR and allows precise control of the aqueous environment of the RC films.

Dispersive charge separation and conformational cooling of P +H A− in reaction centers of Rb. sphaeroides R26: a spontaneous emission study

The decay of the excited state of the primary donor ( 1 P ∗ ) of reaction centers (RC) of Rb. sphaeroides R26 has been monitored via its spontaneous emission. Although there is general agreement that (i) a prominent drop of prompt emission within a few picoseconds results from fast charge separation of the majority of RCs and that (ii) a ∼15 ns delayed emission (amplitude ∼5·10 −5) is due to recombination of P +H A − (H A denoting bacteriopheophytin) when electron transport to the secondary quinone acceptor (Q A) is blocked, the nature of (iii) intermediate fluorescence components with time constants ranging from ∼100 ps to several nanoseconds has been unclear. In this paper these components are studied by manipulating the lifetime of P +H A − via the presence or absence of Q A. Reconstitution of Q A to Q A-free RCs leads to a reduction of this lifetime from 15 ns to 100–200 ps, thus eliminating any delayed emission at later times. In such preparations small components from prompt emission extend up to a nanosecond exhibiting dispersive charge-separation kinetics, which we attribute to an energetic dispersion of the primary radical pair P +B A − (B A is a bacteriochlorophyll). The high-energy wing of this energetic distribution is responsible for charge separation slower than 100 ps, which we find in a fraction of ∼0.3% of the RCs at 280 K. Since at low temperatures correspondingly smaller activation barriers are sufficient for such a retardation, this fraction increases to ∼3% at 85 K. In Q A-free RCs with a long P +H A − lifetime prompt emission still dominates at early times. However, its contribution is smaller than 50% of the total emission at about 600 ps at 280 K and 1 ns at 85 K, respectively. The amplitudes of this delayed emission are time dependent and reveal a relaxation of the average free energy between 1 P ∗ and P +H A − changing from Δ G 0≈−0.21 eV at about 70 ps to Δ G 0≈−0.29 eV after 50 ns. This relaxation is attributed to the slow protein response to charge separation (conformational cooling) and can be described by a Kohlrausch relaxation function with a time constant of 4.21 ns and a stretching exponent of 0.456.

Pump Probe Anisotropy Studies of the Photosynthetic Reaction Center

The photosynthetic reaction center of Rb. sphaeroides is investigated by unique pump-probe anisotropy methods using 13 fs optical pulses tunable throughout the near IR absorption band. The time and wavelength resolved pump probe anisotropy transients yield detailed experimental descriptions of the energy levels, Py+ and Py- of the excitonically coupled special pair (P), and dynamics prior to charge separation, particularly energy transfer from the accessory bacteriochlorophylla's (B) to the special pair (P).

The primary and secondary electron acceptors in bacterial photosynthesis: I. A chronological account of their identification by EPR

A chronological account is presented of the work that led to the identification of the primary and secondary acceptors in photosynthetic reaction centers ofRb. sphaeroides. Both acceptors when reduced give rise to broad EPR signals centered atg = 1.82. When the iron is removed from the reaction center, the EPR line, narrowed by approximately two orders of magnitude, is characteristic of a ubisemiquinone. The broad signal in native reaction centers (RCs) is, therefore, ascribed to an unpaired electron being localized on the ubisemiquinone interacting magnetically with the high spin Fe2+. These findings are consistent with the structure of the RC, as determined a decade later by X-ray diffraction.

Direct evidence of structural changes in reaction centers of Rb. sphaeroides containing suppressor mutations for Asp L213 → Asn: A FTIR study of QB photoreduction

In bacterial reaction centers (RCs), changes of protonation state of carboxylic groups, of quinone-protein interactions as well as backbone rearrangements occuring upon QB photoreduction can be revealed by FTIR difference spectroscopy. The influence of compensatory mutations to the detrimental Asp L213 → Asn replacement on QB−/QB FTIR spectra of Rb. sphaeroides RCs was studied in three double mutants carrying a Asn M44 → Asp, Arg M233 → Cys, or Arg H177 → His suppressor mutation. The proton uptake by Glu L212 upon QB− formation, as reflected by the positive band at 1728 cm−1, is increased in the Asn M44 → Asp and Arg H177 → His suppressor RCs with respect to native RCs, and remains comparable to that observed in Asp L213 → Asn mutant RCs. Only the Arg M233 → Cys suppressor mutation affected the 1728 cm−1 band, reducing its amplitude to near native level. Thus, there is no clear correlation between the apparent extent of proton uptake by Glu L212 and the recovery of the proton transfer RC function. In all of the mutant spectra, several protein (amide I and amide II) and quinone anion (C...O/C...C) modes are perturbed compared to the spectrum of native RCs. These IR data show that all of the compensatory mutations alter the semiquinone-protein interactions and the backbone providing direct evidence of structural changes accompanying the restoration of efficient proton transfer in RCs containing the Asp L213 → Asn lesion.

The spectroscopic and photochemical properties of locked-15,15′-cis-spheroidene in solution and incorporated into the reaction center of Rhodobacter sphaeroides R-26.1

The spectroscopic and photochemical properties of the synthetic carotenoid, locked-15,15′-cis-spheroidene, were studied by absorption, fluorescence, circular dichroism, fast transient absorption and electron spin resonance spectroscopies in solution and after incorporation into the reaction center of Rhodobacter (Rb.) sphaeroides R-26.1. HPLC purification of the synthetic molecule reveals the presence of several di-cis geometric isomers in addition to the mono-cis isomer of locked-15,15′-cis-spheroidene. In solution, the absorption spectrum of the purified mono-cis sample was red-shifted and showed a large cis-peak at 351 nm compared to unlocked all-trans spheroidene. Molecular modeling and semi-empirical calculations reveal how geometric isomerization and structural factors affect the room temperature spectra. The spectroscopic studies of the purified locked-15,15′-mono-cis molecule in solution reveal a more stable manifold of excited states compared to the unlocked spheroidene. Reaction centers of Rb. sphaeroides R-26.1 in which the locked-15,15′-cis-spheroidene was incorporated show no difference in either the spectroscopic properties or photochemistry compared to reaction centers in which unlocked spheroidene was incorporated or to Rb. sphaeroides wild type strain 2.4.1 reaction centers which naturally contain spheroidene. The data suggest that the natural selection of a cis-isomer of spheroidene for incorporation into native reaction centers of Rb. sphaeroides wild type strain 2.4.1 is more determined by the structure or assembly of the reaction center protein than by any special quality of the cis-isomer of the carotenoid that would affect its ability to participate in triplet energy transfer or carry out photoprotection.

Excitonic Interactions between the Reaction Center and Antennae in Purple Photosynthetic Bacteria

The interaction between the light-harvesting antenna (LH1) and the reaction center (RC) in the photosynthetic purple bacteria Rhodobacter sphaeroides R26 and strain 2.4.1 was studied with low-temperature (1.5 K) absorbance-detected magnetic resonance (ADMR) experiments combined with computer simulations. The triplet-minus-singlet (T−S) spectra of chromatophores of Rb. sphaeroides R26 and strain 2.4.1 show an extra positive component at approximately 884 and 887 nm, respectively, and an extra negative component at 897 and 901 nm, respectively, when compared to RCs of Rb. sphaeroides R26 and strain RCO1. Computer simulations using a simple model of the RC encircled by the antenna of LH1 show that the dipole strength and energies of the second-lowest exciton component of the antenna and the RC change when the coupling term between the RC and antenna is included in the Hamiltonian of the system. Thus the difference in the shape of the ADMR-detected T−S spectra of isolated RCs (or strain RCO1 chromatophores) and chromatophores is plausibly caused by a change in the excitonic interaction between the RC and antenna.

Evidence that Ala M260 is hydrogen-bonded to the reduced primary acceptor quinone Q A−. in reaction centers of Rb. sphaeroides

The binding of the primary quinone anion radical Q A −. in reaction centers (Rcs) of Rhodobacter (Rb.) sphaeroides species was investigated with electron spin echo envelope modulation (ESEEM). The ESEEM spectra, at two microwave frequencies, of Zn-substituted RCs of Rb. sphaeroides R26 showed interactions of the unpaired electron of Q A −. with two nitrogen nuclei in the protein matrix. From an analysis of the experimental data the nitrogen nuclear quadrupole resonance parameters were determined: e 2 qQ/ h = 1.52 MHz, η = 0.82 and e 2 qQ/ h = 3.04 MHz, η = 0.66, which are assigned to the 14N δ(1)N group of His M219 and the peptide 14N of Ala M260. The ESEEM spectrum of Q A −. in reaction centers of the Rb. sphaeroides mutant W(M252)Y shows that the nitrogen of Trp M252 is not interacting with Q A −., that of the mutant H(M266)C shows that manipulating the Fe binding site strongly affects the Q A-binding site.

Theoretical analyses on femtosecond time-resolved spectra of initial electron transfer of photosynthetic reaction centers at low temperatures

Abstract In this paper, based on the perturbative density matrix method, we calculate femtosecond (fs) transient spectra of initial electron transfer (ET) in the bacterial reaction centers (RCs) of Rhodobacter (Rb.) capsulatus at 20 K and compare them with experimental data. It is believed that the lowering of temperature will decrease the intermolecular distance in the special pair, which will in turn affect the energy levels of the excitonic-coupled dimer (special pair) P, i.e., P − ∗ , P + ∗ , P − + and P + + where, for instance, P − ∗ and P + + represent the lowest excited and the second charge-separated states of the P, respectively. The absorption spectrum of the P − ∗ band is analyzed to obtain the small vibrational modes and their coupling constants (Huang-Rhys factor). By taking into account the anion and cation states of the bacteriochlorophyll a molecule (B) and the cation state of the bacteriopheophytin a molecule (H), the excitonic-vibronic model including the six electronic states P − ∗ BH, P − + B − H, P − + BH − , PB + H − , P + + B − H and P + + BH − is applied to simulate the fs time-resolved three-dimension spectra of Rb. capsulatus . We find only the three electronic states P − ∗ BH, P + + B − H and P + + BH − are mainly involved in the ET dynamics. We shall theoretically analyze the quantum beats appearing in the fs pump-probe stimulated emission profiles of the D ll mutant RCs of Rb. capsulatus at 10 K. The conventional ET theory assumes that vibrational relaxation is much faster than ET so that vibrational equilibrium is established before ET takes place. However, several recent fs measurements on RCs show quantum beats in their time-resolved profiles. This implies that the non-equilibrated vibrational motions of RCs should be investigated. The D ll mutant RC, lacking of the H ll , is incapable of ET on a time scale of the fs measurements. Thus, analyzing the quantum beats appearing in the fs time-resolved profiles, we shall extract the information of transient vibrational motions and their decays of the electronically lowest excited state P − ∗ . The multi-mode effect on the quantum beats is also investigated.

Reaction centers of Rhodobacter sphaeroides R26 containing C-3 acetyl and vinyl (bacterio)pheophytins at sites HA,B

The native bacteriopheophytin a in reaction centers of Rb. sphaeroides R26 has been exchanged with modified bacteriopheophytins (bacteriochlorins), as well as with plant-type pheophytins (chlorins). Emphasis is on four pigments, which differ by their C-3 substituents (vinyl or acetyl) or their state of oxidation (chlorin or bacteriochlorin). The native BPhe a, which is a member of this group, can be replaced by the other three at both binding sites, HA and HB. However, exchange at HB proceeds more readily. Optical spectra (absorption, cd) show characteristic shifts, and the cd spectra indicate induced interactions between HA,B and BA,B and possibly also with P. Upon flash illumination, all modified reaction centers show reversible electron transfer to QB with recombination times comparable to native reaction centers. Forward rates and electron-transfer yields are also reported for some of the pigments.

FTIR spectroscopy of the photoreduction of the bacteriopheophytin electron acceptor in reaction centers of Rb. sphaeroides and Rps. viridis.

Abstract The photoreduction of the bacteriopheophytin electron acceptor HA in reaction centers from Rhodobacter sphaeroides and Rhodopseudomonas viridis has been monitored by light-induced FTIR difference spectroscopy at 10°C, in the presence of reductant and mediator. The striking similarity of the H A − H A spectra obtained for Rb. sphaeroides and Rps. viridis reflects comparable interactions of the bacteriopheophytin electron acceptor with the protein in both reaction centers and implies that the photoreduction of HA affects conserved amino acid residues. The H A − H A spectra are interpreted by comparison with model compound spectra of the anion radicals of bacteriopheophytin a and b,a nd by analysis of 1 H 2 H isotope effects. The downshift of the 1677 cm−1 mode in Rb. sphaeroides (1681 cm−1 in Rps. viridis) reaction centers with respect to the model compound is interpreted in terms of a strongly perturbed 9-keto carbonyl of HA. This perturbation most probably originates from hydrogen bonding to Glu L104. At least part of the positive signal at 1591 cm−1 in Rb. sphaeroides and at 1601 cm−1 in Rps. viridis is assigned to the 9-keto carbonyl mode of HA−. From 1 H 2 H exchange experiments, it is proposed that the COO1H side chain of Glu L104 contributes to the 1745-1735 cm−1 spectral range with the corresponding COO2H signal displaced to lower frequencies and partly hidden under the 1732 cm−1 band.

Observation of Ultrafast Energy Transfer from the Accessory Bacteriochlorophylls to the Special Pair in Photosynthetic Reaction Centers

The energy transfer from accessory bacteriochlorophyll molecules (B) to the special pair (P) in the reaction center of Rb. capsulatus and Rb. sphaeroides R26 has been time-resolved using 23 fs pulses. In both reaction centers, the B* transient bleaching decays biexponentially, with a fast component of approximately 100 fs which gives a characteristic time scale for excitation energy transfer from B* to P. In Rb. capsulatus, the amplitudes for this component and a 360 {+-} 40 fs decay of uncertain origin reported in previous experiments are approximately equal. Several models for the two components are discussed. 33 refs., 2 figs.

A new approach to experimental investigation of dynamic self-organization in reaction centers of purple bacteria

The results of a study of molecular self-organization processes in the reaction centers (RC) ofRb. Sphaeroides purple bacteria by the method of pulsed optical excitation is presented. The existence of a bistability domain for the parameters of RC recovery kinetics is shown. A good agreement between the theory and experimental results is obtained.

QA binding in reaction centers of the photosynthetic purple bacterium Rhodobacter sphaeroides R26 investigated with electron spin polarization spectroscopy.

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.