Ps Fraction Research Articles

Photoinactivation of photosystem II complexes and photoprotection by non-functional neighbours in Capsicum annuum L. leaves.

Leaf segments from Capsicum annuum plants grown at 100 micromol photons m(-2) s(-1) (low light) or 500 micromol photons m(-2) s(-1) (high light) were illuminated at three irradiances and three temperatures for several hours. At various times, the remaining fraction (f) of functional photosystem II (PS II) complexes was measured by a chlorophyll fluorescence parameter (1/Fo -1/Fm, where Fo and Fm are the fluorescence yields corresponding to open and closed PS II traps, respectively), which was in turn calibrated by the oxygen yield per saturating single-turnover flash. During illumination of leaf segments in the presence of lincomycin, an inhibitor of chloroplast-encoded protein synthesis, the decline of f from 1.0 to about 0.3 was mono-exponential. Thereafter, f declined much more slowly, the remaining fraction (approximately equals 0.2) being able to survive prolonged illumination. The results can be interpreted as being in support of the hypothesis that photoinactivated PS II complexes photoprotect functional neighbours (G. Oquist et al. 1992, Planta 186: 450-460), provided it is assumed that a photoinactivated PS II is initially only a weak quencher of excitation energy, but becomes a much stronger quencher during prolonged illumination when a substantial fraction of PS II complexes has also been photoinactivated. In the absence of lincomycin, photoinactivation and repair of PS II occur in parallel, allowing f to reach a steady-state value that is determined by the treatment irradiance, temperature and growth irradiance. The results obtained in the presence and absence of lincomycin are analysed according to a simple kinetic model which formally incorporates a conversion from weak to strong quenchers, yielding the rate coefficients of photoinactivation and of repair for various conditions, as well as gaining an insight into the influence off on the rate coefficient of photoinactivation. They demonstrate that the method is a convenient alternative to the use of radiolabelled amino acids for quantifying photoinactivation and repair of PS II in leaves.

Temperature dependence of dynamics of solutions of triblock copolymer in a selective solvent

The temperature dependence of dynamic properties of solutions of a triblock copolymer with polystyrene outer blocks and a middle block of fully hydrogenated polybutadiene (Mw=7.0×104gmol−1, mass fraction of PS 0.28) was studied in a selective solvent for the middle block, n-heptane. In dilute solutions, a gradual decomposition of the flower-like micelles was observed on heating. A more complex behavior was observed in semidilute solutions of the copolymer where three dynamic processes can be reliably extracted from the correlation functions. The fast mode corresponds to the collective diffusion mode of physically interconnected micelles, the broad middle mode is probably due to the tracer diffusion of polydispersed clusters (animal-like structures) formed by random association of the triblock copolymer and the slow mode is probably related to the diffusion of aggregates of insoluble polystyrene impurities. The crossover from relaxational to diffusive behavior was found for samples with c>5.5%(w/v). The scattered light intensity measurements revealed a phase transition for the solution with the concentration 4% (w/v) at a temperature of 79±2°C, which was related to the phase separation of polystyrene homopolymer present in a small amount in the copolymer used.

A Comparative Experimental and Theoretical Study between Heteroarm Star and Diblock Copolymers in the Microphase Separated State

A pronounced difference in the behavior of heteroarm star copolymers from the conventional diblock copolymers is analyzed theoretically and experimentally on the example of symmetric polystyrene-poly(2-vinylpyridine) (PS6P2VP6) star block copolymer melts. Microphase separation of PS6P2VP6 stars with volume fraction of PS varying from 0.30 to 0.75 is studied using transmission electron microscopy (TEM). The observed (lamellar, cylindrical and undefined “bicontinuous-like”) structures often turned out to be different from that expected for the corresponding diblock copolymers. The dependence of the long period of lamellar structures of heteroarm star block copolymers on block length is found to deviate in a systematic way from N2/3 law (known for diblock copolymers), especially for the stars of high functionality. The theoretical model proposed to describe an extra-stretching of star block copolymers near the lamellar interface is found to be successful in prediction of block length and star functionality d...

Effects of azide on the S(2) state EPR signals from Photosystem II.

The anion azide, N(3) (-), has been previously found to be an inhibitor of oxygen evolution by Photosystem II (PS II) of higher plants. With respect to chloride activation, azide acts primarily as a competitive inhibitor but uncompetitive inhibition also occurs [Haddy A, Hatchell JA, Kimel RA and Thomas R (1999) Biochemistry 38: 6104-6110]. In this study, the effects of azide on PS II-enriched thylakoid membranes were characterized by electron paramagnetic resonance (EPR) spectroscopy. Azide showed two distinguishable effects on the S(2) state EPR signals. In the presence of chloride, which prevented competitive binding, azide suppressed the formation of the multiline and g = 4.1 signals concurrently, indicating that the normal S(2) state was not reached. Signal suppression showed an azide concentration dependence that correlated with the fraction of PS II centers calculated to bind azide at the uncompetitive site, based on the previously determined inhibition constant. No evidence was found for an effect of azide on the Fe(II)Q(A) (-) signals at the concentrations used. This result is consistent with placement of the uncompetitive site on the donor side of PS II as suggested in the previous study. In chloride-depleted PS II-enriched membranes azide and fluoride showed similar effects on the S(2) state EPR signals, including a notable increase and narrowing of the g = 4.1 signal. Comparable effects of other anions have been described previously and apparently take place through the chloride-competitive site. The two azide binding sites described here correlate with the results of other studies of Lewis base inhibitors.

Preparation of Telechelic Polysilylenes: A General Methodology for the Synthesis of Polysilylene-Based Triblock Copolymers

Telechelic polysilylenes have been developed involving the chemical degradation of poly(di-n-hexylsilylene) with bromine. The molecular weight of the newly formed telechelic PDHS depends on the ratio of bromine to the starting PDHS. The percentage of end groups increases as the molecular weight of the telechelic PDHS is lowered, reaching a maximum of 99% difunctionality. Polystyrene−PDHS−polystyrene triblock copolymers have been prepared by coupling polystyryllithium to the telechelic PDHS in the presence of cryptand [2.1.1]. The effect of molecular weight on the disordering transition of PDHS into a columnar mesophase has been studied, and temperature and enthalpy have been shown to decrease by the presence of the PS end groups and eventually became unobservable at high fractions of PS. The morphologies of the triblock copolymers were studied by atomic force microscopy.

Ultra-small-angle x-ray scattering studies on order-disorder transition in diblock copolymers

The phase behavior near the order-disorder transition (ODT) in diblock copolymers was characterized by ultra-small-angle x-ray scattering (USAXS) method for two polystyrene-block-polyisoprene (PS-b-PI) copolymers having about equal block volume fraction f=0.45 (f is the volume fraction of PS) but different molecular weights. By using USAXS method, the peak width and peak intensity were found to change by about 2 orders of magnitude across the ODT temperature for both PS-b-PI. Furthermore, in a narrow temperature region very close to the ODT, it is revealed that the coexistence of the ordered and the disordered phases occurs at thermal equilibrium for both PS-b-PI. The systematic results in the present study clearly show the thermal fluctuation effects on the phase diagram in the low molecular weight diblock copolymer systems.

Performance of active Photosystem II centers in photoinhibited pea leaves

Effects of photoinhibition on photosynthesis in pea (Pisum sativum L.) leaves were investigated by studying the relationship between the severity of a photoinhibitory treatment (measured as Fv/Fm) and several photoacoustic and chlorophyll a fluorescence parameters. Because of the observed linear relationship between the decline of Fv/Fm and the potential oxygen evolution rate determined by the photoacoustic method, the parameter Fv/Fm was used as an indicator for the severity of photoinhibition. Our analysis revealed that part of the Photosystem II (PS II) reaction centers is inactive in oxygen evolution and is also less sensitive to photoinhibition. Correcting the parameter qP (fraction of open PS II reaction centers) for inactive PS II centers unveiled a strong increase of qP in severely inhibited pea leaves, indicating that the inactivated active centers do no longer contribute to qP and that photoinhibition has an all or none effect on PS II centers. Analysis of qE (energy quenching) demonstrated its initial increase possibly associated with dephosphorylation of LHC II. Analysis of qI (photoinhibition dependent quenching) showed that the half-time of recovery of qI increases steeply below an Fv/Fm of 0.65. This increase of the relaxation half-time corresponds with a decrease of the electron transport rate J and tentatively indicates that the supply of ATP, needed for the recovery, starts to decrease. The data indicate the necessity of correcting for inactive centers in order to make valuable conclusions about effects of photoinhibition on photosynthetic parameters.

Positron spin-relaxation (SR) study of carbon phases, SiC, and fused quartz

The positron spin-relaxation method permits the detection and investigation of positronium (Ps) in condensed matter with very small formation probabilities. In a set-up that makes use of spin-polarized positrons emitted from a source, the effect of reversals of an external magnetic field applied longitudinally to the positron spin polarization on the Doppler broadening of the annihilation photon line is measured. For the first time, the fractions r of Ps-forming positrons and normalized electron densities at the positron in Ps have been determined for natural type-IIa diamond and crystalline SiC . The two materials show a hitherto not observed anomalous dependence of the Doppler broadening on the magnetic field strength that indicates that a large Ps fraction annihilates before becoming thermalized. Fused quartz , fullerite , and highly orientated pyrolytic graphite showed the `normal' field dependence.

Temperature dependence of forward and reverse electron transfer from A1-, the reduced secondary electron acceptor in photosystem I.

Electron-transfer reactions following the formation of P700(+)A1- have been studied in isolated Photosystem I complexes from Synechococcus elongatus between 300 and 5 K by flash absorption spectroscopy. (1) In the range from 300 to 200 K, A1- is reoxidized by electron transfer to the iron-sulfur cluster FX. The rate slows down with decreasing temperature, corresponding to an activation energy of 220 +/- 20 meV in this temperature range. Analyzing the temperature dependence of the rate in terms of nonadiabatic electron-transfer theory, one obtains a reorganization energy of about 1 eV and an edge-to-edge distance between A1 and FX of about 8 A assuming the same distance dependence of the electron-transfer rate as in purple bacterial reaction centers. (2) At temperatures below 150 K, different fractions of PS I complexes attributed to frozen conformational substates can be distinguished. A detailed analysis at 77 K gave the following results: (a) In about 45%, flash-induced electron transfer is limited to the formation and decay of the secondary pair P700(+)A1-. The charge recombination occurs with a t1/2 of about 170 micros. (b) In about 20%, the state P700(+)FX- is formed and recombines with complex kinetics (t1/2 = 5-100 ms). (c) In about 35%, irreversible formation of P700(+)FA- or P700(+)FB- is possible. (3) The transition from efficient forward electron transfer at higher temperatures to heterogeneous photochemistry at low temperatures has been investigated in different glass-forming solvents. The yield of forward electron transfer to the iron-sulfur clusters decreases in a narrow temperature interval. The temperature of the half-maximal effect varies between different solvents and appears to be correlated with their liquid to glass transition. It is proposed that reorganization processes in the surroundings of the reactants which are required for the stabilization of the charge-separated state become arrested near the glass transition. This freezing of protein motions and/or solvent reorganization may affect electron-transfer reactions through changes in the free-energy gap and the reorganization energy. (4) The rate of charge recombination between P700(+) and A1- increases slightly (about 1.5-fold) when the temperature is decreased from 300 to 5 K. This charge recombination characterized by a large driving force is much less influenced by the solvent properties than the forward electron-transfer steps from A1- to FX and FA/B.

Effects of CCl 4 on positronium formation in pure isooctane and in AOT/water/isooctane microemulsions

Previous work, on positron lifetime spectroscopy (LS) measurements on AOT/water/isooctane microemulsions showed that most positronium (Ps) is formed in the aqueous subphase, and predicted that a water insoluble electron scavenger should inhibit much less Ps formation than if Ps was mainly formed in the organic solvent. To test this prediction, Ps inhibition by CCl 4 is examined in pure isooctane and in the micellar system. In the former case, from the combined analysis of LS and Doppler broadening results, CCl 4 is found to strongly inhibit Ps formation ( k = 36.6 M −1) by the successive capture of e −, to form CCl 4 −, and of e +, to form the PsCCl 4 bound-state. As expected, CCl 4 in the micellar system displays a much lower ability to inhibit Ps formation ( k ca. 9 M −1). However, a very good agreement with the data is only obtained by assuming that, besides Ps inhibition in the organic solvent, CCl 4 also promotes some inhibition of that fraction of Ps formed in the aqueous subphase. Quantitatively, the low inhibition constant found for this process (5.1 M −1) is quite consistent with what is expected when considering the fraction of the aqueous positron spurs that overlap with the organic subphase. Another hypothesis is discussed, by which CCl 4 would decrease the probability that the positrons become trapped in the aqueous subphase, due to an increase in the electron density of the organic solvent.

Graft Copolymers of Polypropylene Films. 1. Radiation-induced Grafting of Mixed Monomers

Radiation graft copolymerization of comonomer mixtures of acrylic acid (AAc) and styrene (S) onto polypropylene (PP) films by the mutual method has been investigated. The effect of different factors that may affect the grafting yield, such as inhibitor concentration (Mohr’s salt), solvent composition (MeOH and H2O), radiation dose and dose rate, was studied. It was found that Mohr’s salt was very effective when the content of AAc in the comonomer mixtures was low. However, the addition of 1·25wt% of Mohr’s salt reduced homopolymer formation and enhanced the grafting process. Graft copolymerization in the presence of a solvent mixture composed of methanol and water was found to afford a higher grafting yield than in pure methanol, regardless of the composition of the comonomer mixture used. However, the highest degree of grafting was obtained at a solvent composition of 20% H2O: 80% MeOH and a comonomer mixture of 20% AAc: 80% S. An attempt was made to determine each PAAc and PS fraction by different methods in the graft copolymer obtained. Elemental analysis indicated that the PAAc fraction with respect to PS in the graft copolymer decreased with increasing AAc ratio in the comonomer feed solution. The rough assessment of these fractions by IR spectroscopy showed similar trends. The reactivity ratios of AAc and S monomers determined in the present graft copolymerization system were found to be 0·45 and 1·3, respectively. © of SCI.

Morphology and physical properties of poly(styrene-b-isobutylene-b-styrene) block copolymers

Abstract A series of linear and three-arm star poly(styrene-b-isobutylene-b-styrene) (PS-PIB-PS) block copolymers with varying block compositions was synthesized via living carbocationic polymerization using the initiation system 1,3-di(2-chloro-2-propyl)-5-tert-butylbenzene (or 1,3,5-tris-(2-chloro-2-propyl)benzene)/TiCl4/pyridine/2,6-di-tert-butylpyridine in a 60 40 (v/v) hexane/MeCl solvent mixture at −80°C. High resolution gel permeation chromatography showed that the compositions of the copolymers were complex, consisting of higher molecular weight coupled products, and products of lower molecular weight including, probably, homo-PS. Morphology and physical properties were characteristic of microphase-separated block copolymers, and were affected strongly by the PIB span molecular weight and volume fraction of PS. When the latter was in the range 0.20–0.42 vol%, the morphology was characterized by cylinders of PS in a continuous phase of PIB; however, at least two samples exhibited mixed morphologies in which regions of PS cylinders coexisted with regions of PS spheres and lamellae, respectively. Linear samples showed much better long-range morphological order than star-branched samples. The dynamic mechanical response of block copolymers consisted of separate PIB and PS relaxations whose relative intensities scaled well with copolymer composition. The low-temperature relaxation was broad, which is an inherent characteristic of PIB; the high-temperature PS relaxation was narrow, suggesting well defined phase separation and a sharp PIB-PS interface. Melt rheological studies of representative samples showed that microphase separation persisted up to 265°C. Tensile properties varied with PIB span molecular weight and PS content. An abrupt change occurred from 15–22 vol% PS, indicating a minimum PS content needed for strong network formation. Properties changed from elastomeric within the range 20–37 vol% PS, to ductile in the range 37–45 vol%, to brittle in the range 45–55 vol%. Samples with PS vol% ⩾ 35 yielded improved elastomeric properties as a result of annealing above the glass transition temperature of PS. Within the elastomeric range, the PIB span molecular weight was the dominant factor effecting elongation at break. A high tensile strength of 24 MPa was obtained, and most samples displayed strengths ⩾ 15 MPa.

Cyclic electron flow around Photosystem II in vivo

The oxygen flash yield (YO2) and photochemical yield of PS II (ΦPS II) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, YO2 and ΦPS II followed each other, suggesting a close coupling between the oxidation of water and QA reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432-443). Following a light-to-dark transition, however, the relationship between QA reduction and the fraction of PS II reaction centers capable of evolving O2 became temporarily uncoupled. ΦPS II recovered to the preillumination levels within 5-10 s, while the YO2 required up to 60 s to recover under aerobic conditions. The recovery of YO2 was independent of the redox state of QA, but was accompanied by a 30% increase in the functional absorption cross-section of PS II (σPS II). The hysteresis between YO2 and the reduction of QA during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.

Thermoluminescence investigations on the site of action of o-phthalaldehyde in photosynthetic electron transport

Glow curves from spinach leaf discs infiltrated with o-phthalaldehyde (OPA) show significant similarity to those obtained by DCMU treatment which is known to block the electron flow from QA, the stable acceptor of Photosystem II (PS II). In both the cases, the thermoluminescence (TL) peak II (Q band) was intensified significantly, whereas peaks III and IV (B band) were suppressed. Total TL yield of the glow curve remained constant even when the leaf discs were infiltrated with high concentrations of OPA (4 mM) or with DCMU (100 μM), indicating that even at these high concentrations no significant change in the number of species undergoing charge recombination in PS II occurred. However, studies with thylakoids revealed significant differences in the action of OPA and DCMU on PS II. Although OPA, at a certain concentration and time of incubation, reduced the B band intensity by about 50-70%, and completely abolished the detectable oxygen evolution, it still retained the TL flash yield pattern, and, thus, S state turnover. OPA is known to inhibit the oxidoreductase activity of in vitro Cyt b6/f (Bhagwat et al. (1993) Arch Biochem Biophys 304: 38-44). However, in the OPA treated thylakoids the extent of inhibition of O2 evolution was not reduced even in the presence of oxidized tetramethyl-p-phenylenediamine which accepts electrons from plastoquinol and feeds then directly to Photosystem I. This suggests that OPA inhibition is at a site prior to plastoquinone pool in the electron transport chain, in agreement with it being between QA and QB. However, an unusual feature of OPA inhibition is that even though all oxygen evolution was completely suppressed, a significant fraction of PS II centers were functional and turned over with the same periodicity of four in the absence of any added electron donor, an observation which appears to be similar to that reported by Wydrzynski (Wydrzynski et al. (1985) Biochim Biophys Acta 809: 125-136) with lauroylcholine chloride, a lipid analogue compound. The detailed chemistry of OPA inhibition remains to be studied. Since we dedicate this paper to William A. Arnold, discoverer of delayed light and TL in photosynthesis, we have also included in the Introduction, a brief history of how TL work was initiated at BARC (Bombay, India).

Extension of the PC version of VEPFIT with input and output routines running under Windows

The fitting program VEPFIT has been extended with applications running under the Microsoft-Windows environment facilitating the input and output of the VEPFIT fitting module. We have exploited the Microsoft-Windows graphical users interface by making use of dialog windows, scrollbars, command buttons, etc. The user communicates with the program simply by clicking and dragging with the mouse pointing device. Keyboard actions are limited to a minimum. Upon changing one or more input parameters the results of the modeling of the S-parameter and Ps fractions versus positron implantation energy are updated and displayed. This action can be considered as the first step in the fitting procedure upon which the user can decide to further adapt the input parameters or to forward these parameters as initial values to the fitting routine. The modeling step has proven to be helpful for designing positron beam experiments.

Effect of chloroquine on mast cell membranes

The effect of chloroquine (CQ) on phospholipid turnover and de novo synthesis in isolated rat mast cells (IRMC) was studied by determining the incorporation of 32P and 14C-glycerol into IRMC phospholipids. Incubation of mast cells with chloroquine increased 32P incorporation into PI and PS whilst it decreased 32P incorporation into PC, PE and PA. In mast cells pretreated with CQ and subsequently stimulated with compound 48/80, 32P incorporation into PI, PS and PA fractions was enhanced, while it was decreased into PC and PE, in comparison to 48/80 stimulated IRMC. 14C-glycerol incorporation into total IRMC phospholipids was not significantly changed by CQ and compound 48/80 treatment and neither was any dose-dependent effect of CQ on individual phospholipids detected. Our results indicate that chloroquine, similarly to other cationic amphiphilic drugs, may alter membrane PL turnover without changing de novo synthesis of phospholipids.

Light-harvesting chlorophyll a-b complex requirement for regulation of Photosystem II photochemistry by non-photochemical quenching

Recently, it has been suggested (Horton et al. 1992) that aggregation of the light-harvesting a-b complex (LHC II) in vitro reflects the processes which occur in vivo during fluorescence induction and related to the major non-photochemical quenching (qE). Therefore the requirement of this chlorophyll a-b containing protein complex to produce qN was investigated by comparison of two barley mutants either lacking (chlorina f2) or depressed (chlorina(104)) in LHC II to the wild-type and pea leaves submitted to intermittent light (IL) and during their greening in continuous light.It was observed that qN was photoinduced in the absence of LHC II, i.e. in IL grown pea leaves and the barley mutants. Nevertheless, in these leaves qN had no (IL, peas) or little (barley mutants) inhibitory effect on the photochemical efficiency of QA reduction measured by flash dosage response curves of the chlorophyll fluorescence yield increase induced by a single turn-over flashDuring greening in continuous light of IL pea leaves, an inhibitory effect on QA photoreduction associated to qN developed as Photosystem II antenna size increased with LHC II synthesis. Utilizing data from the literature on connectivity between PS II units versus antenna size, the following hypothesis is put forward to explain the results summarized above. qN can occur in the core antenna or Reaction Center of a fraction of PS II units and these units will not exhibit variable fluorescence. Other PS II units are quenched indirectly through PS II-PS II exciton transfer which develops as the proportion of connected PS II units increases through LHC II synthesis.

Angular correlation studies of positron annihilation in the noble gases

Two-dimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in low-density helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values. The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously determined in lifetime measurements; the widths of the Ps component in helium, neon and argon are consistent with a Ps-atom scattering cross section below 6.8 eV in all three gases of approximately (9-0.5 E) pi a02, where E is the Ps energy in eV.

Divalent cation-induced lipid mixing between phosphatidylserine liposomes studied by stopped-flow fluorescence measurements: effects of temperature, comparison of barium and calcium, and perturbation by DPX.

To understand the mechanism of membrane fusion, it is important to study the processes that mix the lipids of two apposed membranes. We measured the rates of divalent cation-induced aggregation and lipid mixing of bovine brain phosphatidylserine (BBPS) LUV, using light scattering and a resonance energy transfer assay. The lipid and divalent cation solutions were combined by stopped-flow mixing, which permitted measuring the half-times of aggregation and lipid mixing between pairs of liposomes. The collisional quencher DPX [p-xylene-bis(pyridinium bromide)], used in a liposome contents-mixing assay, lowered the main transition temperature (Tm) of BBPS by about 10 degrees C and decreased the temperature threshold for lipid mixing. Since DPX was inside the liposomes for the latter measurements, this implies that perturbations to the inner monolayer affect the reactivity of the liposome. When palmitoyl-oleoyl-PS (POPS) was substituted for BBPS, little or no lipid mixing occurred. Ca(2+)- and Ba(2+)-induced BBPS aggregation and lipid mixing were compared as a function of temperature and divalent cation concentration. Aggregation rates were nearly insensitive to temperature and correlated with the percent of PS bound to either Ba2+ or Ca2+. Above Tm, lipid-mixing rates increased with the Ba2+ and Ca2+ concentrations and temperature, even above the Tm of the Ba2+/PS complex. Arrhenius plots were linear for both ions. The temperature dependence was greater for Ca(2+)- than Ba(2+)-induced reactions, and the slopes were independent of divalent cation concentration. When equivalent fractions of PS were bound with divalent cation at, and above, 20 degrees C, the lipid-mixing rate was greater with Ca2+ than with Ba2+. The faster rate may reflect greater activation entropies and/or greater attempt frequencies at one or more steps in the Ca(2+)-induced process. We conclude that stopped-flow mixing permits better characterization of initial interaction between liposomes, that small changes in the acyl chain region of the PS bilayer or the inner monolayer can have large effects on lipid-mixing rates, and that the differences between Ba(2+)- and Ca(2+)-induced interactions may be related to qualitative differences in the destabilization step.

Properties of inactive Photosystem II centers

A fraction (usually in the range of 10-25%) of PS II centers is unable to transfer electrons from the primary quinone acceptor QA to the secondary acceptor QB. These centers are inactive with respect to O2 evolution since their reopening after photochemical charge separation to the S2OA (-) state involves predominantly a back reaction to S1QA in the few seconds time range (slower phases are also occurring). Several properties of these centers are analyzed by fluorescence and absorption change experiments. The initial rise phase Fo-Fpl of fluorescence induction under weak illumination reflects both the closure of inactive centers and the modulation of the fluorescence yield by the S-states of the oxygen-evolving system: We estimate typical relative amplitudes of these contributions as, respectively, 65 and 35% of the Fo-Fpl amplitude. The half-rise time of this phase is significantly shorter than for the fluorescence induction in the presence of DCMU (in which all centers are involved). This finding is shown to be consistent with inactive centers sharing the same light-harvesting antenna as normal centers, a view which is also supported by comparing the dependence of the fluorescence yield on the amount of closed active or inactive centers estimated through absorption changes. It is argued that the exponential kinetics of the Fo-Fpl phase does not indicate absence of excitation energy transfer between the antennas of inactive and active centers. We show that the acceptor dichlorobenzoquinone does not restore electron transfer in inactive centers, in disagreement with previous suggestions. We confirm, however, the enhancement of steady-state electron flow caused by this quinone and suggest that it acts by relieving a blocking step involved in the reoxidation of a fraction of the plastoquinone pool. Part of the discrepancies between the present results and those from previous literature may arise from the confusion of inactive centers characterized on a single turnover basis and PS II centers that become blocked under steady-state conditions because of deficient reoxidation of their secondary acceptors.