Primary charge separation within P870* in wild type and heterodimer mutants in femtosecond time domain

Primary charge separation dynamics in the reaction center (RC) of purple bacterium Rhodobacter sphaeroides and its P870 heterodimer mutants have been studied using femtosecond time-resolved spectroscopy with 20 and 40fs excitation at 870nm at 293K. Absorbance increase in the 1060-1130nm region that is presumably attributed to P(A)(δ+) cation radical molecule as a part of mixed state with a charge transfer character P*(P(A)(δ+)P(B)(δ-)) was found. This state appears at 120-180fs time delay in the wild type RC and even faster in H(L173)L and H(M202)L heterodimer mutants and precedes electron transfer (ET) to B(A) bacteriochlorophyll with absorption band at 1020nm in WT. The formation of the P(A)(δ+)B(A)(δ-) state is a result of the electron transfer from P*(P(A)(δ+)P(B)(δ-)) to the primary electron acceptor B(A) (still mixed with P*) with the apparent time delay of ~1.1ps. Next step of ET is accompanied by the 3-ps appearance of bacteriopheophytin a(-) (H(A)(-)) band at 960nm. The study of the wave packet formation upon 20-fs illumination has shown that the vibration energy of the wave packet promotes reversible overcoming of an energy barrier between two potential energy surfaces P* and P*(P(A)(δ+)B(A)(δ-)) at ~500fs. For longer excitation pulses (40fs) this promotion is absent and tunneling through an energy barrier takes about 3ps. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

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*.     

Excitation Dynamics in Low Band Gap Donor–Acceptor Copolymers and Blends

Donor–acceptor (D–A) type copolymers show great potential for the application in the active layer of organic solar cells. Nevertheless the nature of the excited states, the coupling mechanism and the relaxation pathways following photoexcitation are yet to be clarified. We carried out comparative measurements of the steady state absorption and photoluminescence (PL) on the copolymer poly[N‐(1‐octylnonyl)‐2,7‐carbazole]‐alt‐5,5‐[4′,7′‐di(thien‐2‐yl)‐2′,1′,3′‐benzothiadiazole] (PCDTBT), its building blocks as well as on the newly synthesized N‐(1‐octylnonyl)‐2,7‐bis‐[(5‐phenyl)thien‐2‐yl)carbazole (BPT‐carbazole). The high‐energy absorption band (HEB) of PCDTBT was identified with absorption of carbazoles with adjacent thiophene rings while the low‐energy band (LEB) originates instead from the charge transfer (CT) state delocalized over the aforementioned unit with adjacent benzothiadiazole group. Photoexcitation of the HEB is followed by internal relaxation prior the radiative decay to the ground state. Adding PC₇₀BM results in the efficient PL quenching within the first 50 ps after excitation. From the PL excitation experiments no evidence for a direct electron transfer from the HEB of PCDTBT towards the fullerene acceptor was found, therefore the internal relaxation mechanisms within PCDTBT can be assumed to precede. Our findings indicate that effective coupling between copolymer building blocks governs the photovoltaic performance of the blends.     

Cofactors involved in light-driven charge separation in photosystem I identified by subpicosecond infrared spectroscopy

Photosystem I is one of the key players in the conversion of solar energy into chemical energy. While the chlorophyll dimer P(700) has long been identified as the primary electron donor, the components involved in the primary charge separation process in PSI remain undetermined. Here, we have studied the charge separation dynamics in Photosystem I trimers from Synechococcus elongatus by femtosecond vis-pump/mid-infrared-probe spectroscopy upon excitation at 700, 710, and 715 nm. Because of the high specificity of the infrared region for the redox state and small differences in the molecular structure of pigments, we were able to clearly identify specific marker bands indicating chlorophyll (Chl) oxidation. Magnitudes of chlorophyll cation signals are observed to increase faster than the time resolution of the experiment (~0.2 ps) upon both excitation conditions: 700 nm and selective red excitation. Two models, involving either ultrafast charge separation or charge transfer character of the red pigments in PSI, are discussed to explain this observation. A further increase in the magnitudes of cation signals on a subpicosecond time scale (0.8-1 ps) indicates the formation of the primary radical pair. Evolution in the cation region with time constants of 7 and 40 ps reveals the formation of the secondary radical pair, involving a secondary electron donor. Modeling of the data allows us to extract the spectra of the two radical pairs, which have IR signatures consistent with A+A?- and P???+A?-. We conclude that the cofactor chlorophyll A acts as the primary donor in PSI. The existence of an equilibrium between the two radical pairs we interpret as concerted hole/electron transfer between the pairs of electron donors and acceptors, until after 40 ps, relaxation leads to a full population of the P???+A?. radical pair.     

Femtosecond energy-transfer processes in the B800-850 light-harvesting complex of Rhodobacter sphaeroides 2.4.1

The B800-to-B850 energy transfer time in the purified B800-850 light-harvesting complex of Rhodobacter sphaeroides 2.4.1 is determined to be 0.7 ps at room temperature. The electronic state dynamics of the principal carotenoid of this species, spheroidene, are examined, both in vivo and in vitro, by direct femtosecond time-resolved experiments and by fluorescence emission yield studies. Evidence is presented which suggests that carotenoid-to-bacteriochlorophyll energy transfer may occur directly from the initially excited carotenoid S2 state, as well as from the carotenoid S1 state. Further support for this conjecture is obtained from calculations of energy transfer rates from the carotenoid S2 state. Previous measurements of in vivo carotenoid and B800 dynamics are discussed in light of the new results, and currently unresolved issues are described.     

Theoretical investigation of thiophene-linked porphyrin-perylene photosensitiser for bulk-heterojunction solar cells

In this work, quantum chemical calculations were applied to investigate a new series of porphyrin-perylene conjugates for their potential use as light-harvesting compounds in bulk-heterojunction solar cells. Molecular design relies on the integration of a electron-donating porphyrin and a electron-accepting perylene units via a spacer containing 0 to 10 thiophene rings. Electronic properties of these push–pull systems were studied using density functional theory (DFT) and time-dependent DFT approaches. Results show that introduction of thiophene-based spacer with different number of the thiophene rings significantly affects the electronic and absorption properties of the molecules. According to its suitable energy gap, efficient charge transfer and appropriate absorption behaviour determined by the calculation, the derivative having a terthiophene linker should be as the optimal compound among all molecules studied herein.     

Femtosecond charge separation in organized assemblies: free-radical reactions with pyridine nucleotides in micelles

Femtosecond laser UV pulse-induced charge separation and electron transfer across a polar interface have been investigated in anionic aqueous micells (sodium lauryl sulfate) containing an aromatic hydrocarbon (phenothiazine). The early events of the photoejection of the electron from the micellized chromophore and subsequent reaction of electron with the aqueous perimicellar phase have been studied by ultrafast infrared and visible absorption spectroscopy. The charge separation (chromophore +...e-) inside the micelle occurs in less than 10(-13) s (100 fs). The subsequent thermalization and localization of the photoelectron in the aqueous phase are reached in 250 fs. This results in the appearance of an infrared band assigned to a nonrelaxed solvated electron (presolvated state). This transient species relaxes toward the fully solvated state of the electron in 270 fs. In anionic aqueous micelles containing pyridine dinucleotides at high concentration (0.025-0.103 M), a single electron transfer can be initiated by femtosecond photoionization of phenothiazine. The one-electron reduction of the oxidized pyridine dinucleotide leads to the formation of a free pyridinyl radical. The bimolecular rate constant of this electron transfer depends on both the pH of the micellar system and the concentration of oxidized acceptor. The free-radical reaction is analyzed in terms of the time dependence of a diffusion-controlled process. In the first 2 ps following the femtosecond photoionization of PTH inside the micelle, an early formation of a free pyridinyl radical is observed. This suggests that an ultrafast free-radical reaction with an oxidized form of pyridine nucleotide can be triggered by a single electron transfer in less than 5 X 10(11) s-1.     

Energy transfer in the major intrinsic light-harvesting complex from Amphidinium carterae

Carbonyl carotenoids are important constituents of the antenna complexes of marine organisms. These carotenoids possess an excited state with a charge-transfer character (intramolecular charge transfer state, ICT), but many details of the carotenoid to chlorophyll energy transfer mechanisms are as yet poorly understood. Here, we employ femtosecond transient absorption spectroscopy to study energy transfer pathways in the intrinsic light-harvesting complex (LHC) of dinoflagellates, which contains the carbonyl carotenoid peridinin. Carotenoid to chlorophyll energy transfer efficiency is about 90% in the 530-550 nm region, where the peridinin S2 state transfers energy with an efficiency of 25-50%. The rest proceeds via the S1/ICT channel, and the major S1/ICT-mediated energy transfer pathway utilizes the relaxed S1/ICT state and occurs with a time constant of 2.6 ps. Below 525 nm, the overall energy transfer efficiency drops because of light absorption by another carotenoid, diadinoxanthin, that contributes only marginally to energy transfer. Instead, its role is likely to be photoprotection. In addition to the peridinin-Chl-a energy transfer, it was shown that energy transfer also occurs between the two chlorophyll species in LHC, Chl-c2, and Chl-a. The time constant characterizing the Chl-c2 to Chl-a energy transfer is 1.4 ps. The results demonstrate that the properties of the S1/ICT state specific for carbonyl carotenoids is the key to ensure the effective harvesting of photons in the 500-600 nm region, which is of vital importance to underwater organisms.     

Carotenoid-to-chlorophyll energy transfer in recombinant major light-harvesting complex (LHCII) of higher plants. I. Femtosecond transient absorption measurements

The energy transfer kinetics from carotenoids to chlorophylls and among chlorophylls has been measured by femtosecond transient absorption kinetics in a monomeric unit of the major light-harvesting complex (LHCII) from higher plants. The samples were reconstituted complexes with different carotenoid contents. The kinetics was measured both in the carotenoid absorption region and in the chlorophyll Q(y) region using two different excitation wavelengths suitable for selective excitation of the carotenoids. Analysis of the data shows that the overwhelming part of the energy transfer from the carotenoids occurs directly from the initially excited S(2) state of the carotenoids. Only a small part (<20%) may possibly take an S(1) pathway. All the S(2) energy transfer from carotenoids to chlorophylls occurs with time constants <100 fs. We have been able to differentiate among the three carotenoids, two luteins and neoxanthin, which have transfer times of approximately 50 and 75 fs for the two luteins, and approximately 90 fs for neoxanthin. About 50% of the energy absorbed by carotenoids is initially transferred directly to chlorophyll b (Chl b), while the rest is transferred to Chl a. Neoxanthin almost exclusively transfers to Chl b. Due to various complex effects discussed in the paper, such as a specific coupling of Chl b and Chl a excited states, the percentage of direct Chl b transfer thus is somewhat lower than estimated by us previously for LHCII from Arabidopsis thaliana. (Connelly, J. P., M. G. Müller, R. Bassi, R. Croce, and A. R. Holzwarth. 1997. Biochemistry. 36:281). We can distinguish three different Chls b receiving energy directly from carotenoids. We propose as a new mechanism that the carotenoid-to-Chl b transfer occurs to a large part via the B(x) state of Chl b and to the Q(x) state, while the transfer to Chl a occurs only via the Q(x) state. We find no compelling evidence in favor of a substantial S(1) transfer path of the carotenoids, although some transfer via the S(1) state of neoxanthin can not be entirely excluded. The S(1) lifetimes of the two luteins were determined to be 15 ps and 3.9 ps. A detailed quantitative analysis and kinetic model of the processes described here will be presented in a separate paper.     

First evidence that cytochrome P450 may catalyze both S-oxidation and epoxidation of thiophene derivatives

Oxidation of 2-phenylthiophene (2PT) by rat liver microsomes, in the presence of NADPH and glutathione (GSH), led to three kinds of metabolites whose structures were established by 1H NMR and mass spectrometry. The first ones were 2PT-S-oxide dimers formed by Diels-Alder type dimerization of 2PT-S-oxide, while the second ones were GSH adducts derived from the 1,4-Micha?l-type addition of GSH to 2PT-S-oxide. The third metabolites were GSH adducts resulting from a nucleophilic attack of GSH to the 4,5-epoxide of 2PT. Oxidation of 2PT by recombinant, human cytochrome P4501A1, in the presence of NADPH and GSH, also led to these three kinds of metabolites. These results provide the first evidence that cytochrome P450 may catalyze the oxidation of thiophene compounds with the simultaneous formation of two reactive intermediates, a thiophene-S-oxide and a thiophene epoxide.     

Excitation dynamics and heterogeneity of energy equilibration in the core antenna of photosystem I from the cyanobacterium Synechocystis sp. PCC 6803

Energy equilibration in the photosystem I core antenna from the cyanobacterium Synechocystis sp. PCC 6803 was studied using femtosecond transient absorption spectroscopy at 298 K. The photosystem I core particles were excited at 660, 693, and 710 nm with 150 fs spectrally narrow laser pulses (fwhm = 5 nm). Global analysis revealed three kinetic processes in the core antenna with lifetimes of 250-500 fs, 1.5-2.5 ps, and 20-30 ps. The first two components represent strongly excitation wavelength-dependent energy equilibration processes while the 20-30 ps phase reflects the trapping of energy by the reaction center. Excitation into the blue and red edge of the absorption band induces downhill and uphill energy flows, respectively, between different chlorophyll a spectral forms of the core. Excitation at 660 nm induces a 500 fs downhill equilibration process within the bulk of antenna while the selective excitation of long-wavelength-absorbing chlorophylls at 710 nm results in a 380 fs uphill energy transfer to the chlorophylls absorbing around 695-700 nm, presumably reaction center pigments. The 1.5-2.5 ps phases of downhill and uphill energy transfer are largely equivalent but opposite in direction, indicating energy equilibration between bulk antenna chlorophylls at 685 nm and spectral forms absorbing below 700 nm. Transient absorption spectra with excitation at 693 nm exhibit spectral evolution within approximately 2 ps of uphill energy transfer to major spectral forms at 680 nm and downhill energy transfer to red pigments at 705 nm. The 20-30 ps trapping component and P(700) photooxidation spectra derived from data on the 100 ps scale are largely excitation wavelength independent. An additional decay component of red pigments at 710 nm can be induced either by selective excitation of red pigments or by decreasing the temperature to 264 K. This component may represent one of the phases of energy transfer from inhomogeneously broadened red pigments to P(700). The data are discussed based on the available structural model of the photosystem I reaction center and its core antenna.     

Photoinduced electron-transfer in perylenediimide triphenylamine-based dendrimers: single photon timing and femtosecond transient absorption spectroscopy.

The excited state dynamics of two generations perylenediimide chromophores substituted in the bay area with dendritic branches bearing triphenylamine units as well as those of the respective reference compounds are investigated. Using single photon timing and multi-pulse femtosecond transient absorption experiments a direct proof of a reversible charge transfer occurring from the peripheral triphenylamine to the electron acceptor perylenediimide core is revealed. Femtosecond pump-dump-probe experiments provide evidence for the ground state dynamics by populating excited vibronic levels. It is found by the means of both techniques that the rotational isomerization of the dendritic branches occurs on a time scale that ranges up to 1 ns. This time scale of the isomerization depends on the size of the dendritic arms and is similar both in the ground and excited state.     

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.     

Energy Transfer among Chlorophylls in Trimeric Light-harvesting ComplexⅡ of Bryopsis corticulans

A study on energy transfer among chlorophylls(Chls)in the trimeric unit of the major light-harvesting complex Ⅱ(LHC Ⅱ)from Bryopsis corriculan,was carried out using time-correlated singlephoton counting.In the chlorophyll Q region of LHC Ⅱ,six molecules characterized as Chlb_(628),Chlb_(646),Chlb_(652)~(654,657),Chla_(664)~(666),Chla_(674)~(677.680)and Chla_(682)~(683) were discriminated according to their absorption spectrumand fluorescence emission spectrum.Then,excited by pulsed light of 628 nm,fluorescence kinetics spectrain the chlorophyll Q region were measured.In accordance with the principles of fluorescence kinetics,thesekinetics data were analyzed with a multi-exponential model.Time constants on energy transfer were obtained.An overwhelming percentage of energy transfer among chlorophylls undergoes a process longer than 97picoseconds(ps),which shows that,before transferring energy to another Chl,the excited Chl might convertenergy to vibrations of a lower state with different multiplicity(intersystem crossing).Energy transfer at thelevel of approximately 10 ps was also obtained,which was interpreted as the excited Chls may go throughinternal conversion before transferring energy to another Chl.Although with a higher standard deviation,timeconstants at the femtosecond level can not be entirely excluded,which can be attributed to the ultrafastprocess of direct energy transfer.Owing to the arrangement and direction of the dipole moment of Chls inLHC Ⅱ,the probability of these processes is different.The fluorescence lifetimes of Chlb_(652)~(654,657),Chla_(664)~(666),Chla_(674)~(677.680)and Chla_(682)~(683)were determined to be 1.44ns,1.43 ns,636 ps and 713 ps,respectively.Thepercentages of energy dissipation in the pathway of fluorescence emission were no more than 40% in thetrimeric unit of LHC Ⅱ.These results are important for a better understanding of the relationship between thestructure and function of LHC Ⅱ.     

Primary light-energy conversion in tetrameric chlorophyll structure of photosystem II and bacterial reaction centers: II. Femto- and picosecond charge separation in PSII D1/D2/Cyt b559 complex

In Part I of the article, a review of recent data on electron-transfer reactions in photosystem II (PSII) and bacterial reaction center (RC) has been presented. In Part II, transient absorption difference spectroscopy with 20-fs resolution was applied to study the primary charge separation in PSII RC (DI/DII/Cyt b 559 complex) excited at 700 nm at 278 K. It was shown that the initial electron-transfer reaction occurs within 0.9 ps with the formation of the charge-separated state P680(+)Chl(D1)(-), which relaxed within 14 ps as indicated by reversible bleaching of 670-nm band that was tentatively assigned to the Chl(D1) absorption. The subsequent electron transfer from Chl(D1)(-) within 14 ps was accompanied by a development of the radical anion band of Pheo(D1) at 445 nm, attributable to the formation of the secondary radical pair P680(+)Pheo(D1)(-). The key point of this model is that the most blue Q(y) transition of Chl(D1) in RC is allowing an effective stabilization of separated charges. Although an alternative mechanism of charge separation with Chl(D1)* as a primary electron donor and Pheo(D1) as a primary acceptor can not be ruled out, it is less consistent with the kinetics and spectra of absorbance changes induced in the PSII RC preparation by femtosecond excitation at 700 nm.     

Picosecond dynamics of primary electron-transfer processes in bacterial photosynthesis.

The primary electron transfer processes in Rhodopseudomonas sphaeroides R-26 were studied as a function of temperature by means of picosecond spectroscopy. The first chemical step of the bacterial photosynthesis involves an electron transfer from the excited state of a bacteriochlorophyll a dimer, (BChl)2, to a bacteriopheophytin (BPh) to form the radical ion pair (BChl)2+. BPh-.. The upper limit for the formation time of this ion-pair was found to be 10 ps, at temperatures in the range 300-4.2 degree K. Similarly, the second chemical step, involving electron transfer from BPh-. to an ubiquinone-iron complex (QFe), was found to have a lifetime of approximately 150 ps, also independent of temperature in the same range. We interpret the absence of temperature dependence as indicating that process 2 proceeds via a tunneling mechanism. Utilizing our results in conjunction with electron tunneling theories, we calculate the distance between BPh-. and Q(Fe) to be 9--13 A. Our results also imply a closer proximity between (BChl)2 and BPh.     

Time-resolved absorption and emission show that the CP43' antenna ring of iron-stressed synechocystis sp. PCC6803 is efficiently coupled to the photosystem I reaction center core

Excitation energy transfer and trapping processes in an iron stress-induced supercomplex of photosystem I from the cyanobacterium Synechocystis sp. PCC6803 were studied by time-resolved absorption and fluorescence spectroscopy on femtosecond and picosecond time scales. The data provide evidence that the energy transfer dynamics of the CP43'-PSI supercomplex are consistent with energy transfer processes that occur in the Chl a network of the PSI trimer antenna. The most significant absorbance changes in the CP43'-PSI supercomplex are observed within the first several picoseconds after the excitation into the spectral region of CP43' absorption (665 nm). The difference time-resolved spectra (DeltaDeltaA) resulting from subtraction of the PSI trimer kinetic data from the CP43'-PSI supercomplex data indicate three energy transfer processes with time constants of 0.2, 1.7, and 10 ps. The 0.2 ps kinetic phase is tentatively interpreted as arising from energy transfer processes originating within or between the CP43' complexes. The 1.7 ps phase is interpreted as possibly arising from energy transfer from the CP43' ring to the PSI trimer via closely located clusters of Chl a in CP43' and the PSI core, while the slower 10 ps process might reflect the overall excitation transfer from the CP43' ring to the PSI trimer. These three fast kinetic phases are followed by a 40 ps overall excitation decay in the supercomplex, in contrast to a 25 ps overall decay observed in the trimer complex without CP43'. Excitation of Chl a in both the CP43'-PSI antenna supercomplex and the PSI trimer completely decays within 100 ps, resulting in the formation of P700(+). The data indicate that there is a rapid and efficient energy transfer between the outer antenna ring and the PSI reaction center complex.     

Ultrafast primary processes in PS I from Synechocystis sp. PCC 6803: roles of P700 and A(0)

The excitation transport and trapping kinetics of core antenna-reaction center complexes from photosystem I of wild-type Synechocystis sp. PCC 6803 were investigated under annihilation-free conditions in complexes with open and closed reaction centers. For closed reaction centers, the long-component decay-associated spectrum (DAS) from global analysis of absorption difference spectra excited at 660 nm is essentially flat (maximum amplitude <10(-5) absorbance units). For open reaction centers, the long-time spectrum (which exhibits photobleaching maxima at approximately 680 and 700 nm, and an absorbance feature near 690 nm) resembles one previously attributed to (P700(+) - P700). For photosystem I complexes excited at 660 nm with open reaction centers, the equilibration between the bulk antenna and far-red chlorophylls absorbing at wavelengths >700 nm is well described by a single DAS component with lifetime 2.3 ps. For closed reaction centers, two DAS components (2.0 and 6.5 ps) are required to fit the kinetics. The overall trapping time at P700 ( approximately 24 ps) is very nearly the same in either case. Our results support a scenario in which the time constant for the P700 --> A(0) electron transfer is 9-10 ps, whereas the kinetics of the subsequent A(0) --> A(1) electron transfer are still unknown.