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.     

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.     

Spectral inhomogeneity of photosystem I and its influence on excitation equilibration and trapping in the cyanobacterium Synechocystis sp. PCC6803 at 77 K.

Ultrafast transient absorption spectroscopy was used to probe excitation energy transfer and trapping at 77 K in the photosystem I (PSI) core antenna from the cyanobacterium Synechocystis sp. PCC 6803. Excitation of the bulk antenna at 670 and 680 nm induces a subpicosecond energy transfer process that populates the Chl a spectral form at 685--687 nm within few transfer steps (300--400 fs). On a picosecond time scale equilibration with the longest-wavelength absorbing pigments occurs within 4-6 ps, slightly slower than at room temperature. At low temperatures in the absence of uphill energy transfer the energy equilibration processes involve low-energy shifted chlorophyll spectral forms of the bulk antenna participating in a 30--50-ps process of photochemical trapping of the excitation by P(700). These spectral forms might originate from clustered pigments in the core antenna and coupled chlorophylls of the reaction center. Part of the excitation is trapped on a pool of the longest-wavelength absorbing pigments serving as deep traps at 77 K. Transient hole burning of the ground-state absorption of the PSI with excitation at 710 and 720 nm indicates heterogeneity of the red pigment absorption band with two broad homogeneous transitions at 708 nm and 714 nm (full-width at half-maximum (fwhm) approximately 200--300 cm(-1)). The origin of these two bands is attributed to the presence of two chlorophyll dimers, while the appearance of the early time bleaching bands at 683 nm and 678 nm under excitation into the red side of the absorption spectrum (>690 nm) can be explained by borrowing of the dipole strength by the ground-state absorption of the chlorophyll a monomers from the excited-state absorption of the dimeric red pigments.     

Light harvesting in photosystem I supercomplexes

In photosynthetic membranes of cyanobacteria, algae, and higher plants, photosystem I (PSI) mediates light-driven transmembrane electron transfer from plastocyanin or cytochrome c6 to the ferredoxin-NADP complex. The oxidoreductase function of PSI is sensitized by a reversible photooxidation of primary electron donor P700, which launches a multistep electron transfer via a series of redox cofactors of the reaction center (RC). The excitation energy for the functioning of the primary electron donor in the RC is delivered via the chlorophyll core antenna in the complex with peripheral light-harvesting antennas. Supermolecular complexes of the PSI acquire remarkably different structural forms of the peripheral light-harvesting antenna complexes, including distinct pigment types and organizational principles. The PSI core antenna, being the main functional unit of the supercomplexes, provides an increased functional connectivity in the chlorophyll antenna network due to dense pigment packing resulting in a fast spread of the excitation among the neighbors. Functional connectivity within the network as well as the spectral overlap of antenna pigments allows equilibration of the excitation energy in the depth of the whole membrane within picoseconds and loss-free delivery of the excitation to primary donor P700 within 20-40 ps. Low-light-adapted cyanobacteria under iron-deficiency conditions extend this capacity via assembly of efficiently energy coupled rings of CP43-like complexes around the PSI trimers. In green algae and higher plants, less efficient energy coupling in the eukaryotic PSI-LHCI supercomplexes is probably a result of the structural adaptation of the Chl a/b binding LHCI peripheral antenna that not only extends the absorption cross section of the PSI core but participates in regulation of excitation flows between the two photosystems as well as in photoprotection.     

Long-wavelength absorbing antenna pigments and heterogeneous absorption bands concentrate excitons and increase absorption cross section

The light-harvesting apparatus of photosynthetic organisms is highly optimized with respect to efficient collection of excitation energy from photons of different wavelengths and with respect to a high quantum yield of the primary photochemistry. In many cases the primary donor is not an energetic trap as it absorbs hypsochromically compared to the most red-shifted antenna pigment present (long-wavelength antenna). The possible reasons for this as well as for the spectral heterogeneity which is generally found in antenna systems is examined on a theoretical basis using the approach of thermal equilibration of the excitation energy. The calculations show that long-wavelength antenna pigments and heterogeneous absorption bands lead to a concentration of excitons and an increased effective absorption cross section. The theoretically predicted trapping times agree remarkably well with experimental data from several organisms. It is shown that the kinetics of the energy transfer from a long-wavelength antenna pigment to a hypsochromically absorbing primary donor does not represent a major kinetic limitation. The development of long-wavelength antenna and spectrally heterogeneous absorption bands means an evolutionary advantage based on the chromatic adaptation of photosynthetic organelles to spectrally filtered light caused by self-absorption.Abbreviations LHC light-harvesting complex - P primary donor - PSI Photosystem I of green plants - PS II Photosystem II of green plants - RC reaction center - X primary acceptor     

Excitation energy transfer in Photosystem I from oxygenic organisms

This Review discusses energy transfer pathways in Photosystem I (PS I) from oxygenic organisms. In the trimeric PS I core from cyanobacteria, the efficiency of solar energy conversion is largely determined by ultrafast excitation transfer processes in the core chlorophyll a (Chl a) antenna network and efficient photochemical trapping in the reaction center (RC). The role of clusters of Chl a in energy equilibration and photochemical trapping in the PS I core is discussed. Dimers of the longest-wavelength absorbing (red) pigments with strongest excitonic interactions localize the excitation in the PS I core antenna. Those dimers that are located closer to the RC participate in a fast energy equilibration with coupled pigments of the RC. This suggests that the function of the red pigments is to concentrate the excitation near the RC. In the PS I holocomplex from algae and higher plants, in addition to the red pigments of the core antenna, spectrally distinct red pigments are bound to the peripheral Chl a/b-binding light-harvesting antenna (LHC I), specifically to the Lhca4 subunit of the LHC I-730 complex. Intramonomeric energy equilibration between pools of Chl b and Chl a in Lhca1 and Lhca4 monomers of the LHC I-730 heterodimer are as fast as the energy equilibration processes within the PS I core. In contrast to the structural stability of the PS I core, the flexible subunit structure of the LHC I would probably determine the observed slow excitation energy equilibration processes in the range of tens of picoseconds. The red pigments in the LHC I are suggested to function largely as photoprotective excitation sinks in the peripheral antenna of PS I. This revised version was published online in August 2006 with corrections to the Cover Date.     

Energy transfer and charge separation kinetics in photosystem I: Part 1: Picosecond transient absorption and fluorescence study of cyanobacterial photosystem I particles

The energy transfer and charge separation kinetics of a photosystem I (PS I) core particle of an antenna size of 100 chlorophyll/P700 has been studied by combined fluorescence and transient absorption kinetics with picosecond resolution. This is the first combined picosecond study of transient absorption and fluorescence carried out on a PS I particle and the results are consistent with each other. The data were analyzed by both global lifetime and global target analysis procedures. In fluorescence major lifetime components were found to be 12 and 36 ps. The shorter-lived one shows a negative amplitude at long wavelengths and is attributed to an energy transfer process between pigments in the main antenna Chl pool and a small long-wavelength Chl pool emitting around 720 nm whereas the longer-lived component is assigned to the overall charge separation lifetime. The lifetimes resolved in transient absorption are 7-8 ps, 33 ps, and [unk]1 ns. The shortest-lived one is assigned to energy transfer between the same pigment pools as observed also in fluorescence kinetics, the middle component of 33 ps to the overall charge separation, and the long-lived component to the lifetime of the oxidized primary donor P700⁺. The transient absorption data indicate an even faster, but kinetically unresolved energy transfer component in the main Chl pool with a lifetime <3 ps. Several kinetic models were tested on both the fluorescence and the picosecond absorption data by global target analysis procedures. A model where the long-wave pigments are spatially and kinetically connected with the reaction center P700 is favored over a model where P700 is connected more closely with the main Chl pool. Our data show that the charge separation kinetics in these PS I particles is essentially trap limited. The relevance of our data with respect to other time-resolved studies on PS I core particles is discussed, in particular with respect to the nature and function of the long-wave pigments. From the transient absorption data we do not see any evidence for the occurrence of a reduced Chl primary electron acceptor, but we also can not exclude that possibility, provided that reoxidation of that acceptor should occur within a time <40 ps.     

Excitation transfer and trapping kinetics in plant photosystem I probed by two-dimensional electronic spectroscopy

Photosystem I is a robust and highly efficient biological solar engine. Its capacity to utilize virtually every absorbed photon’s energy in a photochemical reaction generates great interest in the kinetics and mechanisms of excitation energy transfer and charge separation. In this work, we have employed room-temperature coherent two-dimensional electronic spectroscopy and time-resolved fluorescence spectroscopy to follow exciton equilibration and excitation trapping in intact Photosystem I complexes as well as core complexes isolated from Pisum sativum. We performed two-dimensional electronic spectroscopy measurements with low excitation pulse energies to record excited-state kinetics free from singlet–singlet annihilation. Global lifetime analysis resolved energy transfer and trapping lifetimes closely matches the time-correlated single-photon counting data. Exciton energy equilibration in the core antenna occurred on a timescale of 0.5 ps. We further observed spectral equilibration component in the core complex with a 3–4 ps lifetime between the bulk Chl states and a state absorbing at 700 nm. Trapping in the core complex occurred with a 20 ps lifetime, which in the supercomplex split into two lifetimes, 16 ps and 67–75 ps. The experimental data could be modelled with two alternative models resulting in equally good fits—a transfer-to-trap-limited model and a trap-limited model. However, the former model is only possible if the 3–4 ps component is ascribed to equilibration with a “red” core antenna pool absorbing at 700 nm. Conversely, if these low-energy states are identified with the P₇₀₀ reaction centre, the transfer-to-trap-model is ruled out in favour of a trap-limited model.

     

Antenna structure and excitation dynamics in photosystem I. II. Studies with mutants of Chlamydomonas reinhardtii lacking photosystem II.

Using time-resolved single photon counting, fluorescence decay in photosystem I (PS I) was analyzed in mutant strains of Chlamydomonas reinhardtii that lack photosystem II. Two strains are compared: one with a wild-type PS I core antenna (120 chlorophyll a/P700) and a second showing an apparent reduction in core antenna size (60 chlorophyll a/P700). These data were calculated from the lifetimes of core antenna excited states (75 and 45 ps, respectively) and from pigment stoichiometries. Fluorescence decay in wild type PS I is composed of two components: a fast 75-ps decay that represents the photochemically limited lifetime of excited states in the core antenna, and a minor (less than 10%) 300-800 ps component that has spectral characteristics of both peripheral and core antenna pigments. Temporal and spectral properties of the fast PS I decay indicate that (a) excitations are nearly equilibrated among the range of spectral forms present in the PS I core antenna, (b) an average excitation visits a representative distribution of core antenna spectral forms on all pigment-binding subunits regardless of the origin of the excitation, (c) reduction in core antenna size does not alter the range of antenna spectral forms present, and (d) transfer from peripheral antennae to the PS I core complex is rapid (less than 5 ps).     

Difference picosecond spectroscopy of pigment-protein complexes of photosystem I from higher plants

Using a difference picosecond spectrophotometer with a time resolution of 10 ps, we investigated excitation energy transfer and charge separation in pigment-protein complexes of Photosystem I from bean leaves (chlorophyll/P-700 = 60). Under 20 ps excitation at 650 or 667 nm, the difference absorption spectra in the spectral region 600–720 nm were measured. They are associated with transition of antenna chlorophylls into singlet excited states and P-700 photooxidation. It was shown that the excited states in the whole inhomogeneous antenna were generated within 10 ps and deactivated with three-component kinetics, the t_1/e values being 20–45, 100–300 and over 500 ps. Formation of P-700⁺ has a rise time of 15–30 ps. The fast component of the depletion of the antenna excited states is suggested to be due to transfer of excitation energy from antenna pigments to reaction centers and its trapping. The kinetics of the fast component is independent of excitation energy and a redox state of P-700.     

Excited state dynamics in photosystem I: effects of detergent and excitation wavelength.

Femtosecond transient absorption spectroscopy has been used to investigate the energy transfer and trapping processes in both intact membranes and purified detergent-isolated particles from a photosystem II deletion mutant of the cyanobacterium Synechocystis sp. PCC 6803, which contains only the photosystem I reaction center. Processes with similar lifetimes and spectra are observed in both the membrane fragments and the detergent-isolated particles, suggesting little disruption of the core antenna resulting from the detergent treatment. For the detergent-isolated particles, three different excitation wavelengths were used to excite different distributions of pigments in the spectrally heterogeneous core antenna. Only two lifetimes of 2.7-4.3 ps and 24-28 ps, and a nondecaying component are required to describe all the data. The 24-28 ps component is associated with trapping. The trapping process gives rise to a nondecaying spectrum that is due to oxidation of the primary electron donor. The lifetimes and spectra associated with trapping and radical pair formation are independent of excitation wavelength, suggesting that trapping proceeds from an equilibrated excited state. The 2.7-4.3 ps component characterizes the evolution from the initially excited distribution of pigments to the equilibrated excited state distribution. The spectrum associated with the 2.7-4.3 ps component is therefore strongly excitation wavelength dependent. Comparison of the difference spectra associated with the spectrally equilibrated state and the radical pair state suggests that the pigments in the photosystem I core antenna display some degree of excitonic coupling.     

Spectral and kinetic analysis of the energy coupling in the PS I-LHC I supercomplex from the green alga Chlamydomonas reinhardtii at 77 K

Energy transfer processes in the chlorophyll antenna of the PS I-LHCI supercomplexes from the green alga Chlamydomonas reinhardtii have been studied at 77 K using transient absorption spectroscopy with multicolor excitation in the 640-670 nm region. Comparison of the kinetic data obtained at low and room temperatures indicates that the slow approximately approximately 100 ps excitation equilibration phase that is characteristic of energy coupling of the LHCI peripheral antenna to the PS I core at physiological temperatures (Melkozernov AN, Kargul J, Lin S, Barber J and Blankenship RE (2004) J Phys Chem B 108: 10547-10555) is not observed in the excitation dynamics of the PS I-LHCI supercomplex at 77 K. This suggests that at low temperatures the peripheral antenna is energetically uncoupled from the PS I core antenna. Under these conditions the observed kinetic phases on the time scales from subpicoseconds to tens of picoseconds represent the superposition of the processes occurring independently in the PS I core antenna and the Chl a/b containing LHCI antenna. In the PS I-LHCI supercomplex with two uncoupled antennas the excitation is channeled to the excitation sinks formed at low temperature by clusters of red pigments. A better spectral resolution of the transient absorption spectra at 77 K results in detection of two DeltaA bands originating from the rise of photobleaching on the picosecond time scale of two clearly distinguished pools of low energy absorbing Chls in the PS I-LHCI supercomplex. The first pool of low energy pigments absorbing at 687 nm is likely to originate from the red pigments in the LHCI where the Lhca1 protein is most abundant. The second pool at 697 nm is suggested to result either from the structural interaction of the LHCI and the PS I core or from other Lhca proteins in the antenna. The kinetic data are discussed based on recent structural models of the PS I-LHCI. It is proposed that the uncoupling of pigment pools may be a control mechanism that regulates energy flow in Photosystem I.     

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.     

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.     

Spectral and Kinetic Analysis of the Energy Coupling in the PS I–LHC I Supercomplex from the Green Alga Chlamydomonas reinhardtii at 77 K

Energy transfer processes in the chlorophyll antenna of the PS I–LHCI supercomplexes from the green alga Chlamydomonas reinhardtii have been studied at 77 K using transient absorption spectroscopy with multicolor excitation in the 640–670 nm region. Comparison of the kinetic data obtained at low and room temperatures indicates that the slow ∼ ∼100 ps excitation equilibration phase that is characteristic of energy coupling of the LHCI peripheral antenna to the PS I core at physiological temperatures (Melkozernov AN, Kargul J, Lin S, Barber J and Blankenship RE (2004) J Phys Chem B 108: 10547–10555) is not observed in the excitation dynamics of the PS I–LHCI supercomplex at 77 K. This suggests that at low temperatures the peripheral antenna is energetically uncoupled from the PS I core antenna. Under these conditions the observed kinetic phases on the time scales from subpicoseconds to tens of picoseconds represent the superposition of the processes occurring independently in the PS I core antenna and the Chl a/b containing LHCI antenna. In the PS I–LHCI supercomplex with two uncoupled antennas the excitation is channeled to the excitation sinks formed at low temperature by clusters of red pigments. A better spectral resolution of the transient absorption spectra at 77 K results in detection of two ΔA bands originating from the rise of photobleaching on the picosecond time scale of two clearly distinguished pools of low energy absorbing Chls in the PS I–LHCI supercomplex. The first pool of low energy pigments absorbing at 687 nm is likely to originate from the red pigments in the LHCI where the Lhca1 protein is most abundant. The second pool at 697 nm is suggested to result either from the structural interaction of the LHCI and the PS I core or from other Lhca proteins in the antenna. The kinetic data are discussed based on recent structural models of the PS I–LHCI. It is proposed that the uncoupling of pigment pools may be a control mechanism that regulates energy flow in Photosystem I.     

Kinetics and spectra of photo-induced changes in the absorption of pigment-protein complexes of photosystem 1 in a picosecond range

The energy transfer from the light-harvesting antenna chlorophylls to the reaction center molecules and subsequent charge separation were investigated using a difference picosecond spectrophotometer with selective excitation. The objects were the pigment-protein complexes of photosystem 1 (Chl/P700 = 60) isolated from bean leaves. The difference absorption spectra of the excited states of light-harvesting antenna chlorophylls and the P700 photooxidation were measured. It was shown that the excited states of antenna chlorophylls were generated within 10 ps and deactivated with three-component kinetics: tau 1 = 20--45 ps, tau 2 = 100--300 ps, tau 3 greater than 500 ps. The process of the P700 photooxidation induced by the 650 nm exciting pulse was approximately monoexponential with tau equal to 15--30 ps. It is established that the P700 photooxidation is due to the efficient transfer of excitation energy from antenna chlorophylls to reaction centers.     

Ultrafast primary processes in photosystem I of the cyanobacterium Synechocystis sp. PCC 6803.

Ultrafast primary processes in the trimeric photosystem I core antenna-reaction center complex of the cyanobacterium Synechocystis sp. PCC 6803 have been examined in pump-probe experiments with approximately 100 fs resolution. A global analysis of two-color profiles, excited at 660 nm and probed at 5 nm intervals from 650 to 730 nm, reveals 430 fs kinetics for spectral equilibration among bulk antenna chlorophylls. At least two lifetime components (2.0 and 6.5 ps in our analysis) are required to describe equilibration of bulk chlorophylls with far red-absorbing chlorophylls (>700 nm). Trapping at P700 occurs with 24-ps kinetics. The multiphasic bulk left arrow over right arrow red equilibration kinetics are intriguing, because prior steady-state spectral studies have suggested that the core antenna in Synechocystis sp. contains only one red-absorbing chlorophyll species (C708). The disperse kinetics may arise from inhomogeneous broadening in C708. The one-color optical anisotropy at 680 nm (near the red edge of the bulk antenna) decays with 590 fs kinetics; the corresponding anisotropy at 710 nm shows approximately 3.1 ps kinetics. The latter may signal equilibration among symmetry-equivalent red chlorophylls, bound to different monomers within trimeric photosystem I.     

Pathways of energy flow through the light-harvesting antenna of the photosynthetic purple bacterium rhodobacter sphaeroides

Using low intensity picosecond absorption spectroscopy with independently tunable excitation and probing infrared pulses, we have studied the pathways of energy transport through the light-harvesting antenna pigments of the photosynthetic purple bacterium Rhodobacter sphaeroides. From the observed excited-state rise time of the red-most pigment B896 as a function of excitation wavelength it is concluded that the B850 pigment of LH2 is spectrally heterogeneous. For excitations originating in the B850 pigment this results in a fast channel (9 ps) that is mainly excited in the peak of the B850 absorption band, and a slow channel (35 ps) that is predominantly excited at ~840 nm. Upon excitation of B800, more than 90% of the excitations follow the fast path. From the observed kinetics it is concluded that the majority of the LH2 → LH1 energy transfer takes place within at most a few picoseconds. The rate-limiting step in the whole energy transfer sequence appears to be the B896 → reaction center transfer. The origin of the B850 heterogeneity and the slow 35-ps component is at the moment unclear. Possibly it represents a highly extended form of LH2 in which transfer to LH1 takes a relatively long time, due to a large number of transfer steps.     

Energy transfer processes in the isolated core antenna complexes CP43 and CP47 of photosystem II

The energy equilibration and transfer processes in the isolated core antenna complexes CP43 and CP47 of photosystem II have been studied by steady-state and ultrafast (femto- to nanosecond) time-resolved spectroscopy at room temperature. The annihilation-free femtosecond absorption data can be described by surprisingly simple sequential kinetic models, in which the excitation energy transfer between blue and red states in both antenna complexes is dominated by sub-picosecond processes and is completed in less than 2 ps. The slowest energy transfer steps with lifetimes in the range of 1-2 ps are assigned to transfer steps between the chlorophyll layers located on the stromal and lumenal sides. We conclude that these ultrafast intra-antenna energy transfer steps do not represent a bottleneck in the rate of the primary processes in intact photosystem II. Since the experimental energy equilibration rates are up to a factor of 3-5 higher than concluded previously, our results challenge the conclusions drawn from theoretical modeling.     

Energy transfer in the inhomogeneously broadened core antenna of purple bacteria: a simultaneous fit of low-intensity picosecond absorption and fluorescence kinetics.

The excited state decay kinetics of chromatophores of the purple photosynthetic bacterium Rhodospirillum rubrum have been recorded at 77 K using picosecond absorption difference spectroscopy under strict annihilation free conditions. The kinetics are shown to be strongly detection wavelength dependent. A simultaneous kinetic modeling of these experiments together with earlier fluorescence kinetics by numerical integration of the appropriate master equation is performed. This model, which accounts for the spectral inhomogeneity of the core light-harvesting antenna of photosynthetic purple bacteria, reveals three qualitatively distinct stages of excitation transfer with different time scales. At first a fast transfer to a local energy minimum takes place (approximately 1 ps). This is followed by a much slower transfer between different energy minima (10-30 ps). The third component corresponds to the excitation transfer to the reaction center, which depends on its state (60 and 200 ps for open and closed, respectively) and seems also to be the bottleneck in the overall trapping time. An acceptable correspondence between theoretical and experimental decay kinetics is achieved at 77 K and at room temperature by assuming that the width of the inhomogeneous broadening is 10-15 nm and the mean residence time of the excitation in the antenna lattice site is 2-3 ps.