Bacteriochlorophyll Dimer Research Articles

Manganese oxidation by modified reaction centers from Rhodobacter sphaeroides.

The transfer of an electron from exogenous manganese (II) ions to the bacteriochlorophyll dimer, P, of bacterial reaction centers was characterized for a series of mutants that have P/P(+) midpoint potentials ranging from 585 to 765 mV compared to 505 mV for wild type. Light-induced changes in optical and EPR spectra of the mutants were measured to monitor the disappearance of the oxidized dimer upon electron donation by manganese in the presence of bicarbonate. The extent of electron transfer was strongly dependent upon the P/P(+) midpoint potential. The midpoint potential of the Mn(2+)/Mn(3+) couple was calculated to decrease linearly from 751 to 623 mV as the pH was raised from 8 to 10, indicating the involvement of a proton. The electron donation had a second order rate constant of approximately 9 x 10(4) M(-1) s(-1), determined from the linear increase in rate for Mn(2+) concentrations up to 200 microM. Weak dissociation constants of 100-200 microM were found. Quantitative EPR analysis of the six-line free Mn(2+) signal revealed that up to seven manganese ions were associated with the reaction centers at a 1 mM concentration of manganese. The association and the electron transfer between manganese and the reaction centers could be inhibited by Ca(2+) and Na(+) ions. The ability of reaction centers with high potentials to oxidize manganese suggests that manganese oxidation could have preceded water oxidation in the evolutionary development of photosystem II.

Modelling the bacterial photosynthetic reaction center. V. Assignment of the electronic transition observed at 2200 cm−1 in the special-pair radical-cation as a second-highest occupied molecular orbital to highest occupied molecular orbital transition

Primary charge separation in photoexcited photosynthetic reaction centers produces the radical cation P+ of a bacteriochlorophyll dimer known as the special-pair P. P+ has an intense electronic transition in the vicinity of 1800–5000 cm−1 which is usually assigned to the interchromophore hole-transfer excitation of the dimer radical cation; in principle, this spectrum can give much insight into key steps of the solar-to-electrical energy-conversion process. The extent to which this transition is localized on one-half of the dimer or delocalized over both is of utmost importance; an authoritative deduction of this quantity from purely spectroscopic arguments requires the detailed assignment of the observed high to medium resolution spectra. For reaction centers containing bacteriochlorophylls a or b, a shoulder is observed at 2200 cm−1 on the low-energy side of the main hole-transfer absorption band, a band whose maximum is near 2700 cm−1. Before quantitative analysis of the hole-transfer absorption in these well-studied systems can be attempted, the nature of the processes leading to this shoulder must be determined. We interpret it as arising from an intrachromophore SHOMO to HOMO transition whose intensity arises wholly through vibronic coupling with the hole-transfer band. A range of ab initio and density-functional calculations are performed to estimate the energy of this transition both for monomeric cations and for P+ of Blastochloris viridis, Rhodobacter sphaeroides, Chlorobium limicola, Chlorobium tepidum, Chlamydomonas reinhardtii, Synochocystis S.6803, spinach photosystems I and II, Heliobacillus mobilis, and finally Heliobacterium modesticaldum, with the results found to qualitatively describe the available experimental data. Subsequent papers in this series provide quantitative analyses of the vibronic coupling and complete spectral simulations based on the model developed herein.

Conformation transition in the protein of a photosynthetic reaction center observed at the nanometer range of distances at cryogenic temperatures

Electron spin echo (ESE) spectroscopy is applied to study magnetic dipolar interaction between electron spins in the transient P + Q A − radical pairs (P is the primary donor, a bacteriochlorophyll dimer, and Q A is the primary quinone acceptor) and the 3 PQ A − triplet-radical pairs in bacterial photosynthetic reaction centers of Rhodobacter sphaeroides R26. Distance separation in both pairs is about 29 Å. A well-resolved reversible conformational transition of the reaction center protein holding the P and Q A cofactors was observed between 13 and 20 K. This transition results in a narrowing of the distribution of protein conformations with decreasing temperature, with the width of distance distribution between P and Q A dropping from ca. 4 Å at 20 K to ca. 1 Å at 13 K. This transition implies the existence of low barriers in the protein energy landscape and the presence of cooperatively rearranging domains of the size of several nm both in the protein and in the surrounding glassy environment.

Proton release upon oxidation of tyrosine in reaction centers from Rhodobacter sphaeroides

Markedly different light-induced protonational changes were measured in two reaction center mutants of Rhodobacter sphaeroides. A quadruple mutant containing alterations, at residues L131, M160, M197, and M210, that elevate the midpoint potential of the bacteriochlorophyll dimer was compared to the Y M mutant, which contains these alterations plus a tyrosine at M164 serving as a secondary electron donor [Kálmán et al., Nature 402 (1999) 696]. In the quadruple mutant, a proton uptake of 0.1–0.3 H +/reaction center between pH 6 and 10 resulted from formation of the oxidized bacteriochlorophyll donor and reduced primary quinone. In the Y M mutant, a maximal proton release of −0.5 H +/reaction center at pH 8 was attributed to formation of the tyrosyl radical and modeled using electrostatic and direct proton-releasing mechanisms.

B-side charge separation in bacterial photosynthetic reaction centers: nanosecond time scale electron transfer from HB- to QB.

We report time-resolved optical measurements of the primary electron transfer reactions in Rhodobacter capsulatus reaction centers (RCs) having four mutations: Phe(L181) --> Tyr, Tyr(M208) --> Phe, Leu(M212) --> His, and Trp(M250) --> Val (denoted YFHV). Following direct excitation of the bacteriochlorophyll dimer (P) to its lowest excited singlet state P, electron transfer to the B-side bacteriopheophytin (H(B)) gives P(+)H(B)(-) in approximately 30% yield. When the secondary quinone (Q(B)) site is fully occupied, P(+)H(B)(-) decays with a time constant estimated to be in the range of 1.5-3 ns. In the presence of excess terbutryn, a competitive inhibitor of Q(B) binding, the observed lifetime of P(+)H(B)(-) is noticeably longer and is estimated to be in the range of 4-8 ns. On the basis of these values, the rate constant for P(+)H(B)(-) --> P(+)Q(B)(-) electron transfer is calculated to be between approximately (2 ns)(-)(1) and approximately (12 ns)(-)(1), making it at least an order of magnitude smaller than the rate constant of approximately (200 ps)(-)(1) for electron transfer between the corresponding A-side cofactors (P(+)H(A)(-) --> P(+)Q(A)(-)). Structural and energetic factors associated with electron transfer to Q(B) compared to Q(A) are discussed. Comparison of the P(+)H(B)(-) lifetimes in the presence and absence of terbutryn indicates that the ultimate (i.e., quantum) yield of P(+)Q(B)(-) formation relative to P is 10-25% in the YFHV RC.

Influence of the Protein Environment on the Properties of a Tyrosyl Radical in Reaction Centers from Rhodobacter sphaeroides

The influence of the local environment on the formation of a tyrosyl radical was investigated in modified photosynthetic reaction centers from Rhodobacter sphaeroides. The reaction centers contain a tyrosine residue placed approximately 10 A from a highly oxidizing bacteriochlorophyll dimer. Measurements by both optical and electron paramagnetic resonance spectroscopy revealed spectral features that are assigned as arising primarily from an oxidized bacteriochlorophyll dimer at low pH values and from a tyrosyl radical at high pH values, with a well-defined transition that occurred with a pK(a) of 6.9. A model based on the wild-type structure indicated that the Tyr at M164 is likely to form a hydrogen bond with His M193 and to interact weakly with Glu M173. Substitution of Tyr or Glu for His at M193 increased the pK(a) for the transition from 6.9 to 8.9, while substitution of Gln for His M193 resulted in a higher pK(a) value. Substitution of Glu M173 with Gln resulted in loss of the partial formation of the tyrosyl that occurs in the other mutants at low pH values. The results are interpreted in terms of the ability of the residues to act as proton acceptors for the oxidized tyrosine, with the pK(a) values reflecting those of either the putative proton acceptor or the tyrosine, in accord with general models of amino acid radicals.

The Dependence of the Initial Electron-Transfer Rate on Driving Force in Rhodobacter sphaeroides Reaction Centers

The kinetics of the primary electron transfer following excitation of the bacteriochlorophyll dimer (P) to its lowest excited singlet state were determined for a series of reaction center mutants of Rhodobacter sphaeroides that have P/P+ midpoint potentials 44−147 mV below wild type. These strains were capable of photosynthetic growth and had normal bacteriochlorophyll-to-bacteriopheophytin ratios. Decreasing the P/P+ midpoint potential resulted in an increase in the rate constant of initial electron transfer in each of the mutants tested. At room temperature, the fastest electron-transfer time constant observed was 1.8 ps from a mutant with a midpoint potential 127 mV below wild type. The dependence of the rate on driving force in these measurements and in previous measurements of mutants with high P/P+ midpoint potentials at room temperature was fit to a Marcus equation. Wild type was displaced approximately 100 meV from the peak of this curve. This analysis yielded a reorganization energy between 180 a...

Quantum chemical study of π–π stacking interactions of the bacteriochlorophyll dimer in the photosynthetic reaction center of Rhodobacter sphaeroides

Intermolecular π–π stacking interactions of the bacteriochlorophyll dimer in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides were analyzed by the second order Møller–Plesset perturbation method using the modified 6-31G*(0.25) basis set with diffuse d-polarization by Hobza and co-workers. MP2/6-31G*(0.25) calculations yield an intermolecular interaction energy of −21.50 kcal/mol for the bacteriochlorophyll dimer. Thus, the attractive nature of the π–π stacking interaction of the bacteriochlorophyll dimer in the photosynthetic reaction center from Rhodobacter sphaeroides is, for the first time, firmly established.

X-ray Structure Determination of the Cytochrome c2: Reaction Center Electron Transfer Complex from Rhodobacter sphaeroides

In the photosynthetic bacterium Rhodobacter sphaeroides, a water soluble cytochrome c 2 (cyt c 2) is the electron donor to the reaction center (RC), the membrane-bound pigment–protein complex that is the site of the primary light-induced electron transfer. To determine the interactions important for docking and electron transfer within the transiently bound complex of the two proteins, RC and cyt c 2 were co-crystallized in two monoclinic crystal forms. Cyt c 2 reduces the photo-oxidized RC donor (D +), a bacteriochlorophyll dimer, in the co-crystals in ∼0.9 μs, which is the same time as measured in solution. This provides strong evidence that the structure of the complex in the region of electron transfer is the same in the crystal and in solution. X-ray diffraction data were collected from co-crystals to a maximum resolution of 2.40 Å and refined to an R-factor of 22% ( R free=26%). The structure shows the cyt c 2 to be positioned at the center of the periplasmic surface of the RC, with the heme edge located above the bacteriochlorophyll dimer. The distance between the closest atoms of the two cofactors is 8.4 Å. The side-chain of Tyr L162 makes van der Waals contacts with both cofactors along the shortest intermolecular electron transfer pathway. The binding interface can be divided into two domains: (i) A short-range interaction domain that includes Tyr L162, and groups exhibiting non-polar interactions, hydrogen bonding, and a cation–π interaction. This domain contributes to the strength and specificity of cyt c 2 binding. (ii) A long-range, electrostatic interaction domain that contains solvated complementary charges on the RC and cyt c 2. This domain, in addition to contributing to the binding, may help steer the unbound proteins toward the right conformation.

Effect of protein relaxation on electron transfer from the cytochrome subunit to the bacteriochlorophyll dimer in Rps. sulfoviridis reaction centers within mixed adiabatic/nonadiabatic model

The broad set of nonexponential electron transfer (ET) kinetics in reaction centers (RC) from Rhodopseudomonas sulfoviridis in temperature range 297–40 K are described within a mixed adiabatic/nonadiabatic model. The key point of the model is the combination of Sumi–Marcus and Rips–Jortner approaches which can be represented by the separate contributions of temperature-independent vibrational (v) and temperature-dependent diffusive (d) coordinates to the preexponential factor, to the free energy of reaction Δ G=Δ G v+Δ G d( T) and to the reorganization energy λ= λ v+ λ d( T). The broad distribution of protein dielectric relaxation times along the diffusive coordinate is considered within the Davidson–Cole formalism.

Oscillations in the primary charge separation in bacterial photosynthesis

This paper examines the temporal evolution of an electron in a photosynthetic reaction center. A model based on self-consistent quantum-mechanical equations is suggested to describe the picosecond transfer of an electron from an excited primary donor, the excited bacteriochlorophyll dimer (P*), to bacteriopheophytin (H). This transfer is shown to have an oscillation behavior. The results obtained offer an explanation for oscillations in the emission kinetics of the primary donor excited state.

Electrostatic interactions in an integral membrane protein.

The effects of charge-charge interactions on the midpoint reduction potential (E(m)()) of the primary electron donor (P) in the photosynthetic reaction center of Rhodobacter sphaeroides were investigated by introducing mutations of ionizable amino acids at selected sites. The mutations were designed to alter the electrostatic environment of P, a bacteriochlorophyll dimer, without greatly affecting its structure or molecular orbitals. Two arginine residues at homologous positions in the L and M subunits [residues (L135) and (M164)], Asp (L155), Tyr (L164), and Cys (L247) were changed independently. Arginine (L135) was replaced by Lys, Leu, Gln, or Glu; Arg (M164), by Leu or Glu; Asp (L155), by Asn; Tyr (L164), by Phe; and Cys (L247), by Lys or Asp. The R(L135)E/C(L247)K double mutant also was made. The shift in the E(m)() of P/P(+) was measured in each mutant and was compared with the effect predicted by electrostatics calculations using several different computational approaches. A simple distance-dependent dielectric screening factor reproduced the effects remarkably well. By contrast, microscopic methods that considered the reaction field in the protein and solvent but did not include explicit counterions overestimated the changes in the E(m)() considerably. Including counterions for the charged residues reduced the calculated effects of the mutations in molecular dynamics calculations. The results show that electrostatic interactions of P with ionizable amino acid residues are strongly screened, and suggest that counterions make major contributions to this screening. The screening also could reflect penetration of water or other relaxations not taken into account because of incomplete sampling of configurational space.

Stepwise charge separation and charge recombination in ferrocene-meso,meso-linked porphyrin dimer-fullerene triad.

A meso,meso-linked porphyrin dimer [(ZnP)(2)] as a light-harvesting chromophore has been incorporated into a photosynthetic multistep electron-transfer model for the first time, including ferrocene (Fc), as an electron donor and fullerene (C(60)) as an electron acceptor to construct the ferrocene-meso,meso-linked porphyrin dimer-fullerene system (Fc-(ZnP)(2)-C(60)). Photoirradiation of Fc-(ZnP)(2)-C(60) results in photoinduced electron transfer from the singlet excited state of the porphyrin dimer [(1)(ZnP)(2)] to the C(60) moiety to produce the porphyrin dimer radical cation-C(60) radical anion pair, Fc-(ZnP)(2)(*+)-C(60)(*-). In competition with the back electron transfer from C(60)(*-) to (ZnP)(2)(*+) to the ground state, an electron transfer from Fc to (ZnP)(2)(*+) occurs to give the final charge-separated (CS) state, that is, Fc(+)-(ZnP)(2)-C(60)(*-), which is detected as the transient absorption spectra by the laser flash photolysis. The quantum yield of formation of the final CS state is determined as 0.80 in benzonitrile. The final CS state decays obeying first-order kinetics with a lifetime of 19 micros in benzonitrile at 295 K. The activation energy for the charge recombination (CR) process is determined as 0.15 eV in benzonitrile, which is much larger than the value expected from the direct CR process to the ground state. This value is rather comparable to the energy difference between the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)) and the final CS state (Fc(+)-(ZnP)(2)-C(60)(*-)). This indicates that the back electron transfer to the ground state occurs via the reversed stepwise processes,that is, a rate-limiting electron transfer from (ZnP)(2) to Fc(+) to give the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)), followed by a fast electron transfer from C(60)(*-) to (ZnP)(2)(*+) to regenerate the ground state, Fc-(ZnP)(2)-C(60). This is in sharp contrast with the extremely slow direct CR process of bacteriochlorophyll dimer radical cation-quinone radical anion pair in bacterial reaction centers.

Proton uptake of rhodobacter capsulatus reaction center mutants modified in the primary quinone environment.

Flash-induced absorbance spectroscopy was used to analyze the proton uptake and electron transfer properties of photosynthetic reaction centers (RC) of Rhodobacter capsulatus that have been genetically modified near the primary quinone electron acceptor (Q(A)). M246Ala and M247Ala, which are symmetry-related to the positions of two acidic groups, L212Glu and L213Asp, in the secondary quinone electron acceptor (QB) protein environment, have been mutated to Glu and Asp, respectively. The pH dependence of the stoichiometry of proton uptake upon formation of the P+Q(A)- (H+/P+Q(A)-) and PQ(A) (H+/Q(A)-) (P is the primary electron donor, a noncovalently linked bacteriochlorophyll dimer) states have been measured in the M246Ala --> Glu and the M247Ala --> Asp mutant RC, in the M246Ala-M247Ala --> Glu-Asp double mutant and in the wild type (WT). Our results show that the introduction of an acidic group (Glu or Asp) in the QA protein region induces notable additional proton uptake over a large pH region (approximately 6-9), which reflects a delocalized response of the protein to the formation of Q(A)-. This may indicate the existence of a widely spread proton reservoir in the cytoplasmic region of the protein. Interestingly, the pH titration curves of the proton release caused by the formation of P+ (H+/P+: difference between H+/P+Q(A)- and H+/PQ(A)- curves) are nearly superimposable in the WT and the M246Ala --> Glu mutant RC, but substantial additional proton release is detected between pH 7 and 9 in the M247Ala --> Asp mutant RC. This effect can be accounted for by an increased proton release by the P+ environment in the M247Ala --> Asp mutant. The M247Ala --> Asp mutation reveals the existence of an energetic and conformational coupling between donor and acceptor sides of the RC at a distance of nearly 30A.

Electrostatic Interactions between Charged Amino Acid Residues and the Bacteriochlorophyll Dimer in Reaction Centers from Rhodobacter sphaeroides

The extent of electrostatic contributions from the protein environment was assessed by the introduction of ionizable residues near the bacteriochlorophyll dimer in reaction centers from Rhodobacter sphaeroides. Two mutations at symmetry-related sites, M199 Asn to Asp and L170 Asn to Asp, resulted in a 48 and 44 mV lowering of the midpoint potential, respectively, compared to the wild type at pH 8, while a 75 mV decrease in the midpoint potential was observed for the mutation L168 His to Glu. The decrease relative to wild type was found to be approximately additive, up to 147 mV, for various combinations of the mutations. As the pH was lowered from 9.5 to 6.0, the relative decrease in the midpoint potential became smaller for each of these three mutations. Titration of the pH dependence of the change in midpoint potential of the M199 Asn to Asp mutant compared to wild type yielded a pK(a) value of 7.9 and a change in midpoint potential from low to high pH of 59 mV. The major effect of the mutation on the midpoint potential of the dimer is interpreted as stemming from a negative charge on the residue. An average dielectric constant of approximately 20 was estimated for the local protein environment, consistent with a relatively hydrophobic environment for residue M199. The rate of charge recombination between the primary quinone acceptor and the bacteriochlorophyll dimer decreased in the M199 Asn to Asp mutant at high pH, reflecting the decrease in midpoint potential.

Effect of D 2O and cryosolvents on the redox properties of bacteriochlorophyll dimer and electron transfer processes in Rhodobacter sphaeroides reaction centers

Effects of environmental changes on the reaction pattern of excitation energy trapping and transformation into the “stable” radical pair P +Q A −. have been analyzed in isolated reaction centers of the anoxygenic purple bacterium Rhodobacter sphaeroides. The following results were obtained: (a) replacement of exchangeable protons by deuterons significantly retarded the electron transfer steps of primary charge separation, leading to the radical pair P +I − and of the subsequent reoxidation of I − by the quinone acceptor Q A but has virtually no effect on the midpoint potential of P/P + that was found to be 430±20 mV; (b) addition of 70% (v/v) glycerol causes a shift of E m by about 30 mV towards higher values whereas the kinetics of the electron transfer reactions remain almost unaffected; (c) in the presence of the cryoprotectant DMSO, a combined effect arises, i.e. a retardation of the electron transfer kinetics comparable to that induced by H/D exchange and simultaneously an upshift of the E m value to 475±20 mV, resembling the action of glycerol. These results are discussed within the framework of effects on the midpoint potential due to the dielectric constant of the medium and changes of the charge distribution in the vicinity of the redox groups and the influence of relaxation processes on electron transfer reactions.

Tyrosine oxidation in modified reaction centers from Rhodobacter sphaeroides

A highly oxidizing bacteriochlorophyll dimer was generated by multiple amino acid substitution in reaction centers from Rhodobacter sphaeroides. Tyrosine residues were then introduced into these modified reaction centers at positions that are analogous to those of YZ and YD in photosystem II. Optical and EPR spectra show changes that are consistent with the oxidation of the tyrosine residues by the bacteriochlorophyll dimer. The formation of the tyrosyl radical was found to be pH-dependent, indicating an important role of the surrounding protein environment. Subsequent amino acid substitutions at nearby positions resulted in altered pK values for the pH dependence of the optical and EPR signals characteristic of the oxidized tyrosine. The pH dependence is explained by models of the mechanism of tyrosine oxidation in proteins, in which electron transfer is coupled to the release of the phenolic proton to a proton acceptor, resulting in a neutral tyrosyl radical. These studies provide insight into how electron transfer pathways evolved in photosynthetic complexes.

Studies of electron transport dynamics in photosynthetic reaction centers using fast temperature changes.

Rates of thermoinduced conformational transitions of reaction center (RC) complexes providing effective electron transport were studied in chromatophores and isolated RC preparations of various photosynthesizing purple bacteria using methods of fast freezing and laser-induced temperature jump. Reactions of electron transfer from the primary to secondary quinone acceptors and from the multiheme cytochrome c subunit to photoactive bacteriochlorophyll dimer were used as probes of electron transport efficiency. The thermoinduced transition of the acceptor complex to the conformational state facilitating electron transfer to the secondary quinone acceptor was studied. It was shown that neither the characteristic time of the thermoinduced transition within the temperature range 233-253 K nor the characteristic time of spontaneous decay of this state at 253 K exceeded several tens of milliseconds. In contrast to the quinone complex, the thermoinduced transition of the macromolecular RC complex to the state providing effective electron transport from the multiheme cytochrome c to the photoactive bacteriochlorophyll dimer within the temperature range 220-280 K accounts for tens of seconds. This transition is thought to be mediated by large-scale conformational dynamics of the macromolecular RC complex.

Quinone-Dependent Delayed Fluorescence from the Reaction Center of Photosynthetic Bacteria

Millisecond delayed fluorescence from the isolated reaction center of photosynthetic bacteria Rhodobacter sphaeroides was measured after single saturating flash excitation and was explained by thermal repopulation of the excited bacteriochlorophyll dimer from lower lying charge separated states. Three exponential components (fastest, fast, and slow) were found with lifetimes of 1.5, 102, and 865 ms and quantum yields of 6.4 × 10 −9, 2.2 × 10 −9, and 2.6 × 10 −9 (pH 8.0), respectively. While the two latter phases could be related to transient absorption changes, the fastest one could not. The fastest component, dominating when the primary quinone was prereduced, might be due to a small fraction of long-lived triplet states of the radical pair and/or the dimer. The fast phase observed in the absence of the secondary quinone, was sensitive to pH, temperature, and the chemical nature of the primary quinone. The standard free energy of the primary stable charge pair relative to that of the excited dimer was −910 ± 20 meV at pH 8 and with native ubiquinone, and it showed characteristic changes upon pH and quinone replacement. The interaction energy (∼50 meV) between the cluster of the protonatable groups around GluL212 and the primary semiquinone provides evidence for functional linkage between the two quinone binding pockets. An empirical relationship was found between the in situ free energy of the primary quinone and the rate of charge recombination, with practical importance in the estimation of the free energy levels from the easily available lifetime of the charge recombination. The ratio of the slow and fast components could be used to determine the pH dependence of the free energy level of the secondary stable charge pair relative to that of the excited dimer.

Dipyridamole and its derivatives modify the kinetics of the electron transport in reaction centers from Rhodobacter sphaeroides

A well known vasodilator dipyridamole (DIP), 2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido[5,4-d]pyrimidine, and its derivatives have recently been shown as potential co-activators (modulators) in the phenomenon of multidrug resistance (MDR) in cancer therapy. They inhibit the specific function of a transmembrane P-glycoprotein responsible for the ex-flux of anti-cancer drugs from tumor cells. To clarify molecular mechanisms of the anti-MDR activity of DIP and its two derivatives, RA25 and RA47, we have studied their effects on electron transport in reaction centers (RC) from purple photosynthetic bacteria Rb. sphaeroides, using RC as a model system. Increasing concentrations of DIP and RA47 progressively accelerate the back electron transfer from the primary quinone acceptor Q A to the bacteriochlorophyll dimer Bchl 2 (Bchl 2 +–Q A − recombination). In the absence of o-phenantroline, when both quinone acceptors Q A and Q B are involved in the electron transport, RA47 is more effective than DIP. DIP stabilizes the electron on the secondary quinone acceptor Q B, the effect manifested as the retardation of Bchl 2 +–Q B − recombination. Effects of RA25 are negligible in all cases. The drugs are proposed to change the electron transport affecting the RC structural dynamics and the stabilization of the electron on quinone acceptors through modification of H-bonds in the system.