Picosecond measurements of the primary photochemical events in reaction centers isolated from the facultative green photosynthetic bacterium Chloroflexus aurantiacus: Comparison with the purple bacterium Rhodopseudomonas sphaeroides

SHKUROPATOV ANATOLI, SHUVALOV VLADIMIRA. WAVE PACKET MOTIONS COUPLED TO ELECTRON TRANSFER IN REACTION CENTERS OF CHLOROFLEXUS AURANTIACUS

Transient absorption difference spectroscopy with ~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 BB binding site of accessory bacteriochlorophyll by bacteriopheophytin molecule (ΦB). 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 ~20 fs excitation leads to a coherent formation of the states [Formula: see text] and [Formula: see text] (BA 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 (ΦB absorption bands), and at 1,020–1028 nm ([Formula: see text] 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 ΦB absorption band at 785 nm due to [Formula: see text] formation. These data are consistent with recent measurements for the mutant HM182L Rb. sphaeroides RCs (Yakovlev et al., Biochim Biophys Acta1757:369–379, 2006). Only at a delay of 120 fs was the electron transfer from P* to the A-branch observed with a development of the [Formula: see text] 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 ΦB and BA primary acceptors in the B-branch and A-branch, respectively.

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.

Femtosecond Spectroscopy of the Primary Charge Separation in Reaction Centers of Chloroflexus aurantiacus with Selective Excitation in the QY and Soret Bands

The primary charge separation and electron-transfer processes of photosynthesis occur in the reaction center (RC). Isolated RCs of the green filamentous anoxygenic phototrophic bacterium Chloroflexus aurantiacus were studied at room temperature by using femtosecond transient absorption spectroscopy with selective excitation. Upon excitation in the Q(Y) absorbance band of the bacteriochlorophyll (BChl) dimer (P) at 865 nm, a 7.0 +/- 0.5 ps kinetic component was observed in the 538 nm region (Q(X) band of the bacteriopheophytin (BPheo)), 750 nm region (Q(Y) band of the BPheo), and 920 nm region (stimulated emission of the excited-state of P), indicating that this lifetime represents electron transfer from P to BPheo. The same time constant was also observed upon 740 nm or 800 nm excitation. A longer lifetime (300 +/- 30 ps), which was assigned to the time of reduction of the primary quinone, Q(A), was also observed. The transient absorption spectra and kinetics all indicate that only one electron-transfer branch is involved in primary charge separation under these excitation conditions. However, the transient absorption changes upon excitation in the Soret band at 390 nm reveal a more complex set of energy and electron-transfer processes. By comparison to studies on the RCs of the purple bacterium Rhodobacter sphaeroides, we discuss the possible mechanism of electron-transfer pathway dependence on excitation energy and propose a model of the Cf. aurantiacus RC that better explains the observed results.

Structure and protein binding interactions of the primary donor of the Chloroflexus aurantiacus reaction center

Soret resonance, QX resonance, and QY near-infrared Fourier transform (FT) (pre)resonance Raman spectroscopies were used to determine pigment-protein interactions of specific bacteriochlorin molecules in the reaction center from Chloroflexus aurantiacus. FT Raman spectroscopy, using 1064 nm excitation, was used to selectively obtain preresonance and resonance vibrational Raman spectra of the primary donor (P) of reaction centers (RCs) from Chloroflexus aurantiacus in the Po and P.+ states, respectively. The FT Raman spectrum of RCs in their neutral P (Po) state exhibits bands at 1605, 1632, 1648, and 1696 cm-1 which are attributable to P in its resting neutral state. Specifically, the latter three Raman bands can be assigned to the conjugated C2 acetyl and C9 keto carbonyl groups of the bacteriochlorophyll (BChl) molecules constituting P. The observation of at least three such bands is indicative of a non-monomeric nature of P, consistent with the proposal that it is a dimer of BChl molecules. The 1632 cm-1 band is consistent only with a hydrogen bonded BChl acetyl carbonyl, while the 1648 cm-1 band is assigned to a non-hydrogen bonded acetyl carbonyl. The 1696 cm-1 band is consistent only with a non-hydrogen bonded keto carbonyl group; from the unusually high intensity of this latter band compared to the others, we propose that the 1696 cm-1 band contains contributions from two keto carbonyl groups, both free of hydrogen bonds. From published protein sequence alignments of the L and M subunits of Rhodobacter (Rb.) sphaeroides and Chloroflexus aurantiacus we assign the 1632 cm-1 band as arising from the C2 acetyl carbonyl of the analogous PM constituent of P, which is hydrogen bonded to tyrosine M187 in the Chloroflexus RC, and propose a pigment-protein structural model for the primary donor of Chloroflexus aurantiacus. The FT Raman spectrum of RCs in the P degrees+ state indicates that one component of the 1696 cm-1 band has upshifted 21 cm-1 to 1717 cm-1. Compared to Rb. sphaeroides which showed a 26 cm-1 upshift for the corresponding band, the 21 cm-1 upshift indicates that the + charge is more delocalized over the P.+ species of Chloroflexus; we estimate that ca. 65% of the + charge is localized on one of the two BChl molecules of the Chloroflexus primary donor as compared to ca. 80% for Rb. sphaeroides. The consequences of the proposed structure of the Chloroflexus primary donor in terms of its Po/P.+ redox midpoint potential are discussed.

Temperature dependence of charge recombination from the P+QA- and P+QB- states in photosynthetic reaction centers isolated from the thermophilic bacterium Chloroflexus aurantiacus

The temperature dependence of charge recombination from the P+QA- and from the P+QB- states produced by a flash was studied in reaction centers isolated from the photosynthetic thermophilic bacterium Chloroflexus aurantiacus. P designates the primary electron donor; QA and QB the primary and secondary quinone electron acceptors respectively. In QB-depleted reaction centers the rate constant (kAP) for P+QA- recombination was temperature independent between 0-50 degrees C (17.6 +/- 0.7 s-1 at pH 8 and pH 10). The same value was obtained in intact membranes in the presence of o-phenanthroline. Upon lowering the temperature from 250 K to 160 K, kAP increased by a factor of two and remained constant down to 80 K. The overall temperature dependence of kAP was consistent with an activationless process. Ubiquinone (UQ-3) and different types of menaquinone were used for QB reconstitution. In UQ-3 reconstituted reaction centers charge recombination was monoexponential (rate constant k = 0.18 +/- 0.03 s-1) and temperature independent between 5-40 degrees C. In contrast, in menaquinone-3- and menaquinone-4-reconstituted reaction centers P+ rereduction following a flash was markedly biphasic and temperature dependent. In menaquinone-6-reconstituted reaction centers a minor contribution from a third kinetic phase corresponding to P+QA- charge recombination was detected. Analysis of these kinetics and of the effects of the inhibitor o-phenanthroline at high temperature suggest that in detergent suspensions of menaquinone-reconstituted reaction centers a redox reaction removing electrons from the quinone acceptor complex competes with charge recombination. Instability of the semiquinone anions is more pronounced when QB is a short-chain menaquinone. From the temperature dependence of P+ decay the activation parameters for the P+QB- recombination and for the competing side oxidation of the reduced menaquinone acceptor have been derived. For both reactions the activation enthalpies and entropies change markedly with menaquinone chain length but counterbalance each other, resulting in activation free energies at ambient temperature independent of the menaquinone tail. When reaction centers are incorporated into phospholipid vesicles containing menaquinone-8 a temperature-dependent, monophasic, o-phenanthroline-sensitive recombination from the P+QB- state is observed, which is consistent with the formation of stable semiquinone anions. This result seems to indicate a proper QB functioning in the two-subunit reaction center isolated from Chlorflexus aurantiacus when the complex is inserted into a lipid bilayer.

Applications of ultrafast laser spectroscopy for the study of biological systems

The discovery of mode-locked laser operation now nearly two decades ago has started a development which enables researchers to probe the dynamics of ultrafast physical and chemical processes at the molecular level on shorter and shorter time scales. Naturally the first applications were in the fields of photophysics and photochemistry where it was then possible for the first time to probe electronic and vibrational relaxation processes on a sub-nanosecond timescale. The development went from lasers producing pulses of many picoseconds to the shortest pulses which are at present just a few femtoseconds long. Soon after their discovery ultrashort pulses were applied also to biological systems which has revealed a wealth of information contributing to our understanding of a broadrange of biological processes on the molecular level.It is the aim of this review to discuss the recent advances and point out some future trends in the study of ultrafast processes in biological systems using laser techniques. The emphasis will be mainly on new results obtained during the last 5 or 6 years. The term ultrafast means that I shall restrict myself to sub-nanosecond processes with a few exceptions.

The photochemical reaction center of Chloroflexus aurantiacus is composed of two structurally similar polypeptides

A method has been devised which allowed the isolation of highly purified reaction center from the thermophilic green bacterium, Chloroflexus aurantiacus. The procedure consisted of three chromatography steps. The final step was fast protein liquid chromatography on Mono Q in the presence of nonanoyl-N-methylglucamide (Mega-9). The purified reaction center complex was photochemically active and had an A280/A813 of 1.4 or less. Under non-denaturing conditions, a pigmented protein band having a Mr of 52,000-55,000 was observed in sodium dodecyl sulfate gels. When the isolated complex was heat-dissociated in the presence of sodium dodecyl sulfate, just two polypeptides having very similar Mr (24,000 and 24,500) were observed. Two protein bands were also observed in two-dimensional isoelectric focusing/sodium-dodecyl-sulfate polyacrylamide gel electrophoresis; the PI values of the two polypeptides were 6.5 and 6.7. Partial peptide mapping of the two isolated subunits, using both enzymatic and chemical cleavage techniques, yielded almost identical patterns which indicated a high degree of sequence homology between the two polypeptides. The N-terminal amino acid sequences of the two polypeptides were identical and did not exhibit any homology to reaction center subunits of purple sulfur bacteria. The Chloroflexus reaction center is believed to be composed of one molecule of each polypeptide, the photoactive bacteriochlorophyll a dimer and, as accessory pigments, an additional bacteriochlorophyll a and three bacteriopheophytins. Hence, it appears to be the smallest photochemically active reaction center isolated to date.

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.

Electron donors and acceptors in photosynthetic reaction centers

A review is given of primary and associated electron transport reactions in various division of photosynthetic bacteria and in the two photosystems of plant photosynthesis. Two types of electron acceptor chains are distinguished: type 'Q', found in purple bacteria, Chloroflexus and system II of oxygenic photosynthesis and type 'F', found in green sulfur bacteria, Heliobacterium and photosystem I. Secondary donor reactions are discussed in relation to plant photosystem II.

Electron transport in green photosynthetic bacteria

Green bacteria make up two of the four families of anoxygenic photosynthetic prokaryotes. The two families have similar pigment compositions and membrane fine structure, and both contain a specialized antenna structure known as a chlorosome. The primary photochemistry and electron transport pathways of the two groups are, however, quite distinct. The anaerobic green bacteria (Chlorobiaceae) contain low-potential iron-sulfur proteins as early electron acceptors and can directly reduce NAD(+) in a manner reminiscent of Photosystem I of oxygenic organisms. The facultatively aerobic green bacteria (Chloroflexaceae) contain quinone-type acceptors and have an overall pattern of electron transport very similar to that found in purple bacteria. Many aspects of energy storage in green bacteria, especially photophosphorylation and the role of cytochrome b/c complexes in electron transport, remain poorly understood.

Primary photochemistry in the facultative green photosynthetic bacterium Chloroflexus aurantiacus

The mechanism of primary photochemistry has been investigated in purified cytoplasmic membranes and isolated reaction centers of Chloroflexus aurantiacus. Redox titrations on the cytoplasmic membranes indicate that the midpoint redox potential of P870, the primary electron donor bacteriochlorophyll, is +362 mV. An early electron acceptor, presumably menaquinone has Em 8.1 = -50 mV, and a tightly bound photooxidizable cytochrome c554 has Em 8.1 = +245 mV. The isolated reaction center has a bacteriochlorophyll to bacteriopheophytin ratio of 0.94:1. A two-quinone acceptor system is present, and is inhibited by o-phenanthroline. Picosecond transient absorption and kinetic measurements indicate the bacteriopheophytin and bacteriochlorophyll form an earlier electron acceptor complex.