Proximal Antenna Research Articles

Two types of sensory proliferation patterns underlie the formation of spatially tuned olfactory receptive fields in the cockroach Periplaneta americana.

In the cockroach Periplaneta americana, to represent pheromone source in the receptive space, axon terminals of sex pheromone-receptive olfactory sensory neurons (pSNs) are topographically organized within the primary center, the macroglomerulus, according to the peripheral locations of sex pheromone-receptive single walled (sw)-B sensilla. In this study, we sought to determine when and where pSNs emerge in the nymphal antenna. We revealed two different pSN proliferation patterns that underlie the formation of topographic organization in the macroglomerulus. In nymphal antennae, which lack sw-B sensilla, pSNs are identified in the shorter sensilla, termed sw-A sensilla. Because new sw-A sensilla emerge on the proximal antenna at every molt, topographic organization in the macroglomerulus must be formed by adding axon terminals of newly emerged pSNs to the lateral region in the macroglomerulus at each molt. At the final molt, a huge number of new sw-B sensilla appeared throughout the whole antenna. Sw-B sensilla in the proximal part of the adult antenna were newly formed during the last instar stage, whereas those located in the distal antenna were transformed from sw-A sensilla. This transformation was accompanied by an increase in the number of pSNs. Axon terminals of newborn pSNs in new sw-B sensilla were recruited to the lateral part of the macroglomerulus, whereas those of newborn pSNs in transformed sw-B sensilla were recruited to the macroglomerulus according to the sensillar location. These mechanisms enable an increase in sensitivity to sex pheromone in adulthood while retaining the topographic map formed during the postembryonic development.

The lowest-energy chlorophyll of photosystem II is adjacent to the peripheral antenna: Emitting states of CP47 assigned via circularly polarized luminescence

The identification of low-energy chlorophyll pigments in photosystem II (PSII) is critical to our understanding of the kinetics and mechanism of this important enzyme. We report parallel circular dichroism (CD) and circularly polarized luminescence (CPL) measurements at liquid helium temperatures of the proximal antenna protein CP47. This assembly hosts the lowest-energy chlorophylls in PSII, responsible for the well-known “F695” fluorescence band of thylakoids and PSII core complexes. Our new spectra enable a clear identification of the lowest-energy exciton state of CP47. This state exhibits a small but measurable excitonic delocalization, as predicated by its CD and CPL. Using structure-based simulations incorporating the new spectra, we propose a revised set of site energies for the 16 chlorophylls of CP47. The significant difference from previous analyses is that the lowest-energy pigment is assigned as Chl 612 (alternately numbered Chl 11). The new assignment is readily reconciled with the large number of experimental observations in the literature, while the most common previous assignment for the lowest energy pigment, Chl 627(29), is shown to be inconsistent with CD and CPL results. Chl 612(11) is near the peripheral light-harvesting system in higher plants, in a lumen-exposed region of the thylakoid membrane. The low-energy pigment is also near a recently proposed binding site of the PsbS protein. This result consequently has significant implications for our understanding of the kinetics and regulation of energy transfer in PSII.

Circularly polarized luminescence spectroscopy reveals low-energy excited states and dynamic localization of vibronic transitions in CP43

Circularly polarized luminescence (CPL) spectroscopy is an established but relatively little-used technique that monitors the chirality of an emission. When applied to photosynthetic pigment assemblies, we find that CPL provides sensitive and detailed information on low-energy exciton states, reflecting the interactions, site energies and geometries of interacting pigments. CPL is the emission analog of circular dichroism (CD) and thus spectra explore the optical activity only of fluorescent states of the pigment-protein complex and consequently the nature of the lowest-energy excited states (trap states), whose study is a critical area of photosynthesis research. In this work, we develop the new approach of temperature-dependent CPL spectroscopy, over the 2–120K temperature range, and apply it to the CP43 proximal antenna protein of photosystem II. Our results confirm strong excitonic interactions for at least one of the two well-established emitting states of CP43 named “A” and “B”. Previous structure-based models of CP43 spectra are evaluated in the light of the new CPL data. Our analysis supports the assignments of Shibata et al. [Shibata et al. J. Am. Chem. Soc. 135 (2013) 6903–6914], particularly for the highly-delocalized B-state. This state dominates CPL spectra and is attributed predominantly to chlorophyll a's labeled Chl 634 and Chl 636 (alternatively labeled Chl 43 and 45 by Shibata et al.). The absence of any CPL intensity in intramolecular vibrational sidebands associated with the delocalized “B” excited state is attributed to the dynamic localization of intramolecular vibronic transitions.

Coarse topographic organization of pheromone-sensitive afferents from different antennal surfaces in the American cockroach

In contrast to visual, auditory, taste, and mechanosensory neuropils, in which sensory afferents are topographically organized on the basis of their peripheral soma locations, axons of cognate sensory neurons from different locations of the olfactory sense organ converge onto a small spherical neuropil (glomerulus) in the first-order olfactory center. In the cockroach Periplaneta americana, sex pheromone-sensitive afferents with somata in the antero-dorsal and postero-ventral surfaces of a long whip-like antenna are biased toward the anterior and posterior regions of a macroglomerulus, respectively. In each region, afferents with somata in the more proximal antenna project to more proximal region, relative to the axonal entry points. However, precise topography of afferents in the macroglomerulus has remained unknown. Using single and multiple neuronal stainings, we showed that afferents arising from anterior, dorsal, ventral and posterior surfaces of the proximal regions of an antenna were biased progressively from the anterior to posterior region of the macroglomerulus, reflecting chiasmatic axonal re-arrangements that occur immediately before entering the antennal lobe. Morphologies of individual afferents originating from the proximal antenna matched results of mass neuronal stainings, but their three-dimensional origins in the antenna were hardly predictable on the basis of the projection patterns. Such projection biases made by neuronal populations differ from strict somatotopic projections of antennal mechanosensory neurons in the same species, suggesting a unique sensory mechanism to process information about odor location and direction on a single antenna.

A functional genetic analysis in flour beetles (Tenebrionidae) reveals an antennal identity specification mechanism active during metamorphosis in Holometabola

The antenna was the first arthropod ventral appendage to evolve non-leg identity. Models of antennal evolution have been based on comparisons of antennal and leg identity specification mechanisms in Drosophila melanogaster, a species in which appendages develop from highly derived imaginal discs during the larval period. We test for conservation of the Drosophila antennal identity specification mechanism at metamorphosis in Tribolium castaneum and three other flour beetle species (Tribolium confusum, Tribolium brevicornis and Latheticus oryzae) in the family Tenebrionidae. In Drosophila, loss of function of four transcription factors—homothorax, extradenticle, Distal-less, and spineless—causes large-scale transformations of the antenna to leg identity. Distal-less and spineless function similarly during metamorphosis in T. castaneum. RNA interference (RNAi) targeting homothorax (hth) or extradenticle (exd) caused transformation of the proximal antenna to distal leg identity in flour beetles, but did not affect the identity of the distal antenna. This differs from the functional domain of these genes in early instar Drosophila, where they are required for identity specification throughout the antenna, but matches their functional domain in late instar Drosophila. The similarities between antennal identity specification at metamorphosis in flour beetles and in late larval Drosophila likely reflect the conservation of an ancestral metamorphic developmental mechanism. There were two notable differences in hth/exd loss of function phenotypes between flies and beetles. Flour beetles retained all of their primary segments in both the antenna and legs, whereas flies undergo reduction and fusion of primary segments. This difference in ground state appendage morphology casts doubt on interpretations of developmental ground states as evolutionary atavisms. Additionally, adult Tribolium eyes were transformed to elytron-like structures; we provide a developmental hypothesis for this evolutionarily surprising transformation.

The CP43 Proximal Antenna Complex of Higher Plant Photosystem II Revisited: Modeling and Hole Burning Study. I

The CP43 core antenna complex of photosystem II is known to possess two quasi-degenerate "red"-trap states (Jankowiak, R. et al. J. Phys. Chem. B 2000, 104, 11805). It has been suggested recently (Zazubovich, V.; Jankowiak, R. J. Lumin. 2007, 127, 245) that the site distribution functions of the red states (A and B) are uncorrelated and that narrow holes are burned in the subpopulations of chlorophylls (Chls) from states A and B that are the lowest-energy Chl in their complex and previously thought not to transfer energy. This model of uncorrelated excitation energy transfer (EET) between the quasidegenerate bands is expanded by taking into account both electron-phonon and vibrational coupling. The model is applied to fit simultaneously absorption, emission, zero-phonon action, and transient hole burned (HB) spectra obtained for the CP43 complex with minimized contribution from aggregation. It is demonstrated that the above listed spectra can be well-fitted using the uncorrelated EET model, providing strong evidence for the existence of efficient energy transfer between the two lowest energy states, A and B (either from A to B or from B to A), in CP43. Possible candidate Chls for the low-energy A and B states are discussed, providing a link between CP43 structure and spectroscopy. Finally, we propose that persistent holes originate from regular NPHB accompanied by the redistribution of oscillator strength due to excitonic interactions, rather than photoconversion involving Chl-protein hydrogen bonding, as suggested before ( Hughes J. L. et al. Biochemistry 2006, 45, 12345 ). In the accompanying paper (Reppert, M.; Zazubovich, V.; Dang, N. C.; Seibert, M.; Jankowiak, R. J. Phys. Chem. B 2008, 9934), it is demonstrated that the model discussed in this manuscript is consistent with excitonic calculations, which also provide very good fits to both transient and persistent HB spectra obtained under non-line-narrowing conditions.

Photophysical Behavior and Assignment of the Low-Energy Chlorophyll States in the CP43 Proximal Antenna Protein of Higher Plant Photosystem II

We have employed absorption, circular dichroism (CD), and persistent spectral hole-burning measurements at 1.7 K to study the photoconversion properties and exciton coupling of low-energy chlorophylls (Chls) in the CP43 proximal antenna light-harvesting subunit of photosystem II (PSII) isolated from spinach. These approximately 683 nm states act as traps for excitation energy in isolated CP43. They "bleach" at 683 nm upon illumination and photoconvert to a form absorbing in the range approximately 660-680 nm. We present new data that show the changes in the CD spectrum due to the photoconversion process. These changes occur in parallel with those in absorption, providing evidence that the feature undergoing the apparent bleach is a component of a weakly exciton-coupled system. From our photoconversion difference spectra, we assign four states in the Chl long-wavelength region of CP43, two of which are the known trap states and are both highly localized on single Chls. The other two states are associated with weak exciton coupling (maximally approximately 50 cm(-)(1)) to one of these traps. We propose a mechanism for photoconversion that involves Chl-protein hydrogen bonding. New hole-burning data are presented that indicate this mechanism is distinct to that for narrow-band spectral hole burning in CP43. We discuss the photophysical behavior of the Chl trap states in isolated CP43 compared to their behavior in intact PSII preparations. The latter represent a more intact, physiological complex, and we find no clear evidence that they exhibit the photoconversion process reported here.

Substitution of a Chlorophyll into the Inactive Branch Pheophytin-Binding Site Impairs Charge Separation in Photosystem II

All photosynthetic reaction centers (RCs) have two parallel sets of electron transfer cofactors that cross the membrane. In quinone-type RCs (including photosystem II (PSII)), however, only one pathway (the active branch) is used for electron transfer. Since the electron transfer cofactors of each pathway have nearly identical distance and orientation relationships, it is assumed that local differences in protein environment determine the directionality of electron transfer. To understand further the factors that affect energy distribution among the PSII RC cofactors, we altered the PSII RC cofactor symmetry by replacing the inactive-branch pheophytin (Pheo) with a chlorophyll (Chl). We mutated the D1-L210 residue to a histidine (D1-L210H) to provide a Mg ligand for Chl. Analyses of the pigment composition of D1-L210H RCs indicated that the inactive-branch Pheo had been replaced by a Chl. Comparisons of wild-type and D1-L210 transient absorption spectra confirmed that the red-shifted Pheo Qx absorption band (543.5 nm) belonged to the active-branch Pheo. Surprisingly, intact D1-L210H PSII complexes were unable to evolve oxygen, lacked Chl variable fluorescence, (following a flash), and were unable to photoaccumulate reduced Qa, indicating that electron transfer in D1-L210H PSII complexes was severely perturbed. The kinetics of primary charge separation, however, were not substantially altered in D1-L210H RCs, indicating that the Chl substitution had not perturbed the energetics of the primary electron donor/acceptor pair. Significantly, intact D1-L210H PSII core complexes had a substantially increased and red-shifted Chl fluorescence emission band attributed to fluorescence from Chl's of the distal antenna complex as well as a blue-shifted fluorescence emission peak attributed to Chl's of the proximal antenna complex (77 K). These results are interpreted in terms of a redistribution of the excited-state energy among the pigments of the RC multimer, leading to loss of the excited state via fluorescence in the D1-L210H mutant.

Isolation of CP43 and CP47 photosystem II proximal antenna complexes from plants.

A single-column method to purify the CP43 and CP47 pigment-protein complexes of photo-system (PS)II from higher plants is presented. To validate the isolation procedure, three different species were used (Spinacea oleracea, Beta vulgaris, and Glycine max), and the procedure worked similarly with all three. Oxygen-evolving core complex obtained from highly enriched PSII membrane fragments were used as the starting material. The core complex is treated with the chaotropic agent LiClO4 and the nonionic detergent n-dodecyl beta-D-maltoside. After dialysis against buffer with no detergent or chaotropic agent, the solubilized material is separated by weak anion-exchange chromatography using a TSK-GEL Toyopearl DEAE 650s column. CP43 complex does not bind to the column and elutes with the first pigmented fractions. When the eluate becomes colorless, the column is subjected to a 0-175 mM LiClO4 linear gradient. The main pigment elution band corresponds to CP47 complex. The last pigmented elution band contains both reaction center-CP47 and reaction center complexes.

Association of His117 in the D2 protein of photosystem II with a chlorophyll that affects excitation-energy transfer efficiency to the reaction center.

His117 of the D2 protein of photosystem II (PS II) is a conserved residue in the second transmembrane region of the protein and has been suggested to bind chlorophyll. Nine site-directed mutations were introduced at residue 117, using both photosystem I (PS I)-containing and PS I-less background strains of the cyanobacterium Synechocystis sp. PCC 6803. Of these nine, four (H117C, H117M, H117N, and H117T) were photoautotrophic in the PS I-containing background. The other mutants (H117F, H117L, H117P, H117R, and H117Y) did not accumulate appreciable amounts of PS II in their thylakoids. The type of residues that can functionally replace His117 support the notion of His117 serving as a chlorophyll ligand. The properties of the H117N and H117T mutants were characterized in more detail. Whereas the properties of the H117N mutant were close to those of wild type, in the H117T mutant the 77-K fluorescence emission spectrum shows a much smaller amplitude at 695 nm than expected on the basis of the amount of PS II that is present. Moreover, in H117T, the amount of light needed to half-saturate O2-evolution rates was twofold higher than in the control strain, and the variable fluorescence yield was quenched. However, O2 evolution rates at saturating light intensity and electron-transport kinetics were normal in the mutant. Also, the radical accessory chlorophyll (Chlz+) formed by donation of an electron to the PS-II reaction center could be generated normally by illumination at low temperature in the H117T mutant. We conclude that the chlorophyll associated with residue 117 of the D2 protein is important for efficient excitation transfer between the proximal antenna and the PS II reaction center. A possible mechanism involving a chlorophyll cation to explain the quenching in the H117T mutant is discussed.

Langmuir-Blodgett and X-ray diffraction studies of isolated photosystem II reaction centers in monolayers and multilayers: physical dimensions of the complex.

The photosystem II (PSII) reaction center (RC) is a hydrophobic intrinsic protein complex that drives the water-oxidation process of photosynthesis. Unlike the bacterial RC complex, an X-ray crystal structure of the PSII RC is not available. In order to determine the physical dimensions of the isolated PSII RC complex, we applied Langmuir techniques to determine the cross-sectional area of an isolated RC in a condensed monolayer film. Low-angle X-ray diffraction results obtained by examining Langmuir-Blodgett multilayer films of alternating PSII RC/Cd stearate monolayers were used to determine the length (or height; z-direction, perpendicular to the plane of the original membrane) of the complex. The values obtained for a PSII RC monomer were 26 nm2 and 4.8 nm, respectively, and the structural integrity of the RC in the multilayer film was confirmed by several approaches. Assuming a cylindrical-type RC structure, the above dimensions lead to a predicted volume of about 125 nm3. This value is very close to the expected volume of 118 nm3, calculated from the known molecular weight and partial specific volume of the PSII RC proteins. This same type of comparison was also made with the Rhodobacter sphaeroides RC based on published data, and we conclude that the PSII RC is much shorter in length and has a more regular solid geometric structure than the bacterial RC. Furthermore, the above dimensions of the PSII RC and those of PSII core (RC plus proximal antenna) proteins protruding outside the plane of the PSII membrane into the lumenal space as imaged by scanning tunneling microscopy (Seibert, Aust. J. Pl. Physiol. 22, 161-166, 1995) fit easily into the known dimensions of the PSII core complex visualized by others as electron-density projection maps. From this we conclude that the in situ PSII core complex is a dimeric structure containing two copies of the PSII RC.

Pigment content of D1-D2-cytochrome b559 reaction center preparations after removal of CP47 contamination: an immunological study.

Isolated D1-D2-cytochrome b559 photosystem II reaction center preparations with pigment stoichiometry higher than 4 chlorophylls per 2 pheophytins can be contaminated with CP47 proximal antenna complex. Reaction center prepared by a modification of the Nanba-Satoh procedure and containing about 6 chlorophylls per 2 pheophytins showed immuno-cross-reactivity when probed with a monoclonal antibody raised against the CP47 polypeptide. Furthermore, they could be fractionated successfully by Superose-12 sieve chromatography into two different populations. The first few fractions off the column contained a more definitive 435 nm shoulder corresponding to increased chlorophyll content, and showed strong immuno-cross-reactivity with the CP47 antibody. The peak fractions off the column displayed a less prominent 435 nm shoulder, and did not cross-react with the antibody. Moreover, when a 6-chlorophyll preparation was mixed with Sepharose beads coupled to CP47 antibody, the eluted material corresponded to a preparation of about 4 chlorophylls per 2 pheophytins and did not show any cross-reaction with the antibody against CP47. The amount of CP47 protein in the 6-chlorophyll preparation as quantitated using Coomassie Blue staining or from gel blots was sufficient to account for most of the extra 2 chlorophylls. We conclude that D1-D2-cytochrome b559 preparations containing more than 4 chlorophylls per 2 pheophytins can be contaminated with small amounts of CP47-D1-D2-Cyt b559 complex and that native photosystem II reaction centers contain 4 core chlorophylls per 2 pheophytins.

Temperature dependent hole burning of the 684 nm chlorophyll a of the isolated reaction center of Photosystem II: confirmation of the linker model

Currently the nature of the additional chlorophyll a (Chl a ) molecules which take the number of Chl a per isolated D1-D2-cyt b 559 complex beyond the value of four for the bacterial reaction center is a subject of much importance and interest. Chang et al. [J. Phys. Chem. 98 (1994) 7725], who studied preparations of this complex with significantly different Chl a contents, concluded that the 684 nm absorbing Chl a contribute to the additional Chl a and proposed that they are of the linker type, serving to shuttle energy from the proximal antenna complex to the reaction center. Implicit in this model is that the 684 nm Chl a communicate with the primary electron donor Chl a (P680) via energy transfer and that the resulting detrapping of the Q y -state of the 684 nm Chl a should depend strongly on temperature. Temperature dependent transient hole spectra are presented that are in complete accord with this prediction. Theoretical calculations on the kinetics and temperature dependence of the hole profile of the 680 nm absorption band are presented and provide convincing support for the linker model. The data are argued to be inconsistent with other models.

Loss of chloroplast transcripts for proteins associated with photosystem II: an early event during heat-bleaching in Euglena gracilis.

A shift in the ratio of chlorophyll (Chl) a and Chl b is an early indicator of heat bleaching in Euglena gracilis. This observation prompted us to consider whether or not changes in steady-state levels of chloroplast transcripts and in transcriptional activity could limit the synthesis of Chl a-binding proteins in bleaching plastids. We found that the mature transcripts for CP47 and CP43, the Chl a-binding apoproteins of the proximal antenna of photosystem II, decline sharply very early during bleaching. Our study also shows that transcription of psbB and psbC, the chloroplast genes encoding CP47 and CP43, remains essentially unchanged during the same interval. We conclude that posttranscriptional events, such as mRNA stability, could play a major role in initiating an irreversible loss of chloroplast function in Euglena at a moderately elevated temperature. Lack of these transcripts would eventually impair the assembly of photosystem II in thylakoids.

Changes in in vivo fluorescence quenching in rye and barley as a function of reduced PSII light harvesting antenna size

The relationship between the size of the light harvesting antenna to photosystem II (LHCII) and quenching of non‐photochemical and dark level fluorescence was studied in wild‐type rye (Secale cereale L. cv. Musketeer) and barley (Hordeum vulgare L. cv. Gunilla) as well as in the barley chlorophyll b‐less chlorina F2 mutant (H. vulgare L. cv. Dornaria, chlorina‐F2). Exposure for 10 min to an irradiance of 500 μmol m−2 s−1 resulted in a strong (0.71–0.73) non‐photochemical (qs) quenching of the fluorescence yield in wild‐type (WT) material, while the barley chlorina F2‐mutant was quenched to 75% of this level. Relaxation of qs in darkness revealed a fast initial decay, related to relaxation of the high‐energy‐state dependent (qE) part of qs. Etiolated seedlings of rye and barley exposed to intermittent light (IML) for 36 cycles of 2 min light and 118 min darkness had suppressed Chl b and LHCII‐production in both WT rye and barley, while the barley chlorina F2‐mutant became totally devoid of all LHCII‐polypeptides. It was found that the levels of qs and qs were similar in control grown barley chlorina F2 and IML‐grown WT rye and barley, but qs was reduced by 30 to 35% and qs by 50 to 65%, respectively, as compared to control‐grown. WT plants. No significant qs could be detected in IML‐grown barley chlorina F2. It is clear, from these changes in in vivo fluorescence quenching in rye and barley that a significant level of qs is detectable even in the absence of LHCII. Only when the proximal antennae are totally absent, does qE completely disappear. We conclude that the presence of LHCII is not an absolute requirement for qE‐quenching and suggest that distal as well as proximal antenna may contribute to qE in vivo.

Exciton Level Structure and Dynamics in the CP47 Antenna Complex of Photosystem II

Persistent nonphotochemical and population bottleneck hole-burning results obtained as a function of burn wavelength are reported for the CP47 proximal antenna protein complex of photosystem II. Attention is focused on the lower energy chlorophyll a Q[sub y] states. Results are presented for the CP47 complex from two preparations. The Chl a content per CP47 complex was determined, spectroscopically, to be 14 [+-] 2. On the basis of the analysis of the hole spectra and the 4.2 K static fluorescence spectrum, the lowest energy state of CP47 lies at 690 nm (fluorescence origin at 691 nm). The width of the weak 690-nm absorption band from inhomogeneous broadening is 100 cm[sup [minus]1]. The linear electron-phonon coupling of the 690-nm state is weak with a Huang-Rhys factor (S) of about 0.2 and a mean phonon frequency ([Omega][sub m]) of 120 cm[sup [minus]1], which explains why the Stokes shift (2S[Omega][sub m]) is so small. The 690-nm state is found to be excitonically correlated with a hitherto unobserved state at 687 nm. However, the combined absorption intensity of the 690- and 687-nm states was determined to be equivalent to only 1 Chl a molecule. Results are presented which illustrate that these two states are fragilemore » (i.e., their associated chlorophyll a molecules are disrupted). Thus, it is possible that the correct number of Chl a molecules is 2, not 1. 53 refs., 9 figs., 1 tab.« less

KINETIC STUDIES ON THE STABILIZATION OF THE PRIMARY RADICAL PAIR P680+ Pheo‐ IN DIFFERENT PHOTOSYSTEM II PREPARATIONS FROM HIGHER PLANTS*

Abstract— The stabilization of the primary radical pair P680+ pheophytin (Pheo)‐ through rapid electron transfer from Pheo to the special plastoquinone of photosystem II (PS II), QA, was analyzed on the basis of time‐resolved (40 ps) UV‐absorption changes detected in different PS II preparations from higher plants. Lifetime measurements of1Chi* fluorescence by single photon counting and a numerical analysis of the redox reactions revealed (1) at exciton densities required for light saturation of the stable charge separation, annihilation processes dominate the excited state decay leading to very similar lifetimes of 1Chi* in systems with open and closed reaction centers and (2) the difference of absorption changes induced by actinic flashes of comparatively high photon density in samples with open and photochemically closed reaction centers, respectively, provides a suitable measure of the rate constant of QA formation. Conclusion 2 was confirmed in PS II membrane fragments by measurements at three wavelengths (280 nm, 292 nm and 325 nm) where the difference spectrum of Q‐A formation exhibits characteristic features. The numerical evaluation of the experimental data led to the following results: (1) the rate constant of Q‐A formation was found to be (300 ± 100 ps)‐1 in PS II membrane fragments and PS II core complexes deprived of the distal and proximal antenna and (2) an iron depletion treatment of membrane fragments does not affect these kinetics. The implications of these results are briefly discussed in terms of the PS II reaction pattern.

Pico- and nanosecond fluorescence kinetics of Photosystem II reaction centre and its complex with CP47 antenna

Spectrally resolved pico- and nanosecond fluorescence kinetics of two types of Photosystem II core complex: D1/D2/cyt b559 reaction centres (RCs) and RCs together with CP47 proximal antenna have been studied at room temperature and at 77 K. The kinetics at room temperature were measured with the RCs being in different functional states. In the photoactive RCs at room temperature a picosecond decay with the lifetime components 13 ± 3 ps and 110 ± 30 ps is followed by the complex nanosecond kinetics. In the case of RC + CP47 complexes picosecond decays are slower: 25 ± 10 ps and 190 ± 30 ps, but nanosecond decay has similar behaviour. The data are analyzed by a simple three-state kinetic model postulating the formation of the primary radical pair in an unrelaxed form and allowing a back-recombination from that state. If this is correct, it is the proof that in the charge-separated state nuclear coordinate relaxation takes place on the picosecond time-scale. The following conclusions considering the nature and temporal characteristics of light excitations have been made: (i) At room temperature excitations are in equilibrium between P680 and the accessory chlorophylls including CP47 antenna and, therefore, only the average trapping time could be observed. This time is equal to 13 ± 3 ps in RCs and 25 ± 10 ps in RC + CP47 complexes. (ii) All other decays at room temperature (except probably part of the 5 ± 0.5 ns component) are of a recombination origin and reflect complex relaxation of the metastable radical pair state. (iii) At low temperatures an energetically directed excitation transfer, qualitatively very similar to the one observed in the core antenna of some purple bacteria takes place. This energy transfer is relatively slow with an apparent transfer time 10–20 ps at 77 K. (iv) Not only P680, but also P + 680 and Pheo − are very efficient quenchers of excitations.