Eric M. Franken

Reaction centers of Rhodobacter sphaeroides R-26 with selective replacement of bacteriopheophytin by pheophytin a: I. Characterisation of steady-state absorbance and circular dichroism, and of the P+QA− state

Abstract Bacteriopheophytin (BPheo) a of reaction centers (RCs) of Rhodobacter sphaeroides R-26 has been exchanged with pheophytin (Pheo) a . By varying the incubation temperature of the pigment exchange procedure two types of RCs were obtained, with either only the BPheo in the B-chain (BPheo B ) or both BPheo B and BPheo A replaced by Pheo a . For the two RC types absorption and CD spectra at 6 K as well as P + Q A − difference spectra at 10 K are compared with those of native RCs. The most pronounced differences are observed in the Q Y and Q X regions of the (B)Pheos. The P + Q A − decay halftime is for RCs with Pheo a in both chains 10–15 ms longer than for native RCs and RCs that still have a BPheo in the A-chain, at all temperatures between 10 and 290 K. At low temperatures all three RC types showed biphasic P + Q A − recombination.

ADMR study of the effect of oligomerisation on the carotenoid triplets and on triplet-triplet transfer in light harvesting complex II (LHC II) of spinach

Carotenoid triplets in isolated light harvesting complex (LHC) II of spinach at different concentrations were studied by absorbance-detected magnetic resonance (ADMR). Going from high to low LHC II concentrations, a change was observed in the intensity of the ADMR spectra of the |D|+|E| transition recorded at 507 nm relative to that recorded at 525 nm, from approx. 0.5 to approx. 1.0. The relative intensity of the 2|E| transition did not change. The change in relative intensity of the ADMR singla is due to a change of the ADMR signal intensity of the |D|+|E| transition that is detected at 525 nm. The effect is ascribed to an aggregation of trimeric LHC II into an oligomeric form of LHC II. Taking into account the narrowing of the zero-field resonance bands with oligomerisation, and the absence of bandshifts, the relative increase of the signal intensity of the |D|+|E| transition detected at 525 nm can be explained by assuming that the oligomer consists of a multiple of trimers, between which ‘inter-trimer’ energy transfer occurs. This yields an increase in the number of triplets that is transferred to the carotenoid having its triplet absorption maximum at 525 nm. Our new results indicate that the carotenoids are bound to Chl a monomers, and not dimers as proposed earlier (Van der Vos, R., Carbonera, D. and Hoff, A.J. (1991) Appl. Magn. Res. 2, 179–202).

ELECTRON TRANSPORT AND TRIPLET FORMATION IN MEMBRANE FRAGMENTS OF THE GREEN SULFUR BACTERIUM PROSTHECOCHLORIS AESTUARII

Abstract Under conditions where normal electron transport is blocked, at high pH in the presence of dithionite, three triplets could be observed upon flash illumination of membrane fragments of the green sulfur bacterium Prosthecochloris aestuarii. The first triplet decayed in 7.5 μs and is assigned to carotenoid. The second triplet decays in 67 μs and is assigned to bacteriochlorophyll (BChl) a of the Fenna-Matthews-Olson (FMO) complex, since a triplet with the same spectrum and lifetime was also formed in the isolated FMO complex. The third triplet decayed in 165 μs and is assigned to BChl a of the reaction center core complex, based on its main bleaching at 837 nm. There is insufficient evidence to decide whether this triplet is located on the primary electron donor P840 or on a long-wavelength antenna BChl a of the core. A multiple-flash experiment indicated the presence of two photo-oxidizable hemes per reaction center (RC), both having a difference spectrum centered around 553 nm. The oxidation time was 25 μs for both cytochromes. However, a 75-μs delay was observed for the oxidation of the second heme, indicating that another process must take place before this reaction can occur. This result, together with the observed low efficiency of oxidation of the second heme, suggests the presence of a four-heme cytochrome (as observed in some other species of green sulfur bacteria), with only one heme in direct contact with the RC, rather than a model with two cytochromes symmetrically attached to the RC, as proposed by others. The observed delay can then be explained by a relatively slow heme-to-heme electron transfer. The cytochrome oxidation time of both hemes increased with viscosity, suggesting that some molecular motion is involved in the oxidation process.

Reaction centers of Rhodobacter sphaeroides R-26 with selective replacement of bacteriopheophytin a by pheophytin a: II. Temperature dependence of the quantum yield of P+QA− and 3P formation

Abstract The quantum yield of the formation of the charge-separated state P+QA− in reaction centers (RCs) of Rhodobacter sphaeroides R-26, in which the bacteriopheophytins in both the active (A) and the inactive (B) branch are replaced by pheophytin (Pheo) a (ΦA,B-exchanged RCs), shows a positive temperature dependence: it is 38±5% between 10 and 60 K, increases with temperature to 72±5% at 200 K and shows a minor additional increase above this temperature. The temperature dependence of the quantum yield of P+QA− formation in ΦA,B-exchanged RCs is modelled in the framework of a reaction scheme with the energy level of P+PheoA− placed above P+BA− (Shkuropatov, A.Ya. and Shuvalov, V.A. (1993) FEBS Lett. 322, 168–172), by the introduction of direct electron transfer from BA− to QA, assisted by a superexchange-mechanism via P+PheoA−. The observed triplet formation of ΦA,B-exchanged RCs with pre-reduced QA at cryogenic temperatures (quantum yield≤12%) is attributed to a residual fraction of RCs in which only ΦB was exchanged for Pheo a. The lack of triplet formation in pre-reduced ΦA,B-exchanged RCs is consistent with our kinetic model, since this predicts that at low temperatures the state P+PheoA− is not populated.

Magnetic Field Effects: MARY, MIMS and MODS*

The principles and methodology of magnetic field effects (MFE) on the yield of photosynthetic reactions are briefly reviewed. Two basicallly different mechanisms are described: the MFE driven by photochemical radical pair production and subsequent spinselective recombination (Radical Pair Mechanism, RPM), and the MFE produced by the magnetic-field induced mixing of the three sublevéis of a molecular triplet state (Magnetic field Induced Mixing of Sublevéis, MIMS). A number of experimental methods for measuring so-called MARY (MAgnetic field effect on the Reaction Yield) curves are described. It is shown that the MFE is a powerful tool for measuring highly accurate Triplet-minus-Singlet absorbance difference spectra (Magneto-Optical Difference Spectroscopy, MODS). A number of recent applications of the MFE in the study of photosynthetic reactions is discussed, comprising also an investigation of OpticallyDetected Magnetic Resonance (ODMR) in low magnetic field, which suggests that a significant spin-orbit coupling in the triplet state of the primary electron donor leads to level anti-crossing.

Optically detected magnetic field effects on reaction centers of Rhodobacter sphaeroides 2.4.1 and its Tyr M210 → Trp mutant

Abstract In this work, the magnetic field effects (MFE) on the triplet yield in reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides 2.4.1 and its Tyr M210 → Trp mutant are compared. The MFE is measured between 5 and 225 K by monitoring the absorbance. Using monochromatic polarised and unpolarised light, linear dichroic (LD-)MFE curves were obtained. Simulations of the (LD-) MFE measured at low temperatures (25 K) are presented, which suggest that the exchange interaction in the mutant and in the wild type have opposite sign. This is explained by considering the free energy differences between the excited primary donor state 1P∗ and the charge-separated state. The magnetic field dependence of the MFE of Q-depleted mutant RCs at low magnetic fields is similar for temperatures between 25 and 225 K, implying that kT is practically temperature independent. This suggests that the temperature dependence of the triplet yield is due to a change in radical pair singlet recombination rate kS. The exchange interaction between the oxidised primary donor and reduced intermediary bacteriopheophytin acceptor of mutant RCs is twice as large as that of the wild type. This is attributed to an upward change of the energy level of the charge-separated state caused by the mutation.

The Energy Level of P + B A - in Plant Pheophytin-Exchanged Bacterial Reaction Centers Probed by the Temperature Dependence of Delayed Fluorescence

The role of the accessory bacteriochlorophyll (BChl) BA between the primary electron donor P and the bacteriopheophytin (BPheo) electron acceptor HA in photosynthetic reaction centers (RC) is still subject of debate. The free energy gap between the states P* and P+B A - will have a decisive influence on the mechanism of the first electron transfer steps. We have recently shown [1] that P* and P+B A - accumulates in pheophytin (Pheo) exchanged RCs at 5K and that electron transfer to this state occurs at essentially the same rate as in native RCs. According to these results, the free energy level of P* and P+B A - has to lie below that of P* (Scheme 1.). The measurement of the temperature dependence of the fluorescence quantum yield resulting from thermal repopulation of the state P’ therefore offers a direct access to determine the free energy level of P* and P+B A - relative to that of P*.

MAGNETIC INTERACTION BETWEEN QA-. AND THE TRIPLET STATE OF THE PRIMARY DONOR IN MODIFIED REACTION CENTERS OF THE PHOTOSYNTHETIC BACTERIUM RHODOBACTER SPHAEROIDES R26

Abstract The interaction between the reduced primary acceptor quinone (QA−.), and the triplet state of the primary donor 3D is investigated with time-resolved continuous-wave EPR. The trough at high-field in the QA−. electron-spin polarized X-band EPR-spectrum at early delay times after the laser flash [De Groot et al. (1985) Biochim. Biophys. Acta 808, 13–20] is studied as a function of temperature and of the delay, in zinc-reconstituted reaction centers, with and without carotenoid. In all cases investigated a decrease in 3D concentration is accompanied by a simultaneous attenuation of the high-field trough in the QA−. EPR spectrum. These observations confirm the hypothesis that the line-shape of QA−. at short delay times is influenced by a magnetic interaction with 3D. The line-shape of the QA−. electron-spin polarized EPR spectrum directly after the laser flash, could be very well simulated using an extension of the model of Hore et al. [Hore, P.J. et al. (1993) Biochim. Biophys. Acta 1141, 221–230], with a dipolar coupling between QA−. and 3D of −0.125 mT.