The primary electron donor of photosystem II of the cyanobacterium Acaryochloris marina is a chlorophyll d and the water oxidation is driven by a chlorophyll a/chlorophyll d heterodimer

We present a theoretical analysis of the flash-induced absorbance difference spectrum assigned to the formation of the secondary radical pair P(+)QA(-) in photosystem II of the chlorophyll d-containing cyanobacterium Acaryochloris marina. An exciton Hamiltonian determined previously for chlorophyll a-containing photosystem II complexes is modified to take into account the occupancy of certain binding sites by chlorophyll d instead of chlorophyll a. Different assignments of the reaction center pigments to chlorophyll a or d from the literature are investigated in the calculation of the absorbance difference spectrum. A quantitative explanation of the experimental data requires one chlorophyll a molecule per reaction center, located at the site of P(D1). The remaining sites are occupied by chlorophyll d and pheophytin a. By far, the lowest site energy is found for the accessory chlorophyll of the D1 branch, Chl(D1), which represents the sink of excitation energy and therefore the primary electron donor. The cationic state is stabilized at the chlorophyll a, which drives the oxidation of water.     

Characterization of the water oxidizing complex of photosystem II of the Chl d-containing cyanobacterium Acaryochloris marina via its reactivity towards endogenous electron donors and acceptors

Acaroychloris (A.) marina is a unique oxygen evolving organism that contains a large amount of chlorophyll d (Chl d) and only very few Chl a molecules. This feature raises questions on the nature of the photoactive pigment, which supports light-induced oxidative water splitting in Photosystem II (PS II). In this study, flash-induced oxygen evolution patterns (FIOPs) were measured to address the question whether the S(i) state transition probabilities and/or the redox-potentials of the water oxidizing complex (WOC) in its different S(i) states are altered in A. marina cells compared to that of spinach thylakoids. The analysis of the obtained data within the framework of different versions of the Kok model reveals that in light activated A. marina cells the miss probability is similar compared to spinach thylakoids. This finding indicates that the redox-potentials and kinetics within the WOC, of the reaction center (P680) and of Y(Z) are virtually the same for both organisms. This conclusion is strongly supported by lifetime measurements of the S(2) and S(3) states. Virtually identical time constants were obtained for the slow phase of deactivation. Kinetic differences in the fast phase of S(2) and S(3) decay between A. marina cells and spinach thylakoids reflect a shift of the E(m) of Y(D)/Y(D)(ox) to lower values in the former compared to the latter organisms, as revealed by the observation of an opposite change in the kinetics of S(0) oxidation to S(1) by Y(D)(ox). A slightly increased double hit probability in A. marina cells is indicative of a faster Q(A)(-) to Q(B) electron transfer in these cells compared to spinach thylakoids.     

Pulsed high-frequency EPR study on the location of carotenoid and chlorophyll cation radicals in photosystem II

When the primary electron-donation pathway from the water-oxidation complex in photosystem II (PS II) is inhibited, chlorophyll (Chl(Z) and Chl(D)), beta-carotene (Car) and cytochrome b(559) are alternate electron donors that are believed to function in a photoprotection mechanism. Previous studies have demonstrated that high-frequency EPR spectroscopy (at 130 GHz), together with deuteration of PS II, yields resolved Car(+) and Chl(+) EPR signals (Lakshmi et al. J. Phys. Chem. B 2000, 104, 10 445-10 448). The present study describes the use of pulsed high-frequency EPR spectroscopy to measure the location of the carotenoid and chlorophyll radicals relative to other paramagnetic cofactors in Synechococcus lividus PS II. The spin-lattice relaxation rates of the Car(+) and Chl(+) radicals are measured in manganese-depleted and manganese-depleted, cyanide-treated PS II; in these samples, the non-heme Fe(II) is high-spin (S = 2) and low-spin (S = 0), respectively. The Car(+) and Chl(+) radicals exhibit dipolar-enhanced relaxation rates in the presence of high-spin (S = 2) Fe(II) that are eliminated when the Fe(II) is low-spin (S = 0). The relaxation enhancements of the Car(+) and Chl(+) by the non-heme Fe(II) are smaller than the relaxation enhancement of Tyr(D)(*) and P(865)(+) by the non-heme Fe(II) in PS II and in the reaction center from Rhodobactersphaeroides, respectively, indicating that the Car(+)-Fe(II) and Chl(+)-Fe(II) distances are greater than the known Tyr(D)(*)-Fe(II) and P(865)(+)-Fe(II) distances. The Car(+) radical exhibits a greater relaxation enhancement by Fe(II) than the Chl(+) radical, consistent with Car being an earlier electron donor to P(680)(+) than Chl. On the basis of the distance estimates obtained in the present study and by analogy to carotenoid-binding sites in other pigment-protein complexes, possible binding sites are discussed for the Car cofactors in PS II. The relative location of Car(+) and Chl(+) radicals determined in this study provides valuable insight into the sequence of electron transfers in the alternate electron-donation pathways of PS II.     

Examination of the photophysical processes of chlorophyll d leading to a clarification of proposed uphill energy transfer processes in cells of Acaryochloris marinas

A comprehensive study of the photophysical properties of chlorophyll (Chl) d in 1:40 acetonitrile-methanol solution is performed over the temperature range 170-295 K. From comparison of absorption and emission spectra, time-dependent density-functional calculations and homologies with those of Chl a, we assign the key features of the absorption and fluorescence spectra. Possible photophysical energy relaxation mechanisms are summarized, and thermal equilibration processes are studied in detail by monitoring the observed emission profiles and quantum yields as a function of excitation energy. In particular, we concentrate on emission subsequent to excitation in the extreme far-red tail of the Qy absorption spectrum, with this emission partitioned into contributions from hot-band absorptions as well as uphill energy transfer processes that occur subsequent to absorption. No unusual photophysical processes are detected for Chl d; it appears that all intramolecular relaxation processes reach thermal equilibration on shorter timescales than the fluorescence lifetime even at 170 K. The results from these studies are used to reinterpret a previous study of photochemical processes observed in intact cells and their acetone extracts of the photosynthetic system of Acaryochloris marina. In the study of Mimuro et al., light absorbed by Chl d at 736 nm is found to give rise to emission by another species, believed to also be Chl d, at 703 nm; this uphill energy transfer process is easily rationalized in terms of the thermal equilibration processes that we deduced for Chl d. However, no evidence is found in the experimental results of Mimuro et al. to support claims that (nonequilibrium) uphill energy transfer is additionally observed to Chl a species that emit at 670-680 nm. This finding is relevant to broader issues concerning the nature of the special pair in photosystem II of A. marina because suggestions that it is comprised of Chl a can only be correct if nonthermal uphill energy transfer processes from Chl d are operative.     

Separation and determination of minor photosynthetic pigments by reversed-phase HPLC with minimal alteration of chlorophylls.

Reversed-phase HPLC conditions for separation of chlorophyll (Chl) a, Chl a' (the C132-epimer of Chl a), pheophytin (Pheo) a (the primary electron acceptor of photosystem (PS) II), and phylloquinone (PhQ) (the secondary electron acceptor of PS 1), have been developed. Pigment extraction conditions were optimized in terms of pigment alteration and extraction efficiency. Pigment composition analysis of light-harvesting complex II, which would not contain Chl a' nor Pheo a, showed the Chl a'/Chl a ratio of 3-4 x 10(-4) and the Pheo a/Chl a ratio of 4-5 x 10(-4), showing that the conditions developed here were sufficiently inert for Chl analysis. Preliminary analysis of thylakoid membranes with this analytical system gave the PhQ/Chl a' ratio of 0.58 +/- 0.03 (n = 4), in line with the stoichiometry of one molecule of Chl a' per PS I.     

Modulation of photosystem II chlorophyll fluorescence by electrogenic events generated by photosystem I.

In an attempt to uncover electric field interactions between PS I and PS II during their functioning, fluorescence induction curves were measured on hydroxylamine-treated thylakoids of Chenopodium album under conditions ensuring low and high levels of photogenerated membrane potentials. In parallel experiments with Peperomia metallica chloroplasts, the photocurrents were measured with patch-clamp electrodes and served as indicator of electrogenic activity of thylakoid membranes in continuous light. Inhibition of linear electron flow at PS II donor side by hydroxylamine (0.1 mM) eliminated a slow rise of chlorophyll fluorescence to a peak level and suppressed photoelectrogenesis. Activation of PS I-dependent electron transport using cofactors of either cyclic (phenazine methosulfate) or noncyclic electron transport (reduced TMPD or DCPIP in combination with methyl viologen) restored photoelectrogenesis in hydroxylamine-treated chloroplasts and led to reappearance of slow components in the fluorescence induction curve. Exposure of thylakoids to valinomycin reduced the peak fluorescence in the presence of KCl but not in the absence of KCl. Combined application of valinomycin and nigericin in the presence of KCl exerted stronger suppression of fluorescence than valinomycin alone but was ineffective in the absence of KCl. In samples treated with hydroxylamine and PS I cofactors (DCPIP/ascorbate and methyl viologen), preillumination with a single-turnover flash or a multiturnover pulse shifted the induction curves of both membrane potential and chlorophyll fluorescence to shorter times, which confirms the supposed influence of PS I-generated electrical field on PS II fluorescence. A model is presented that describes modulating effect of the membrane potential on chlorophyll fluorescence and roughly simulates the fluorescence induction curves measured at low and high membrane potentials.     

Reversed-phase HPLC determination of chlorophyll a' and naphthoquinones in photosystem I of red algae: existence of two menaquinone-4 molecules in photosystem I of Cyanidium caldarium.

Chlorophyll (Chl) a', the C13(2)-epimer of Chl a, is one of the two Chl molecules constituting the primary electron donor (P700) of photosystem (PS) I of a thermophilic cyanobacterium Synechococcus elongatus. To examine whether PS I of other oxygenic photosynthetic organisms in general contain one Chl a' molecule in P700, the pigment composition of thylakoid membranes and PS I preparations isolated from red algae Porphyridium purpureum and Cyanidium caldarium was examined by reversed-phase HPLC with particular attention to Chl a' and phylloquinone (PhQ), the secondary electron acceptor of PS I. The two red algae contained one Chl a' molecule at the core part of PS I. In PS I of C. caldarium, two menaquinone-4 (MQ-4) molecules were detected in place of PhQ used by higher plants and cyanobacteria. The 1:2:1 stoichiometry among Chl a', PhQ (MQ-4) and P700 in PS I of the red algae indicates that one Chl a' molecule universally exists in PS I of oxygenic photosynthetic organisms, and two MQ-4 molecules are associated with PS I of C. caldarium.     

Chemical environment heterogeneity around the two chlorophyll a′ molecules in photosystem I

Treatment of whole and fractionated plant tissues with hydrophilic and hydrophobic solvent mixtures of varied volume ratio liberates two chlorophyll (Chl) a′ molecules from the photosystem (PS)I core when the solvent hydrophilicity exceeds a critical level, whereas only one molecule is extracted in hydrophobic media. The PSI core proteins, PSI-A and PSI-B, which form a heterodimer, appear to bind one Chl a′ molecule each, in local environments significantly different regarding their hydrophilicity or hydrophobicity.     

Normal-phase HPLC separation of possible biosynthetic intermediates of pheophytin a and chlorophyll a'.

Normal-phase HPLC conditions have been developed for separating the C17(3) isoprenoid isomers, which are expected to be formed as biosynthetic intermediates of chlorophyll (Chl) a, Chl a' (C13(2)-epimer of Chl a), pheophytin (Pheo) a and protochlorophyll (PChl). The application of these conditions to pigment composition analysis of greening etiolated barley leaves allowed us to detect, for the first time, the C17(3) isomers of Chl a', a possible constituent of the primary electron donor of photosystem (PS) I, P700, and those of Pheo a, the primary electron acceptor of PS II, in the very early stage of greening. The C17(3) isomer distribution patterns were approximately the same between Chl a and Chl a', but significantly different between Pheo a and Chl a', probably reflecting the similarity and difference, respectively, in the biosynthetic pathways of these pigment pairs.     

Isolation and characterization of a photosystem I-associated antenna (LHC I) and a photosystem I—core complex from the chlorophyll c-containing alga Pleurochloris meiringensis (Xanthophyceae)

A photosystem (PS) I holocomplex was isolated from Pleurochloris meiringensis Vischer (Xanthophyceae) using sucrose density centrifugation. This complex exhibited a fluorescence emission maximum at 715 nm, which is in accordance with the long wavelength emission of whole cells. The complex was further dissociated into a core complex and a light-harvesting protein (LHC I). The core protein contains mainly Chl a and β-carotene, is 8.25 times enriched in P700 and has its main emission maximum at 715 nm. Therefore, the longest wavelength emission of P. meiringensis is due to the PS I core, which is in contrast to higher plants. The LHC I differs from LHC II with regard to its polypeptide pattern as well as its spectral properties. The arrangement of antennae is discussed in relation to the regulation of energy transfer between the photosystems.     

Changes in the structure of chlorophyll-protein complexes and excitation energy transfer during photoinhibitory treatment of isolated photosystem I submembrane particles

The activity of light-induced oxygen consumption, absorption spectra, low temperature (77 K) chlorophyll fluorescence emission and excitation spectra were studied in suspensions of photosystem (PS) I submembrane particles illuminated by 2000 microE m(-2) s(-1) strong white light (WL) at 4 degrees C. A significant stimulation of oxygen uptake was observed during the first 1-4 h of photoinhibitory treatment, which rapidly decreased during further light exposure. Chlorophyll (Chl) content gradually declined during the exposure of isolated PSI particles to strong light. In addition to the Chl photobleaching, pronounced changes were found in Chl absorption and fluorescence spectra. The position of the major peak in the red part of the absorption spectrum shifted from 680 nm towards shorter wavelengths in the course of strong light exposure. A 6-nm blue shift of that peak was observed after 5-h illumination. Even more pronounced changes were found in the characteristics of Chl fluorescence. The magnitude of the dominating long-wavelength emission band at 736 nm located in untreated particles was five times reduced after 2-h exposure, whereas the loss in absolute Chl contents did not exceed 10% of its initial value. The major peak in low-temperature Chl fluorescence emission spectra shifted from 736 to 721 nm after 6-h WL treatment. Individual Chl-protein complexes differed in the response of their absorption spectra to strong WL. Unlike light-harvesting complexes (LHC), LHCI-680 and LHC-730, which did not exhibit changes in the major peak position, its maximum was shifted from 678 to 671 nm in CPIa complex after PSI submembrane particles were irradiated with strong light for 6 h. The results demonstrated that excitation energy transfer represents the stage of photosynthetic utilization of absorbed quanta which is most sensitive to strong light in isolated PSI particles.     

Raman properties of chlorophyll d, the major pigment of Acaryochloris marina: studies using both Raman spectroscopy and density functional theory

The Raman spectroscopy of purified chlorophyll (Chl) d extracted from Acaryochloris marina has been measured over the wide region of 250-3200 cm(-1) at 77 K following excitation of its Soret band at 488 nm and analyzed with the aid of hybrid density-functional vibrational analyses. A Raman peak specific to Chl d, which arises from the formyl group 3(1) C=O stretching, was clearly observed at 1659 cm(-1) with medium intensity. Peaks due to other C=O stretching vibrations of the 13(1) keto-, 13(3) ester- and 17(3) groups were also observed. Four very strong peaks were observed in the range of 1000-1600 cm(-1), assigned to the CC stretching and mixtures of the CH3 bend and CN stretching. CCC and NCC bending contribute to medium intensity peaks at 986 and 915 cm(-1). Out-of-plane CH bending at Chl d methine sites 10, 5 and 20 contribute to observed peaks at 885, 864 and 853 cm(-1), respectively. A few modes involving the MgN stretching and MgNC bending motions were observed in the very low frequency range. Density functional theory (DFT) calculations have been used to make assignments on the observed Raman spectrum and the DFT results have been found to be in good agreement with the experimental results.     

DCMU-INDUCED STIMULATION OF PHOTOSYSTEM I ELECTRON TRANSPORT. INVOLVEMENT OF MEMBRANE STRUCTURAL CHANGES

DCMU-induced stimulation of the rate of photosystem I (PS I) electron transport in DCIPH₂→ MV photoreaction occurs through the action of DCMU on the rate-limiting step which contains the site of electron donation of DCIPH₂ (Ramanujam et al. , 1981). The magnitude of stimulation of the rate by 50 μ M DCMU decreased with increasing concentration of chlorophyll (Chl), implying that DCMU is stoichiometrically related to Chl with respect to the stimulation of the PS I rate.
DCMU-induced stimulation was sensitive to the ionic condition of the thylakoids, the effect being reduced at low cation concentration. Cation-induced scattering changes in thylakoid suspension were partially reversed by DCMU, and the percent Chl in the 10 K fraction of the thylakoid decreased upon addition of DCMU, indicating that grana structure is disrupted by DCMU. Hydroquinone-mediated reduction of cytochrome f in thylakoids in the dark was accelerated in the presence of DCMU. The DCMU effect was not observed in isolated PS I particles.
It is concluded that DCMU binds to the thylakoid membranes and brings about structural changes leading to unstacking of the thylakoids accompanied by an altered interaction of the electron transfer chain components with the added electron donor. This binding of DCMU must have an affinity lower than the well-known binding of DCMU to photosystem II (PS II), because the concentration required is markedly higher.     

Photosystem II fluorescence: slow changes--scaling from the past

With the advent of photoelectric devices (photocells, photomultipliers) in the 1930s, fluorometry of chlorophyll (Chl) a in vivo emerged as a major method in the science of photosynthesis. Early researchers employed fluorometry primarily for two tasks: to elucidate the role in photosynthesis, if any, of other plant pigments, such as Chl b, Chl c, carotenoids and phycobilins; and to use it as a convenient inverse measure of photosynthetic activity. In pursuing the latter task, it became apparent that Chl a fluorescence emission is influenced (i) by redox active Chl a molecules in the reaction center of photosystem (PS) II (photochemical quenching); (ii) by an electrochemical imbalance across the thylakoid membrane (high energy quenching); and (iii) by the size of the peripheral antennae of weakly fluorescent PSI and strongly fluorescent PSII in response to changes in the ambient light (state transitions). In this perspective we trace the historical evolution of our awareness of these concepts, particularly of the so-called 'State Transitions'.     

A Synthetic Chlorophyll Dimer Appending Fullerene: Effect of Chlorophyll Pairing on (Photo)redox Properties

Accurately mimicking structure and function of natural chlorophyll (Chl) assemblies is very challenging. Herein, we report the synthesis of a fullerene-appended Chl dimer being capable of intramolecular photoinduced charge separation (CS) with a unique structure reminiscent of reaction centers (RCs) in phototrophs. Structural analyses revealed that the Chl dimer adopts a bird-like structure in which two Chl components overlapped partially with one of the four pyrrole rings in a Chl ring similar to in a Chl pair in the natural RC complexes. A comparative study including voltammetry and spectrometric analyses using the Chl dimer and its corresponding monomer with and without a fullerene moiety was performed to gain insight into the effect of Chl pairing on (photo)redox properties. Our results suggest that the present dimer motif that closely resemble the Chl pair in natural RCs lead to more facile oxidation and lower energy of the CS state of the Chl dimer than those of the Chl monomer, resulting in its photoredox behavior different from that of the monomer Chl.     

Application of molecular orbital calculations to interpret the chlorophyll spectral forms in pea photosystem II

The energy and oscillator strength of electronic transitions of chlorophyll (Chl)-amino acid complexes were calculated by using molecular orbital methods. The energies varied widely with coordinated amino acids and the difference between the maximum and minimum energy was about 830 cm-1. This energy difference was comparable with the spreading of absorption bands for light-harvesting Chl-protein complexes of photosystem II (LHC II) of green plants. The feature of the Qy band for pea LHC II was interpreted with the aid of the calculated energies and oscillator strengths. Four spectral components of the band were assigned to individual Chl-amino acid complexes.     

CHLOROPHYLL b: AN INTEGRAL COMPONENT OF PHOTOSYSTEM I OF HIGHER PLANT CHLOROPLASTS

Abstract— An undissociated photosystem I complex may be isolated from spinach thylakoids by mild gel electrophoresis (CP1a) or Triton X-100. CP1a has a Chl a / b ratio of 11 and a Chl/P700 ratio of 120. while the Triton X-100 PS I complex (Chl a / b ratio of 5.9) has a larger antenna unit size (Chl/P700 ratio of 180). None of the Chl a / b -proteins of the main light-harvesting complex (apoproteins of 30–27 kD) are present in CP1a, and they account for less than 10% of the total chlorophyll in the Triton X-100 PS I complex. Instead, these PS I complexes have specific, but as yet little characterized, Chi a / b -proteins (apoproteins in the 26–21 kD range). With both PS I complexes, Chi b transfers light excitation to the 735 nm low temperature fluorescence band characteristic of photosystem I. We suggest that Chi b is an integral but minor component of photosystem I.     

Time-resolved two-photon spectroscopy of photosystem I determines hidden carotenoid dark-state dynamics

We present time-resolved fs two-photon pump-probe data measured with photosystem I (PS I) of Thermosynechococcus elongatus. Two-photon excitation (lambda(exc)/2 = 575 nm) in the spectral region of the optically forbidden first excited singlet state of the carotenoids, Car S1, gives rise to a 800 fs and a 9 ps decay component of the Car S1 --> S(n) excited-state absorption with an amplitude of about 47 +/- 16% and 53 +/- 10%, respectively. By measuring a solution of pure beta-carotene under exactly the same conditions, only a 9 ps decay component can be observed. Exciting PS I at exactly the same spectral region via one-photon excitation (lambda(exc) = 575 nm) also does not show any sub-ps component. We ascribe the observed constant of 800 fs to a portion of about 47 +/- 16% beta-carotene states that can potentially transfer their energy efficiently to chlorophyll pigments via the optically dark Car S1 state. We compared these data with conventional one-photon pump-probe data, exciting the optically allowed second excited state, Car S2. This comparison demonstrates that the fast dynamics of the optically forbidden state can hardly be unravelled via conventional one-photon excitation only because the corresponding Car S1 populations are too small after Car S2 --> Car S1 internal conversion. A direct comparison of the amplitudes of the Car S1 --> S(n) excited-state absorption of PS I and beta-carotene observed after Car S2 excitation allows determination of a quantum yield for the Car S1 formation in PS I of 44 +/- 5%. In conclusion, an overall Car S2 --> Chl energy-transfer efficiency of approximately 69 +/- 5% is observed at room temperature with 56 +/- 5% being transferred via Car S2 and probably very hot Car S1 states and 13 +/- 5% being transferred via hot and "cold" Car S1 states.     

P680 (P(D1)P(D2)) and Chl(D1) as alternative electron donors in photosystem II core complexes and isolated reaction centers

Low temperature (77-90 K) measurements of absorption spectral changes induced by red light illumination in isolated photosystem II (PSII) reaction centers (RCs, D1/D2/Cyt b559 complex) with different external acceptors and in PSII core complexes have shown that two different electron donors can alternatively function in PSII: chlorophyll (Chl) dimer P(680) absorbing at 684 nm and Chl monomer Chl(D1) absorbing at 674 nm. Under physiological conditions (278 K) transient absorption difference spectroscopy with 20-fs resolution was applied to study primary charge separation in spinach PSII core complexes excited at 710 nm. It was shown that the initial electron transfer reaction takes place with a time constant of ~0.9 ps. This kinetics was ascribed to charge separation between P(680)* and Chl(D1) absorbing at 670 nm accompanied by the formation of the primary charge-separated state P(680)(+)Chl(DI)(-), as indicated by 0.9-ps transient bleaching at 670 nm. The subsequent electron transfer from Chl(D1)(-) occurred within 13-14 ps and was accompanied by relaxation of the 670-nm band, bleaching of the Pheo(D1) Q(x) absorption band at 545 nm, and development of the anion-radical band of Pheo(D1)(-) at 450-460 nm, the latter two attributable to formation of the secondary radical pair P(680)(+)Pheo(D1)(-). The 14-ps relaxation of the 670-nm band was previously assigned to the Chl(D1) absorption in isolated PSII RCs [Shelaev, Gostev, Nadtochenko, Shkuropatov, Zabelin, Mamedov, Semenov, Sarkisov and Shuvalov, Photosynth. Res. 98 (2008) 95-103]. We suggest that the longer wavelength position of P(680) (near 680 nm) as a primary electron donor and the shorter wavelength position of Chl(D1) (near 670 nm) as a primary acceptor within the Q(y) transitions in RC allow an effective competition with an energy transfer and stabilization of separated charges. Although an alternative mechanism of charge separation with Chl(D1)* as the primary electron donor and Pheo(D1) as the primary acceptor cannot be ruled out, the 20-fs excitation at the far-red tail of the PSII core complex absorption spectrum at 710 nm appears to induce a transition to a low-energy state P(680)* with charge-transfer character (probably P(D1)(δ+)P(D2)(δ-)) which results in an effective electron transfer from P(680)* (the primary electron donor) to Chl(D1) as the intermediary acceptor.     

Effects of acid and alkali on the light absorption, energy transfer and protein secondary structures of core antenna subunits CP43 and CP47 of photosystem II

The effects of acid and alkali treatment on the light absorption, energy transfer and protein secondary structure of the photosystem II core antenna CP43 and CP47 of spinach were investigated by the absorption spectra, fluorescence emission spectra and ciruclar dichroism spectra. It has been found that acid treatment caused the appearance of absorption characteristic of pheophytin a (Pheo a), whereas alkali treatment induced a new absorption peak at 642 nm. The energy transfer between β-carotene and chlorophyll a (Chl a) in CP43 was easily disturbed by alkali, whereas in CP47 was readily affected by acid. As to the effects on the secondary structure of proetins in CP43 and CP47, effects of acid were far less than those of alkali. Both acid and alkali disturbed the microenvironment of Chl a and interfered exciton interaction between Chl a molecules. It was suggested that acid and alkali affect the light absorption, energy transfer and protein secondary structure of CP43 and CP47 in a differenty way. H⁺ can permeate into the internal space of α-helix, change Chl a into Pheo a and disturb the microenvironment of pgiments without damaging the secondary structure of protein, whereas OH⁻ can induce the protein unfolding at first, then saponify Chl a to chlorophyllide and disturb the microenvironment of pigments.