The effect of exchange of bacteriopheophytin a with plant pheophytin a on charge separation in Y(M210)W mutant reaction centers of Rhodobacter sphaeroides at low temperature

The bacteriopheophytin a molecules at the H(A) and H(B) binding sites of reaction centers (RCs) of the Y(M210)W mutant of Rhodobacter sphaeroides were chemically exchanged with plant pheophytin a. The Y(M210)W mutation slows down the formation of H(A)(-), presumably by raising the free energy level of the P(+)B(A)(-) state above that of P* due to increasing the oxidation potential of the primary electron donor P and lowering the reduction potential of the accessory bacteriochlorophyll B(A). Exchange of the bacteriopheophytins with pheophytin a on the contrary lowers the redox potential of H(A), inhibiting its reduction. A combination of the mutation and pigment exchange was therefore expected to make the A-side of the RC incapable of electron transfer and cause the excited state P* to deactivate directly to the ground state or through the B-side, or both. Time-resolved absorption difference spectroscopy at 10 K on the RCs that were modified in this way showed a lifetime of P* lengthened to about 500 ps as compared to about 200 ps measured in the original Y(M210)W RCs. We show that the decay of P* in the pheophytin-exchanged preparations is accompanied by both return to the ground state and formation of a new charge-separated state, the absorption difference spectrum of which is characterized by bleachings at 811 and 890 nm. This latter state was formed with a time constant of ca. 1.7 ns and a yield of about 30%, and lasted a few nanoseconds. On the basis of spectroscopic observations these bands at 811 and 890 nm are tentatively attributed to the presence of the P(+)B(B)(-) state, where B(B) is the accessory bacteriochlorophyll in the "inactive" B-branch of the cofactors. The B(B) molecules in Y(M210)W RCs are suggested to be spectrally heterogeneous, absorbing in the Q(y) region at 813 or 806 nm. The results are discussed in terms of perturbation of the free energy level of the P(+)B(B)(-) state and absorption properties of the B(B) bacteriochlorophyll in the mutant RCs due to a long-range effect of the Y(M210)W mutation on the protein environment of the B(B) binding pocket.     

Mutation of the Chlamydomonas reinhardtii analogue of residue M210 of the Rhodobacter sphaeroides reaction center slows primary electron transfer in Photosystem II

Primary charge separation within Photosystem II (PS II) is much slower (time constant 21 ps) than the equivalent step in the related reaction center (RC) found in purple bacteria ( 3 ps). In the case of the bacterial RC, replacement of a specific tyrosine residue within the M subunit (at position 210 in Rhodobacter sphaeroides), by a leucine residue slows down charge separation to 20 ps. Significantly the analogous residue in PS II, within the D2 polypeptide, is a leucine not a tyrosine (at position D2-205, Chlamydomonas reinhardtii numbering). Consequently, it has been postulated [Hastings et al. (1992) Biochemistry 31: 7638–7647] that the rate of electron transfer could be increased in PS II by replacing this leucine residue with tyrosine. We have tested this hypothesis by constructing the D2-Leu205Tyr mutant in the green alga, Chlamydomonas reinhardtii, through transformation of the chloroplast genome. Primary charge separation was examined in isolated PS II RCs by time-resolved optical spectroscopy and was found to occur with a time constant of 40 ps. We conclude that mutation of D2-Leu205 to Tyr does not increase the rate of charge separation in PS II. The slower kinetics of primary charge separation in wild type PS II are probably not due to a specific difference in primary structure compared with the bacterial RC but rather a consequence of the P680 singlet excited state being a shallower trap for excitation energy within the reaction center.     

X-ray crystal structure of the YM210W mutant reaction centre from Rhodobacter sphaeroides

The X-ray crystal structure of a reaction centre from Rhodobacter sphaeroides with a mutation of tyrosine M210 to tryptophan (YM210W) has been determined to a resolution of 2.5 A. Structural conservation is very good throughout the body of the protein, with the tryptophan side chain adopting a position in the mutant complex closely resembling that of the tyrosine in the wild-type complex. The spectroscopic properties of the YM210W reaction centre are discussed with reference to the structural data, with particular focus on evidence that the introduction of the bulkier tryptophan in place of the native tyrosine may cause a small tilt of the macrocycle of the B(L) monomeric bacteriochlorophyll.     

Primary processes of charge separation in reaction centers of YM210L/FM197Y and YM210L mutants of <Emphasis Type="Italic">Rhodobacter sphaeroides</Emphasis>

Difference femtosecond absorption spectroscopy with 20-fsec temporal resolution was applied to study a primary stage of charge separation and transfer processes in reaction centers of YM210L and YM210L/FM197Y site-directed mutants of the purple bacterium Rhodobacter sphaeroides at 90 K. Photoexcitation was tuned to the absorption band of the primary electron donor P at 880 nm. Coherent oscillations in the kinetics of stimulated emission of P* excited state at 940 nm and of anion absorption of monomeric bacteriochlorophyll B_A⁻ at 1020 nm were monitored. The absence of tyrosine YM210 in RCs of both mutants leads to strong slowing of the primary reaction P* → P⁺B_A⁻ and to the absence of stabilization of separated charges in the state P⁺B_A⁻. Mutation FM197Y increases effective mass of an acetyl group of pyrrole ring I in the bacteriochlorophyll molecule P_B of the double mutant YM210L/FM197Y by a hydrogen bond with OH-TyrM197 group that leads to a decrease in the frequency of coherent nuclear motions from 150 cm⁻¹ in the single mutant YM210L to ∼100 cm⁻¹ in the double mutant. Oscillations with 100–150 cm⁻¹ frequencies in the dynamics of the P* stimulated emission and in the kinetics of the reversible formation of P⁺B_A⁻ state of both mutants reflect a motion of the P_B molecule relatively to P_A in the area of mutual overlapping of their pyrrole rings I. In the double mutant YM210L/FM197Y the oscillations in the P* emission band and the B_A⁻ absorption band are conserved within a shorter time ∼0.5 psec (1.5 psec in the YM210L mutant), which may be a consequence of an increase in the number of nuclei forming a wave packet by adding a supplementary mass to the dimer P.     

Spectroscopic resolution of the picosecond reduction kinetics of the secondary electron acceptor A1 in photosystem I

Forward electron transfer in photosystem I from Synechocystis sp. PCC 6803 has been studied in the picosecond time range with transient absorption spectroscopy in the blue and near-UV spectral regions. From the direct measurement, at 380-390 nm, of the reduction kinetics of the phylloquinone secondary acceptor A1 and from the absence of spectral evolution between 100 ps and 2 ns, we conclude that electron transfer, from the chlorophyll a primary acceptor A0, to A1 occurs directly and completely with a time constant of about 30 ps.     

Spectral heterogeneity and time-resolved spectroscopy of excitation energy transfer in membranes of Heliobacillus mobilis at low temperatures.

Transient absorption difference spectra in the Qy absorption band from membranes of Heliobacillus mobilis were recorded at 140 and 20 K upon 200 fs laser pulse excitation at 590 nm. Excitation transfer from short wavelength absorbing forms of bacteriochlorophyll g to long wavelength bacteriochlorophyll g occurred within 1-2 ps at both long wavelength bacteriochlorophyll g occurred within 1-2 ps at both temperatures. In addition, a slower energy transfer process with a time constant of 15 ps was observed at 20 K within the pool of long wavelength-absorbing bacteriochlorophyll g. Energy transfer from long wavelength antenna pigments to the primary electron donor P798 was observed, yielding the primary charge-separated state P798+A0-. The time constant for this process was 30 ps at 140 K and about 70 ps at 20 K. A decay component with smaller amplitude and a lifetime of up to hundreds of picoseconds was observed that was centered around 814 nm at 20 K. Kinetic simulations using simple lattice models reproduce the observed decay kinetics at 295 and 140 K, but not at 20 K. The kinetics of energy redistribution within the spectrally heterogeneous antenna system at low temperature argue against a simple "funnel" model for the organization of the antenna of Heliobacillus mobilis and favor a more random spatial distribution of spectral forms. However, the relatively high rate of energy transfer from long wavelength antenna bacteriochlorophyll g to the primary electron donor P798 at low temperature is difficult to explain with either of these models.     

Mechanism of charge separation and their stabilization in bacterial reaction centers

The nuclear wavepacket formed by 20-fs excitation on the P* potential energy surface in native and mutant (YM210W and YM210L) reaction centers of Rhodobacter (Rb.) sphaeroides and Chloroflexus (C.) aurantiacus RCs was found to be reversibly transferred to the P+BA- surface at 120, 380, and 640-fs delays (monitored by measurements of BA- absorption at 1020-1028 nm). The reaction centers of YM210W(L) mutant show the most simple pattern of fs oscillations with a period of 230 fs in stimulated emission from P* and in the product P+BA-. The mechanisms of the electron transfer pathway between P* and BA and of the stabilization of the state P+BA- in bacterial reaction centers are discussed.     

On the efficiency of energy transfer and the different pathways of electron transfer in mutant reaction centers of Rhodobacter sphaeroides

The efficiency of energy transfer from the monomeric pigments to the primary donor was determined from 77 K steady-state fluorescence excitation spectra of three mutant reaction centers, YM210L, YM210F and LM160H / FM197H. For all three reaction centers this efficiency was not 100% and ranged between 55 and 70%. For the YM210L mutant it was shown using pump-probe spectroscopy with B band excitation at 798 nm that the excitations which are not transferred to P give rise to efficient charge separation. The results can be interpreted with a model in which excitation of the B absorbance band leads to direct formation of the radical pair state B_A ⁺H _A ^– in addition to energy transfer to P. It is also possible that some P⁺B_A ^– is formed from B^*. In previous publications we have demonstrated the operation of such alternative pathways for transmembrane electron transfer in a YM210W mutant reaction center [van Brederode et al. (1996) The Reaction center of Photosynthetic Bacteria, pp 225–238; (1997a,b) Chem Phys Lett 268: 143–149; Biochemistry 36: 6855–6861]. The results presented here demonstrate that these alternative mechanisms are not peculiar to the YM210W reaction center.     

Determination of the excitation migration time in Photosystem II consequences for the membrane organization and charge separation parameters

The fluorescence decay kinetics of Photosystem II (PSII) membranes from spinach with open reaction centers (RCs), were compared after exciting at 420 and 484 nm. These wavelengths lead to preferential excitation of chlorophyll (Chl) a and Chl b, respectively, which causes different initial excited-state populations in the inner and outer antenna system. The non-exponential fluorescence decay appears to be 4.3+/-1.8 ps slower upon 484 nm excitation for preparations that contain on average 2.45 LHCII (light-harvesting complex II) trimers per reaction center. Using a recently introduced coarse-grained model it can be concluded that the average migration time of an electronic excitation towards the RC contributes approximately 23% to the overall average trapping time. The migration time appears to be approximately two times faster than expected based on previous ultrafast transient absorption and fluorescence measurements. It is concluded that excitation energy transfer in PSII follows specific energy transfer pathways that require an optimized organization of the antenna complexes with respect to each other. Within the context of the coarse-grained model it can be calculated that the rate of primary charge separation of the RC is (5.5+/-0.4 ps)(-1), the rate of secondary charge separation is (137+/-5 ps)(-1) and the drop in free energy upon primary charge separation is 826+/-30 cm(-1). These parameters are in rather good agreement with recently published results on isolated core complexes [Y. Miloslavina, M. Szczepaniak, M.G. Muller, J. Sander, M. Nowaczyk, M. R?gner, A.R. Holzwarth, Charge separation kinetics in intact Photosystem II core particles is trap-limited. A picosecond fluorescence study, Biochemistry 45 (2006) 2436-2442].     

Electrochromic detection of a coherent component in the formation of the charge pair P(+)H(L)(-) in bacterial reaction centers

We demonstrate coupling of an intraprotein electron transfer reaction to coherent vibrational motions. The kinetics of charge separation toward the radical pair state P(+)H(L)(-) were studied in reaction centers of Rhodobacter sphaeroides at 15 K. The electrochromic shift of the bacteriochlorophyll monomers is the most prominent spectral feature associated with this charge displacement. The newly reported absolute absorption spectrum of the P(+)H(L)(-) state is discussed in terms of this shift. In wild-type reaction centers, the rise kinetics of the electrochromic shift display a small but significant 30 cm(-)(1) periodic modulation (period of approximately 1 ps). This modulation is also present in FL181Y mutant reaction centers, where overall charge separation is somewhat more rapid than in the wild-type reaction center. In contrast, in YM210L mutant reaction centers, where the charge separation is much slower, the modulation is absent. The conclusion that the motion along the reaction coordinate has a 30 cm(-)(1) coherent component is discussed in light of possible mechanisms of electron transfer.     

Is bicarbonate in Photosystem II the equivalent of the glutamate ligand to the iron atom in bacterial reaction centers?

Photosystem II of oxygen-evolving organisms exhibits a bicarbonate-reversible formate effect on electron transfer between the primary and secondary acceptor quinones, QA and QB. This effect is absent in the otherwise similar electron acceptor complex of purple bacteria, e.g., Rhodobacter sphaeroides. This distinction has led to the suggestion that the iron atom of the acceptor quinone complex in PS II might lack the fifth and sixth ligands provided in the bacterial reaction center (RC) by a glutamate residue at position 234 of the M-subunit in Rb. sphaeroides RCs (M232 in Rps. viridis). By site-directed mutagenesis we have altered GluM234 in RCs from Rb. sphaeroides, replacing it with valine, glutamine and glycine to form mutants M234EV, M234EQ and M234EG, respectively. These mutants grew competently under phototrophic conditions and were tested for the formate-bicarbonate effect. In chromatophores there were no detectable differences between wild type (Wt) and mutant M234EV with respect to cytochrome b-561 reduction following a flash, and no effect of bicarbonate depletion (by incubation with formate). In isolated RCs, several electron transfer activities were essentially unchanged in Wt and M234EV, M234EQ and M234EG mutants, and no formate-bicarbonate effect was observed on: (a) the fast or slow phases of recovery of the oxidized primary donor (P+) in the absence of exogenous donor, i.e., the recombination of P+Q-A or P+Q-B, respectively; (b) the kinetics of electron transfer from Q-A to QB; or (c) the flash dependent oscillations of semiquinone formation in the presence of donor to P+ (QB turnover). The absence of a formate-bicarbonate effect in these mutants suggests that GluM234 is not responsible for the absence of the formate-bicarbonate effect in Wt bacterial RCs, or at least that other factors must be taken into account. The mutant RCs were also examined for the fast primary electron transfer along the active (A-)branch of the pigment chain, leading to reduction of QA. The kinetics were resolved to reveal the reduction of the monomer bacteriochlorophyll (tau = 3.5 ps), followed by reduction of the bacteriopheophytin (tau = 0.9 ps). Both steps were essentially unaltered from the wild type. However, the rate of reduction of QA was slowed by a factor of 2 (tau = 410 +/- 30 and 47 +/- 30 ps for M234EQ and M234EV, respectively, compared to 220 ps in the wild type).(ABSTRACT TRUNCATED AT 400 WORDS)     

Energy and electron transfer in photosystem II reaction centers with modified pheophytin composition

Energy and electron transfer in Photosystem II reaction centers in which the photochemically inactive pheophytin had been replaced by 13(1)-deoxo-13(1)-hydroxy pheophytin were studied by femtosecond transient absorption-difference spectroscopy at 77 K and compared to the dynamics in untreated reaction center preparations. Spectral changes induced by 683-nm excitation were recorded both in the Q(Y) and in the Q(X) absorption regions. The data could be described by a biphasic charge separation. In untreated reaction centers the major component had a time constant of 3.1 ps and the minor component 33 ps. After exchange, time constants of 0.8 and 22 ps were observed. The acceleration of the fast phase is attributed in part to the redistribution of electronic transitions of the six central chlorin pigments induced by replacement of the inactive pheophytin. In the modified reaction centers, excitation of the lowest energy Q(Y) transition produces an excited state that appears to be localized mainly on the accessory chlorophyll in the active branch (B(A) in bacterial terms) and partially on the active pheophytin H(A). This state equilibrates in 0.8 ps with the radical pair. B(A) is proposed to act as the primary electron donor also in untreated reaction centers. The 22-ps (pheophytin-exchanged) or 33-ps (untreated) component may be due to equilibration with the secondary radical pair. Its acceleration by H(B) exchange is attributed to a faster reverse electron transfer from B(A) to. After exchange both and are nearly isoenergetic with the excited state.     

Primary light-energy conversion in tetrameric chlorophyll structure of photosystem II and bacterial reaction centers: I. A review

The purpose of the review is to show that the tetrameric (bacterio)chlorophyll ((B)Chl) structures in reaction centers of photosystem II (PSII) of green plants and in bacterial reaction centers (BRCs) are similar and play a key role in the primary charge separation. The Stark effect measurements on PSII reaction centers have revealed an increased dipole moment for the transition at approximately 730 nm (Frese et al., Biochemistry 42:9205-9213, 2003). It was found (Heber and Shuvalov, Photosynth Res 84:84-91, 2005) that two fluorescent bands at 685 and 720 nm are observed in different organisms. These two forms are registered in the action spectrum of Q(A) photoreduction. Similar results were obtained in core complexes of PSII at low temperature (Hughes et al., Biochim Biophys Acta 1757: 841-851, 2006). In all cases the far-red absorption and emission can be interpreted as indication of the state with charge transfer character in which the chlorophyll monomer plays a role of an electron donor. The role of bacteriochlorophyll monomers (B(A) and B(B)) in BRCs can be revealed by different mutations of axial ligand for Mg central atoms. RCs with substitution of histidine L153 by tyrosine or leucine and of histidine M182 by leucine (double mutant) are not stable in isolated state. They were studied in antennaless membrane by different kinds of spectroscopy including one with femtosecond time resolution. It was found that the single mutation (L153HY) was accompanied by disappearance of B(A) molecule absorption near 802 nm and by 14-fold decrease of photochemical activity measured with ms time resolution. The lifetime of P(870)* increased up to approximately 200 ps in agreement with very low rate of the electron transfer to A-branch. In the double mutant L153HY + M182HL, the B(A) appears to be lost and B(B) is replaced by bacteriopheophytin Phi(B) with the absence of any absorption near 800 nm. Femtosecond measurements have revealed the electron transfer to B-branch with a time constant of approximately 2 ps. These results are discussed in terms of obligatory role of B(A) and Phi(B) molecules located near P for efficient electron transfer from P*.     

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.     

Selective perturbation of the second electron transfer step in mutant bacterial reaction centers

In order to specifically perturb the primary electron acceptor B(A) -- a monomeric bacteriochlorophyll (BChl) a -- involved in bacterial photosynthetic charge separation (CS), the protein environment of B(A) in the reaction center (RC) of Rhodobacter sphaeroides was modified by site-directed mutagenesis. Isolated RCs were characterized by redox titrations, low temperature optical spectroscopy, ENDOR/TRIPLE resonance spectroscopy and femtosecond time-resolved spectroscopy. Two mutations were studied: In the GS(M203) mutant a serine is introduced near the ring E keto group of B(A), while in FY(L146) a phenylalanine near the ring A acetyl group of B(A) is replaced by tyrosine. In all mutations the oxidation potential of the primary electron donor P as well as the electronic structure of both the P(*+) radical cation and the radical anion of the secondary electron acceptor, H(A)(*-), are not significantly altered compared to the wild type (WT), while changes of the optical absorption spectra at 77 K in the BChl Q(X) and Q(Y) regions are observed. The GS(M203) mutation only leads to a minor retardation of the CS reactions at room temperature, whereas for FY(L146) significant deviations from the native electron transfer (ET) rates could be detected: In addition to a faster first (2.9 ps) and a slower second (1 ps) ET step, a new 8-ps time constant was found in the FY(L146) mutant, which can be ascribed to a fraction of RCs with slowed down secondary ET. The results allow us to address the functional role of the acetyl group of B(A) and question the role of the free energy changes as the main determining factor of ET rates in RCs. It is concluded that structural rearrangements alter the electronic coupling between the pigments and thereby influence the rate of fast CS.     

Manipulating the direction of electron transfer in the bacterial reaction center by swapping Phe for Tyr near BChl(M) (L181) and Tyr for Phe near BChl(L) (M208)

We have investigated the primary charge separation processes in Rb. capsulatus reaction centers (RCs) bearing the mutations Phe(L181) --> Tyr, Tyr(M208) --> Phe, and Leu(M212) --> His. In the YFH mutant, decay of the excited primary electron donor P occurs with an 11 +/- 2 ps time constant and is trifurcated to give (1) internal conversion to the ground state ( approximately 10% yield), (2) charge separation to the L side of the RC ( approximately 60% yield), and (3) electron transfer to the M-side bacteriopheophytin BPh(M) ( approximately 30% yield). These results relate previous work in which the ionizable residues Lys (at L178) and Asp (at M201) have been used to facilitate charge separation to the M side of the RC, and the widely studied L181 and M208 mutants. One conclusion that comes from this work is that the Tyr (M208) --> Phe and Gly(M201) --> Asp mutations near the L-side bacteriochlorophyll (BChl(L)) raise the free energy of P(+)BChl(L)(-) by comparable amounts. The results also suggest that the free energy of P(+)BChl(M)(-) is lowered more substantially by a Tyr at L181 than a Lys at L178. The results on the YFH mutant further demonstrate that the free energy differences between the L- and M-side charge-separated states play a significant role in the directionality of charge separation in the wild-type RC, and place limits on the contributing role of differential electronic matrix elements on the two sides of the RC.     

Dynamics of energy transfer and trapping in the light-harvesting antenna of Rhodopseudomonas viridis.

By low intensity picosecond absorption spectroscopy it is shown that the exciton lifetime in the light-harvesting antenna of Rhodopseudomonas (Rps.) viridis membranes with photochemically active reaction centers at room temperature is 60 +/- 10 ps. This lifetime reflects the overall trapping rate of the excitation energy by the reaction center. With photochemically inactive reaction centers, in the presence of P+, the exciton lifetime increases to 150 +/- 15 ps. Prereducing the secondary electron acceptor QA does not prevent primary charge separation, but slows it down from 60 to 90 +/- 10 ps. Picosecond kinetics measured at 77 K with inactive reaction centers indicates that the light-harvesting antenna is spectrally homogeneous. Picosecond absorption anisotropy measurements show that energy transfer between identical Bchlb molecules occurs on the subpicosecond time scale. Using these experimental results as input to a random-walk model, results in strict requirements for the antenna-RC coupling. The model analysis prescribes fast trapping (approximately 1 ps) and an approximately 0.5 escape probability from the reaction center, which requires a more tightly coupled RC and antenna, as compared with the Bchla-containing bacteria Rhodospirillum (R.) rubrum and Rhodobacter (Rb.) sphaeroides.     

Primary photosynthetic processes in Heliobacterium chlorum at 15K

Absorbance changes induced by 25-ps laser flashes were measured in membranes of Heliobacterium chlorum at 15 K. Absorbance difference spectra, measured at various times after the flash showed negative bands in the Q_y region at 812, 793 and 665 nm. The first of these bands was attributed to the formation of excited singlet states of a long-wavelength form of antenna bacteriochlorophyll g (BChl g 808). Absorbance changes of shorter wavelength absorbing antenna BChls g were at least an order of magnitude smaller, indicating rapid excitation energy transfer (i.e. within the time resolution of the apparatus) from these BChls to BChl g 808. Excited BChl g 808 showed a bi-exponential decay with time constants of 50 and 200 ps. The bands at 793 and 665 nm may be attributed to the primary charge separation and reflect the photooxidation of the primary electron donor P-798 and photoreduction of a primary electron acceptor absorbing near 670 nm, presumably a BChl c or Chl a-like pigment. The bleaching of this pigment reversed with a time constant of 300 ps at 15 K and of 800 ps at 300 K. This indicates that electron transfer from the primary to the secondary electron acceptor is approximately 2.5 times faster at 15 K than at room temperature.Abbreviations BChl bacteriochlorophyll - FWHM full width at half maximum - P-798 primary electron donor - Tris tris(hydroxymethyl)amino methane