Transient states in reaction centers containing reduced bacteriopheophytin

Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts

Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation. Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor ("one-way" ET), a form of positive design. Curiously, negative design strategies that explicitly avoid unwanted side reactions have been underexplored. We report here that electronic polarization of a molecular chromophore can be used as both a positive and negative design element in a light-driven reaction. Intriguingly, prudent engineering of polarized excited states can steer a "U-turn" ET-where the excited electron density of the donor is initially pushed away from the acceptor-to outcompete a conventional one-way ET scheme. We directly compare one-way vs. U-turn ET strategies via a linked donor-acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing. These results demonstrate that oppositely polarized electronically excited states can be employed to steer photoexcited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead ends.

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

We studied the accumulation of long-lived charge-separated states in reaction centers isolated from Rhodobacter sphaeroides, using continuous illumination, or trains of single-turnover flashes. We found that under both conditions a long-lived state was produced with a quantum yield of about 1%. This long-lived species resembles the normal P(+)Q(-) state in all respects, but has a lifetime of several minutes. Under continuous illumination the long-lived state can be accumulated, leading to close to full conversion of the reaction centers into this state. The lifetime of this accumulated state varies from a few minutes up to more than 20 min, and depends on the illumination history. Surprisingly, the lifetime and quantum yield do not depend on the presence of the secondary quinone, Q(B). Under oxygen-free conditions the accumulation was reversible, no changes in the normal recombination times were observed due to the intense illumination. The long-lived state is responsible for most of the dark adaptation and hysteresis effects observed in room temperature experiments. A simple method for quinone extraction and reconstitution was developed.

Electron transfer in reaction centers of Rhodobacter sphaeroides and Rhodobacter capsulatus monitored by fluorescence of the bacteriochlorophyll dimer

Spectral and kinetic characteristics of fluorescence from isolated reaction centers of photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodobacter capsulatus were measured at room temperature under rectangular shape of excitation at 810 nm. The kinetics of fluorescence at 915 nm reflected redox changes due to light and dark reactions in the donor and acceptor quinone complex of the reaction center as identified by absorption changes at 865 nm (bacteriochlorophyll dimer) and 450 nm (quinones) measured simultaneously with the fluorescence. Based on redox titration and gradual bleaching of the dimer, the yield of fluorescence from reaction centers could be separated into a time-dependent (originating from the dimer) and a constant part (coming from contaminating pigment (detached bacteriochlorin)). The origin was also confirmed by the corresponding excitation spectra of the 915 nm fluorescence. The ratio of yields of constant fluorescence over variable fluorescence was much smaller in Rhodobacter sphaeroides (0.15±0.1) than in Rhodobacter capsulatus (1.2±0.3). It was shown that the changes in fluorescence yield reflected the disappearance of the dimer and the quenching by the oxidized primary quinone. The redox changes of the secondary quinone did not have any influence on the yield but excess quinone in the solution quenched the (constant part of) fluorescence. The relative yields of fluorescence in different redox states of the reaction center were tabulated. The fluorescence of the dimer can be used as an effective tool in studies of redox reactions in reaction centers, an alternative to the measurements of absorption kinetics.

Control of electron transfer between the L- and M-sides of photosynthetic reaction centers

An aspartic acid residue has been introduced near ring V of the L-side accessory bacteriochlorophyll (BCHlL) or the photosynthetic reaction center in a rhodobacter capsulatus mutant in which a His also replaces Leu 212 on the M-polypeptide. The initial stage of charge separation in the G(M201)D/L(M212)H double mutant yields approximately 70 percent electron transfer to the L-side cofactors, approximately 15 percent rapid deactivation to the ground state, and approximately 15 percent electron transfer to the so-called inactive M-side bacteriopheophytin (BPhM). It is suggested here that the Asp introduced at M201 modulates the reduction potential of BCHlL, thereby changing the energetics of charge separation. The results demonstrate that an individual amino acid residue can, through its influence on the free energies of the charge-separated states, effectively dictate the balance between the forward electron transfer reactions on the L-side of the RC, the charge-recombination processes, and electron transfer to the M-side chromophores.

Photochemical trapping of a bacteriopheophytin anion in site-specific reaction-center mutants from the photosynthetic bacterium Rhodobacter sphaeroides

The mutant YY in the reaction center of Rhodobacter sphaeroides, in which Phe181 on the L chain has been replaced by Tyr, and the double mutant FY, with Tyr210 on the M chain replaced by Phe and Phe181 on the L chain replaced by Tyr, have been constructed by site-directed mutagenesis. The studies described here were performed to complement a previous mutational analysis of mutant FF with Tyr210 replaced by Phe. Both new strains grow photoheterotrophically. The optical absorption spectra of reaction centers isolated from these mutants have band shifts attributable to the monomer bacteriochlorophylls in the vicinity of the substitutions. Photochemical trapping of the bacteriopheophytin anion (I-) indicates that the bacteriopheophytin on the B branch is reduced to a much greater extent in FF and FY as compared to YY and wild-type YF. Low temperature (77 K) absorption spectra clearly show that in the wild-type (YF) and YY reaction centers only the 545-nm-absorbing bacteriopheophytin is reduced while in the FF and FY reaction centers both the 535-nm and 545-nm-absorbing bacteriopheophytins are reduced. A simple kinetic analysis is used to explain these results. This analysis suggests that, in order for the observed trapping results to occur, a decrease in the 'cycling' time must take place, that is changes in the rate(s) of charge recombination must accompany the already known decrease in the forward electron transfer rate.

Light-induced charge separation in Rhodopseudomonas viridis reaction centers monitored by Fourier-transform infrared difference spectroscopy: the quinone vibrations

Static FTIR light-induced difference spectra have been recorded for reaction centers from Rhodopseudomonas viridis in the following charge-separated states: P+QA(-)-PQA, P+QB(-)-PQB, I(-)-I, I-QA(-)-IQA, and I-QA(2-)-IQA. A comparison of the I(-)-I difference spectra with the I-QA(-)-IQA difference spectra reveals new bands which can be assigned to QA- vibrations; these vibrations are also observed in the P+QA(-)-PQA and P+QB(-)-PQB difference spectra. Through an analysis of all of the static difference spectra, the electron-transfer pathway can be monitored in the infrared from the primary donor, P, to the secondary acceptor, QB, via the intermediate acceptor, I, and the primary acceptor, QA. The difference spectra are dominated by absorbance changes of prosthetic groups, with very few identifiable contributions from amino acids and little overall structural change in the protein backbone, involving only one or two residues for the various charge-separated states. Oxidation of the primary donor in the reaction center shows the characteristic absorbance changes of the 9-keto and 10-ester carbonyl groups observed upon oxidation of bacteriochlorophyll b in a non-hydrogen-bonded environment [Ballschmiter, K. H., & Katz, J. J. (1969) J. Am. Chem. Soc. 91, 2661-2677]. Reduction of the quinones in the reaction center yields absorbance changes of the carbonyls observed during reduction of quinones in a hydrogen-bonded environment [Bauscher, M., Nabedryk, E., Bagley, K., Breton, J., & Mäntele, W. (1990) FEBS Lett. 261, 191-195].(ABSTRACT TRUNCATED AT 250 WORDS)

Initial electron-transfer in the reaction center from Rhodobacter sphaeroides

The initial electron transfer steps in the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides have been investigated by femtosecond time-resolved spectroscopy. The experimental data taken at various wavelengths demonstrate the existence of at least four intermediate states within the first nanosecond. The difference spectra of the intermediates and transient photodichroism data are fully consistent with a sequential four-step model of the primary electron transfer: Light absorption by the special pair P leads to the state P*. From the excited primary donor P*, the electron is transferred within 3.5 +/- 0.4 ps to the accessory bacteriochlorophyll B. State P+B- decays with a time constant of 0.9 +/- 0.3 ps passing the electron to the bacteriopheophytin H. Finally, the electron is transferred from H- to the quinone QA within 220 +/- 40 ps.

Measurement of the extent of electron transfer to the bacteriopheophytin in the M-subunit in reaction centers of Rhodopseudomonas viridis

We have measured the extent of flash-induced electron transfer from the bacteriochlorophyll dimer, P, to the bacteriopheophytin in the M-subunit, HM, in reaction centers of Rhodopseudomonas viridis. This has been done by measuring the transient states produced by excitation of reaction centers trapped in the PHL (-)HM state at 90 K. Under these conditions the normal forward electron transfer to the bacteriopheophytin in the L-subunit, HL, is blocked and the yield of transient P(+)HM (-) can be estimated with respect to the lifetime of P(*). Under these conditions flash induced absorbance decreases of the bacteriochlorophyll dimer 990 nm band suggest that a transient P(+) state is formed with a quantum yield of 0.09±0.06 compared to that formed during normal photochemistry. These transient measurements provide an upper limited on the yield of a transient P(+) HM (-) state. An estimate of 0.09 as the yield of the P(+) HM (-) state is consistent with all current observations. This estimate and the lifetime of P(*) suggest that the electron transfer rate from P(*) to HM, kM, is about 5 × 10(9) sec(-1) (τM = 200ps). These measurements suggest that the a branching ratio kL/kM is on the order of 200. The large value of the branching ratio is remarkable in view of the structural symmetry of the reaction center. This measurement should be useful for electron transfer calculations based upon the reaction center structure.

Primary photochemistry of reaction centers from the photosynthetic purple bacteria

Photosynthetic organisms transform the energy of sunlight into chemical potential in a specialized membrane-bound pigment-protein complex called the reaction center. Following light activation, the reaction center produces a charge-separated state consisting of an oxidized electron donor molecule and a reduced electron acceptor molecule. This primary photochemical process, which occurs via a series of rapid electron transfer steps, is complete within a nanosecond of photon absorption. Recent structural data on reaction centers of photosynthetic bacteria, combined with results from a large variety of photochemical measurements have expanded our understanding of how efficient charge separation occurs in the reaction center, and have changed many of the outstanding questions.

Picosecond kinetics of the initial photochemical electron-transfer reaction in bacterial photosynthetic reaction centers

The absorption changes that occur in reaction centers of the photosynthetic bacterium Rhodopseudomonas sphaeroides during the initial photochemical electron-transfer reaction have been examined. Measurements were made between 740 and 1300 nm at 295 and 80 K by using a pulse-probe technique with 610-nm, 0.8-ps flashes. An excited singlet state of the bacteriochlorophyll dimer P* was found to give rise to stimulated emission with a spectrum similar to that determined previously for fluorescence from reaction centers. The stimulated emission was used to follow the decay of P*; its lifetime was 4.1 +/- 0.2 ps at 295 K and 2.2 +/- 0.1 ps at 80 K. Within the experimental uncertainty, the absorption changes associated with the formation of a bacteriopheophytin anion, Bph-, develop in concert with the decay of P* at both temperatures, as does the absorption increase near 1250 nm due to the formation of the cation of P, P+. No evidence was found for the formation of a bacteriochlorophyll anion, Bchl-, prior to the formation of Bph-. This is surprising, because in the crystal structure of the Rhodopseudomonas viridis reaction center [Deisenhofer, J., Epp, O., Miki, K., Huber, R., & Michel, H. (1984) J. Mol. Biol. 180, 385-398] a Bchl is located approximately in between P and the Bph. It is possible that Bchl- (or Bchl+) is formed but, due to kinetic or thermodynamic constraints, is never present at a sufficient concentration for us to observe. Alternatively, a virtual charge-transfer state, such as P+Bchl-Bph or PBchl+Bph-, could serve to lower the energy barrier for direct electron transfer between P* and the Bph.

Excitation transport and trapping in a synthetic chlorophyllide substituted hemoglobin: orientation of the chlorophyll S1 transition dipole

Excitation transport in synthetic zinc chlorophyllide substituted hemoglobin has been observed by pico -second time-resolved fluorescence depolarization experiments. In this hybrid molecular system, two zinc chlorophyllide molecules are substituted into the beta-chains of hemoglobin, while deoxy hemes remain in the alpha-chains. The rate of excitation transfer between the two chlorophyllides is analyzed in terms of the distance and orientation dependences predicted by the F orster dipole-dipole theory. In this analysis, the beta-beta interchromophore geometry is assumed to be that of the deoxyhemoglobin crystal structure. When combined with steady-state fluorescence depolarization data of the complementary hybrid containing zinc chlorophyllide in the alpha-chains, these experiments provide the necessary information to determine the orientation of the S1 transition dipole moment in the zinc chlorophyllide molecule. We also find that the fluorescence lifetime of the zinc chlorophyllide is 1.42 ns when the heme is in the deoxy state but 3.75 ns when the heme is ligated to carbon monoxide. This is explained by irreversible excitation transfer from the S1 state of the zinc chlorophyllide to the lower energy excited states present in deoxy heme.