INHIBITION OF ELECTRON TRANSPORT IN CHLOROPLASTS BY CHAOTROPIC AGENTS AND THE USE OF MANGANESE AS AN ELECTRON DONOR TO PHOTOSYSTEM II*

Abstract— Incubating spinach chloroplasts with various chaotropic agents results in inhibition of photosynthetic electron transport between water and Photosystem II similar to the inhibition caused by washing chloroplasts with a high concentration of Tris buffer. Partial restoration of NADP photoreduction and fluorescence of variable yield is achieved by adding hydroquinone or Mn²⁺, either of which donates electrons to Photosystem II in the inhibited chloroplasts. The inhibitory treatments cause the release of Mn from its bound state in the chloroplast, thus allowing the measurement of the ESR signal of Mn²⁺. The ESR measurement is used to follow the photooxidation of Mn²⁺ as it donates electrons to photosystem II.     

EFFECTS OF PHOTOSYSTEM II INHIBITORS ON ELECTRON PARAMAGNETIC RESONANCE SIGNAL II OF SPINACH CHLOROPLASTS

Abstract— In isolated spinach chloroplasts the light-induced electron paramagnetic resonance signal (signal II) associated with the oxygen evolving photosystem (photosystem II) decays slowly and incompletely in the dark. Tris-washing, hydroxylamine, or carbonylcyanide m -chlorophenylhydrazone (CCCP) enhance the decay of signal II, which can still be induced by red (645 nm) but not by far-red (735 nm) radiation. Although 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) alone has no effect on signal II, it blocks the induction of signal II in the presence of hydroxylamine or CCCP. These data suggest that signal II is an indicator of an oxidized intermediate on the water-splitting side of photosystem II.     

LIGHT-ABSORPTION AND ELECTRON-TRANSPORT BALANCE BETWEEN PHOTOSYSTEM II AND PHOTOSYSTEM I IN SPINACH CHLOROPLASTS

Abstract— The distribution of absorbed light and the turnover of electrons by the two photosystems in spinach chloroplasts was investigated. This was implemented upon quantitation of photochemical reaction centers, chlorophyll antenna size and composition of each photosystem (PS), and rate of light absorption in situ. In spinach chloroplasts, the photosystem stoichiometry was PSIIJPSII_α/PSII_β/PSI= 1.3/0.4/1.0. The number (N) of chlorophyll (a+b) molecules associated with each PS was N(PSII_α)/N(PSII_β)/N(PSI)=230/100/200, i.e. about 65% of all Chl is associated with PSII and about 35% with PSI. Light absorption by PSII in vivo is selectively attenuated at the molecular, membrane and leaf levels, (a) The rate of light absorption by PSII was only 0.85 that of PSI because of the lower rate of light absorption by Chl b as compared to Chl a (approximately 80% of all Chl b in the chloroplast is associated with PSII). (b) The exclusive localization of PSII_α in the membrane of the grana partition regions and of PSI in intergrana lamellae resulted in a differential “sieve effect” or “flattening of absorbance” by the photosystems in the two membrane regions. Due to this phenomenon, the rate of light absorption by PSII was lower than that of PSI by 15-20%. (c) Selective filtering of sunlight through the spinach leaf results in a substantial distortion of the effective absorbance spectra and concomitant attenuation of light absorption by the two photosystems. Such attenuation was greater for PSII than for PSI because the latter benefits from light absorption in the 700-730 nm region. It is concluded that, in spite of its stoichiometric excess in spinach chloroplasts, light absorption by PSII is not greater than that by PSI due to the different molecular composition of the two light-harvesting antenna systems, due to the localization of PSII in the grana, and also because of the light transmission properties through the leaf. The elevated PSII/PSI reaction center ratio of 1.7 and the association of 65% of all Chl with PSII help to counter the multilevel attenuation of light absorption by PSII and ensure a balanced PSII/PSI electron turnover ratio of about 1:1.     

QUANTITATION OF PHOTOSYSTEM II ACTIVITY IN SPINACH CHLOROPLASTS. EFFECT OF ARTIFICIAL QUINONE ACCEPTORS

Quantitation of photosystem II (PSII) activity in spinach chloroplasts is presented. Rates of PSII electron-transport were estimated from the concentration of PSII reaction-centers (Chl/PSII = 380:1 when measured spectrophotometrically in the ultraviolet [ΔA₃₂₀] and green [ΔA_540–550] regions of the spectrum) and from the rate of light utilization by PSII under limiting excitation conditions. Rates of PSII electron-transport were measured under the same light-limiting conditions using 2,5-dimethylbenzoquinone or 2,5-dichlorobenzoquinone as the PSII artificial electron acceptors. Evaluation is presented on the limitations imposed in the measurement of PSII electron flow to artificial quinones in chloroplasts. Limitations include the static quenching of excitation energy in the pigment bed by added quinones, the fraction of PSII centers (PSII_β) with low affinity to native and added quinones, and the loss of reducing equivalents to molecular oxygen. Such artifacts lowered the yield of steady-state electron transport in isolated chloroplasts and caused underestimation of PSII electron-transport capacity. The limitations described could explain the low PSII concentration estimates in higher plant chloroplasts (Chl/PSII = 600 ± 50) resulting from proton flash yield and/or oxygen flash-yield measurements. It is implied that quantitation of PSII by repetitive flash-yield methods requires assessment of the slow turnover of electrons by PSII_β and, in the presence of added quinones, assessment of the PSII quantum yield.     

NEW PHOTOSYSTEM I ELECTRON ACCEPTORS: IMPROVEMENT OF HYDROGEN PHOTOPRODUCTION BY CHLOROPLASTS *

Abstract Four novel electron carriers (two zwitterionic bipyridyls, dicarboxyl colbalticinium and sodium metatungstate), which are negatively charged in their reduced form, have been tested as photo-system I acceptors and as mediators of H₂ evolution. Measurements of O₂ uptake, anaerobic photoreduction rates and stationary concentrations of reduced species under continuous illumination indicate that Coulombic interactions control the electron transfer between the photosynthetic membrane and the mediators. Both rates of forward transfer and back reaction (electron cycling) seem to depend on the charge of the electron carrier. The low concentration of anionic species in the diffuse layer associated with the membrane could explain our results. Hydrogen evolution rates obtained with these four mediators used as electron relays between the photosynthetic membrane and colloidal platinum catalyst are higher than with methylviologen. This improvement of the conversion efficiency parallels the high steady state accumulation of reduced carriers favoured by their negative charge. It is also shown that these synthetic mediators, except metatungstate, are able to evolve hydrogen with an hydrogenase isolated from Desulfovibrio desulfuricans.     

INHIBITION AND UNCOUPLING OF ELECTRON TRANSPORT IN ISOLATED CHLOROPLASTS BY THE HERBICIDE 4,6-DINITRO-O-CRESOL

Abstract— Dinitrophenols are known to affect photosynthetic electron transfer. It is shown that the widely used herbicide 4,6-dinitro-o-cresol is a potent inhibitor of the Hill reaction in isolated chloroplasts. By studying different parts of Photosystem II dependent electron transport it is indicated that this herbicide inhibits at the same site as 3-(3.4-dichlorophenyl)-l,1-dimethylurea.
However, the Photosystem I dependent Mehler reaction ascorbate/dichlorophenolindophenol→ diquat is stimulated at higher concentrations of the herbicide. This stimulation does not occur when an uncoupler is added to the reaction medium. There is also no stimulation of the ascorbatep-tetra-methyl-p-phenylene diamine → diquat Mehler reaction. This suggests that 4,6-dinitro-o-cresol uncouples electron transport in the Photosystem I dependent Mehler reaction, when added at higher     

BIMANES—26. AN ELECTRON TRANSFER REACTION BETWEEN PHOTOSYSTEM I1 AND MONOBROMOBIMANE INDUCES STATIC CHLOROPHYLL a QUENCHING IN SPINACH CHLOROPLASTS

Abstract— Monobromobimane in chloroplasts lowers both the quantum yield of system II photochemistry and the yield of chlorophyll a fluorescence. Illumination of the chloroplasts in the presence of monohromobimane is an absolute prerequisite to the manifestation of this phenomenon, which proceeds via the Photosystem II intermediate, the semiquinone radical anion, Q_A-. The latter transfers an electron to monobromobimane to yield an anion radical (mBBr^·), which may either lose bromide ion to yield a reactive radical (mB^·), or acquire a proton and undergo further reduction, eventually forming syn-(methyl, methyl) bimane. In turn, mB reacts with the protein of the light-harvesting complex, to form a product which acts as static excitation energy quencher in the chlorophyll pigment bed of photosystem 11. Polarographic reduction of monobromobimane shows an adsorption wave at O V and two reduction waves. Prolonged reduction in water at -0.5 V yields syn-(methyl, methyl) bimane (which is further reduced at more negative potentials) and bromide ion. Thus, both electrochemical and chloroplast-induced reduction produce syn-(methyl, methyl) bimane. Monobromobimane may then serve as a Photosystem II activated probe in elucidating the conformation of intrinsic thylakoid membrane polypeptides.     

PROTEIN COMPOSITION OF SPINACH CHLOROPLASTS AND THEIR PHOTOSYSTEM I AND PHOTOSYSTEM II SUBFRAGMENTS*

Abstract— The proteins of spinach chloroplasts and their subfragments containing photosystem I and photosystem II, obtained by Triton X-100 treatment or French-pressure rupture, were separated by sodium dodecyl sulfate (SDS)-acrylamide electrophoresis at pH 7·0 in phosphate buffer. The individual protein bands were identified where possible by comparing them with known, isolated and characterized proteins from chloroplasts, and their molecular weights were determined. The protein composition of the chloroplast fragments were correlated to the functional properties of these fragments. Distinct patterns were obtained for photosystem I and photosystem II particles. The major protein of photosystem II is expressed in the 23 kilodalton range and photosystem I proteins seem to be clustered mainly in the 50–70 kilodalton range.     

INHIBITION OF OXYGEN EVOLUTION IN CHLOROPLAST PHOTOSYSTEM II BY THE PROTEIN-MODIFYING AGENT TETRANITROMETHANE

Abstract— The protein-modifying agent tetranitromethane (TNM) reacts with tyrosine residues and -SH groups. It was found to inhibit photo synthetic electron transport on the water splitting side of photosystem II (P. V. Sane and U. Johanningmeier, Z. Naturforsch. 35c, 293–297, 1979). In the present work the inhibition by TNM is studied in detail using photosystem II submembrane fractions. It is shown that the action of TNM with membrane-bound proteins could imply the modification of tyrosine residues. At concentrations below 30 μ M and with short incubation periods (<2 min), TNM produces the release of the extrinsic polypeptides involved in the stabilization of the water-splitting complex, this being correlated with inhibition of electron transport at a site prior to H₂O₂ electron donation even though the inhibition cannot be prevented by the addition of Cl or Ca²⁺, which are known cofactors for oxygen evolution. As the incubation period or the concentration of TNM is increased, photosynthetic pigments are bleached, starting with aggregates absorbing at relatively long wavelengths. The inhibition by low concentrations of TNM differs from the effect of most of the previously reported inhibitors acting at the oxygen-evolving complex of photosystem II.     

STRUCTURE OF PHOTOSYSTEM I AND PHOTOSYSTEM II OF PLANT CHLOROPLASTS*,†,§

Abstract— The action of Triton X-100 upon photosynthetic membranes which are devoid of carotenoids produces a small Photosystem I particle (HP700 particle) which is active in N ADP photoreduction and has a [Chl]/[P700] ratio of 30. The properties of the HP700 particle indicate that it is a reaction center complex which is served by an accessory complex containing the additional light-harvesting chlorophyll of Photosystem I as well as the cytochromes and plastoquinone. When Photosystem II particles obtained by the action of Triton X-100 are further washed with a solution 0.5 M in sucrose and 0.05 M in Tris buffer (pH 8.0), chlorophyll-containing material is released. After centrifugation, the supernatant contains about 1 per cent of the chlorophyll and contains three types of particles which can be separated by sucrose density gradient centrifugation. One of these particles, designated TSF-2b, has the same pigment composition as the original Photosystem II fragment, contains cytochrome 559, and shows Photosystem II activity (DCMU-sensitive diphenylcarbazide-supported photoreduction of 2,6-dichlorophenolindophenol). The other two particles (TSF-2a and TSF-2a′) have a [Chl a]/[Chl b] ratio of 8, have a low concentration of xanthophylls, and show a [Chl]/[Cyt 5591 ratio of about 20. Only the TSF-2a particle is active in the Photosystem II reaction described above. On the basis of these data, it is proposed that the Photosystem II unit consists of a reaction center complex which contains Chl a, Cyt 559, and an acceptor for the photochemical reaction. The reaction center complex would be served by an accessory complex which contains the light-harvesting pigments, Chl a. Chi b, and xanthophyils.     

PICOSECOND TRANSIENT BEHAVIOR OF REACTION-CENTER PARTICLES OF PHOTOSYSTEM I ISOLATED FROM SPINACH CHLOROPLASTS. ENERGY AND ELECTRON TRANSFER UPON SINGLE AND MULTIPLE PHOTON EXCITATION

Abstract— Both [15-¹³C] and [14-¹³C] all-trans-retinals were synthesized. Bacteriorhodopsin containing [14-¹³C]retinal as a chromophore, when solubilized with octyl-β-D-glucoside, showed characteristic resonances at 125 and 118 ppm from tetramethyl silane. The former was assigned to the signal from free retinal and the latter from protonated Sehiff base. When the bacteriorhodopsin was denatured in sodium dodecyl sulfate, the signal at 118 ppm disappeared, while the signal at 125 ppm rather increased.
In the case of bacteriorhodopsin containing [15-¹³C]retinal, when solubilized with Triton X-100, a characteristic resonance at 169 ppm was distinguishable as a shoulder peak superimposed on the broad signal of carbonyl carbons and it was assigned to the signal from the protonated Sehiff base. The other signal observed at 191 ppm was from free retinal.
These results suggested that the Sehiff base of bacteriorhodopsin is protonated in the dark.     

EXCITON TRANSFER OUT OF OPEN PHOTOSYSTEM II REACTION CENTERS

Abstract A simple photochemical model of photosystem II consisting of antenna chlorophyll and a reaction center was used to examine the phenomenon of exciton detrapping, i.e. the transfer of excitation energy from open reaction centers back to the antenna. η, the ratio of the probability of detrapping when the reaction centers are all open, Ψ_t(o) to the probability when the centers are closed, Ψ_t(x) was used as a variable parameter to examine the various pathways of energy dissipation in a system in which P, the yield of photochemistry, and R, the ratio of the maximum to the minimum yields of fluorescence, were assumed to be known (e.g. R= 4.0 and P= 0.90). It is shown that η must fall within a range of values between 0 and R (1 –P) and that, for given values of R and P, Ψ_t(o) and the ratio of the rate constant for photochemistry at the reaction center, k_p, to the rate constant for energy transfer back to the antenna, k_t, can be determined for any assumed value of η. Even though detrapping occurs at open reaction centers, it is the magnitude of the yield of nonradiative decay at closed reaction centers, Ψ_a(x) which sets the upper limit on η. Equations for the overall yields of fluorescence and nonradiative decay in the antenna chlorophyll and of nonradiative decay at the reaction center chlorophyll, under conditions of both open and closed reaction centers, were derived in conventional probability terms and in terms of R, P and η. As η increases within its range of permissible values, energy dissipation in the antenna decreases and nonradiative decay at the reaction center increases. The determination of a specific value of η or of the ratio k_pk_t would require additional information such as the value of the maximum yield of fluorescence and the ratio of the rate constants for fluorescence and nonradiative decay in the antenna chlorophyll. The characteristics of a system in which there is no nonradiative decay in the reaction center (i.e. k_d= 0), in which case R (1 –P) = 1.0, were also examined. In this case the yield of detrapping has no influence on energy dissipation in the system. Finally, the question of heterogeneity in PSII was considered. It is suggested that Ψ_d(x) may be greater in PSII_β than in PSII_α so that the probability of detrapping could be greater in the PSII_α fraction.     

BIPHASIC ENERGY CONVERSION KINETICS AND ABSORBANCE DIFFERENCE SPECTRA OF PHOTOSYSTEM II OF CHLOROPLASTS. EVIDENCE FOR TWO DIFFERENT PHOTOSYSTEM II REACTION CENTERS

Abstract— The continuous illumination induced kinetics of photochemical energy conversion at system II have been measured with isolated and 3-(3, 4-dichlorophenyl)-l, l-dimethylurea (DCMU) poisoned chloroplasts by means of absorbance difference spectroscopy in the UV and by the area growth over the fluorescence induction curve at room temperature. An optimal set of conditions was found in order to isolate absorbance changes caused by the reduction of the primary electron acceptor Q of PS II by suppressing other electron transfer processes. The light induced kinetics of Q- accumulation in the absorbance change measurements were found to be biphasic and strictly correlated with the kinetics of the area growth measured under the same conditions. From the resolution of the biphasic kinetics at different wavelengths in the UV region of the spectrum, it was found that both kinetic components in the system II photochemistry involve the reduction of a plastoquinone molecule to its plastosemiquinone anion. From the two kinetic components one was fast and non-exponential and the other relatively slow with an exponential time course. The initial rate difference in the kinetics of the two components was by a factor of approximately 3. A difference by a factor of about three was also found in the flash saturation curves of the two kinetic components.
The results are explained by the hypothesis that in higher plant chloroplasts there are system II reaction centers embedded in a large pigment matrix with statistical energy transfer, and system II reaction centers embedded in separate, in terms of excitation energy transfer, units. The effective absorption cross section per reaction center for the centers in the statistical pigment bed is approximately 3 times larger than that of the reaction centers in the separate system II units. The two types of system II reaction centers have different yields of excitation trapping and charge stabilization properties.     

酰基二茂铁在液相中的电子传递性质

酰基二茂铁在液相中的电子传递性质蒋朝阳邰子厚＊季斌（南京大学配位化学国家重点实验室，南京２１００９３）关键词：酰基二茂铁三相氧化还原体系电子传递二茂铁及其衍生物在生物模拟和光电材料中有着极高的应用价值［１－４］。本文合成了一系列的酰基二茂铁ＦｃＸ（Ｘ...     

INCREASED ENERGY TRANSFER FROM PHYCOBILISOMES TO PHOTOSYSTEM II IN HIGH LIGHT ADAPTED Anabaena cylindrica

Excitation energy transfer from phycobilisomes to photosystem II in high-light adapted cells of Anabaena cylindrica was studied by fluorescence spectroscopy and compared to that of low-light adapted cells. Measurements were made on membrane fragments containing phycobilisomes, photosystem I and II, isolated in 0.75 M K-phosphate. Relative efficiency of 430 to 590 nm light in the excitation of F₆₈₀ chlorophyll fluorescence was compared in low and high light adapted cells, respectively. The values indicate that light energy absorbed by phycobilisomes is transferred to photosystem II antenna chlorophylls with higher efficiency in high-light adapted cells than in low-light adapted cells. Partial dissociation and uncoupling of energy transfer caused by low ion concentration were different in the membrane fragments isolated from the two kinds of cells and indicated a higher aggregation state of pigment-protein complexes of phycobilisomes in high-light adapted A. cylindrica cells.     

RUTHENIUM RED INHIBITION OF OXYGEN EVOLUTION AND SPECIFIC RELEASE OF THE EXTRINSIC 16 kDa POLYPEPTIDE IN A PHOTOSYSTEM II PREPARATION

The inhibitory effect of the dye ruthenium red was studied in photosystem II-enriched submembrane fractions. A number of distinct types of interaction were found, which differed in their concentration range and required incubation time. Ruthenium red instantaneously quenches the initial chlorophyll a fluorescence level (F₀) and the maximum fluorescence level (F_m) by enhancing radiationless deactivation in the chlorophyll light harvesting complex. Associated with this quenching of fluorescence is an instantaneous decrease in the quantum yield of oxygen evolution. Ruthenium red also inhibited the light saturated rate of oxygen evolution and the variable fluorescence, monitored 80 µs after a saturating excitation-flash. These inhibitions increased with incubation time and became greater than 50% within 5 min. Although ruthenium red was known to affect Ca²⁺ or Cl^? sites specifically, the inhibitory action was more pronounced than simple Ca²⁺ or Cl^? depletion. Incubation with ruthenium red for 5 min blocks the Z P680⁺ → Z⁺ P680 charge transfer reaction. Upon mixing with the photosystem II preparation, ruthenium red induced specific release of the extrinsic 16 kDa polypeptide associated with water-splitting without release of Mn. It is proposed that the inhibitor produces an ionic imbalance which alters the configuration of the donor side of photosystem II.     

酪胺酸-色胺酸缩聚二肽分子内电子转移速率常数的从头算研究

用量子化学从头算方法对色氨酸－酪氨酸缩聚二肽体系进行电子转移动力学参数的计算。用ＵＨＦ／６－３１Ｇ方法分别优化给体,受体和桥体的几何构型,用线性反应坐标构造了给体和受体分子间电子转移的双势阱,得到两透热势能面在Ｒｃ约为０处交叉,表面气相反应为无能垒过程。     

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.