Identification of the 120 mus phase in the decay of delayed fluorescence in spinach chloroplasts and subchloroplast particles as the intrinsic back reaction. The dependence of the level of this phase on the thylakoids internal pH

Antioxidant Activity of Beef, Pork and Chicken Burgers before and after Cooking and after In Vitro Intestinal Digestion

The aim of the present work was to evaluate and compare in vitro the antioxidant activity of raw, cooked and cooked-digested pork, beef and chicken burgers. The cooking process influenced the antioxidant capacity of the meat by decreasing the values of ABTS, FRAP and the content of free thiols. Conversely, a positive effect was observed after in vitro gastrointestinal digestion which increased the biological activity of the meat, characterised by greater antioxidant activity. The type of meat influenced the chemical composition and biological capacity of the burgers. In fact, both before and after the cooking process, beef burgers showed higher thiol content and, consequently, a higher oxidative stability of proteins than chicken and pork burgers. In vitro gastrointestinal digestion also improved the nutraceutical quality of beef burgers, which showed higher ABTS values and thiol content than pork burgers, which showed higher FRAP values. This work aims to support the potential of meat constituents as a natural antioxidant component that is essential to counteract the oxidative stress responsible for imbalances in the human organism and several cardiovascular diseases.

Novel Therapeutic Approaches to Invasive Candidiasis: Considerations for the Clinician

Invasive candidiasis (IC), due to the yeast pathogen Candida, is still a major cause of in-hospital morbidity and mortality. The limited number of antifungal drug classes and the emergence of multi-resistant Candida species, such as Candida auris and some Candida glabrata isolates, is concerning. However, recent advances in antifungal drug development provide promising perspectives for the therapeutic approach of IC. Notably, three novel antifungal agents, currently in Phase II/III clinical trials, are expected to have an important place for the treatment of IC in the future. Rezafungin is a novel echinocandin with prolonged half-life. Ibrexafungerp and fosmanogepix are two first-in-class antifungal drugs with broad spectrum activity against Candida spp., including C. auris and echinocandin-resistant species. These novel antifungal agents also represent interesting alternative options because of their acceptable oral bioavailability (ibrexafungerp and fosmanogepix) or their large interdose interval (once weekly intravenous administration for rezafungin) for prolonged and/or outpatient treatment of complicated IC. This review discusses the potential place of these novel antifungal drugs for the treatment of IC considering their pharmacologic properties and their preclinical and clinical data.

Jean Lavorel (1928-2021): a great scientist, a model of integrity

This paper is a tribute to a great scientist and an authentic "honest man" that was Jean Lavorel (1928-2021). He was a pioneer in research on the primary events of photosynthesis in algae, plants, and photosynthetic bacteria. He focused his attention on chlorophyll fluorescence and luminescence, and also on oxygen evolution, both experimentally (with laboratory-built refined apparatus) and theoretically. Many of his results are classical now. Besides a survey of his main achievements, most of them obtained by him alone, different reminiscences on the researcher and the person he was illustrate the rich personality of Jean Lavorel.

Significant biophysical and genetic properties of maize inbred lines and hybrids with erect top leaves

This study confirms the hypothesis that there are elite maize inbred lines and hybrids with erect top leaves, which have a dominant property of an efficient photosynthetic and fluorescent model that is successfully used in modern breeding programmes and the production hybrid seed and commercial maize. This statement is supported by the displayed results on the erect top leaves, the dynamics of grain dry-down during the maturation period and photosynthetic and florescence parameters: the temperature dependence of the delayed chlorophyll fluorescence intensity, the Arrhenius criterion for the determination of critical temperatures (phase transition temperatures) and the activation energies. The presented results show that properties of observed maize inbreds and their hybrids are based on the nature of conformational and functional changes that occur in their thylakoid membranes and other chemical tissues structures of grain and intact leaves, as well as, on positive effects in maize breeding. Moreover, other relevant significant breeding and seed production properties (commercial maize quality over grain structure, physical and chemical parameters) of maize inbred lines and their hybrids were analyzed in the present study.

Observation of nanosecond laser induced fluorescence of in vitro seawater phytoplankton

Seawater has been irradiated using a train of 70 ns flashes from a 440 nm laser source. This wavelength is on resonance with the blue absorption peak of Chlorophyll pigment associated with the photosystem of in vitro phytoplankton. The resulting fluorescence at 685 nm is instantaneously recorded during each laser pulse using a streak camera. Delayed fluorescence is observed, yielding clues about initiation of the photosynthetic process on a nanosecond time scale. Further data processing allows for determination of the functional absorption cross section, found to be 0.0095 A(2), which is the first reporting of this number for in vitro phytoplankton. Unlike other flash-pump studies of Chlorophyll, using a LED or flashlamp-based sources, the short laser pulse used here does not reveal any pulse-to-pulse hysteresis (i.e., variable fluorescence), indicating that the laser pulses used here are not able to drive the photosynthetic process to completion. This is attributed to competition from a back reaction between the photoexcited photosystem II and the intermediate electron acceptor. The significance of this work as a new type of deployable ocean fluorimeter is discussed, and it is believed the apparatus will have applications in thin-layer phytoplankton research.

Efficiency and role of loss processes in light-driven water oxidation by PSII

Its superior quantum efficiency renders PSII a model for biomimetic systems. However, also in biological water oxidation by PSII, the efficiency is restricted by recombination losses. By laser-flash illumination, the secondary radical pair, P680(+)Q(-) (A) (where P680 is the primary Chl donor in PSII and Q(A), primary quinone acceptor of PSII), was formed in close to 100% of the PSII. Investigation of the quantum efficiency (or yield) of the subsequent steps by time-resolved delayed (10 micros to 60 ms) and prompt (70 micros to 700 ms) Chl fluorescence measurements on PSII membrane particles suggests that (1) the effective rate for P680(+) Q(-) (A) recombination is approximately 5 ms(-1) with an activation energy of approximately 0.34 eV, circumstantially confirming dominating losses by reformation of the primary radical pair followed by ground-state recombination. (2) Because of compensatory influences on recombination and forward reactions, the efficiency is only weakly temperature dependent. (3) Recombination losses are several-fold enhanced at lower pH. (4) Calculation based on delayed-fluorescence data suggests that the losses depend on the state of the water-oxidizing manganese complex, being low in the S(0)-->S(1) and S(1)-->S(2) transition, clearly higher in S(2)-->S(3) and S(3)-->S(4)-->S(0). (5) For the used artificial electron acceptor, the efficiency is limited by acceptor-side processes/S-state decay at high/low photon-absorption rates resulting in optimal efficiency at surprisingly low rates of approximately 0.15-15 photons s(-1) (per PSII). The pH and S-state dependence can be rationalized by the basic model of alternate electron-proton removal proposed elsewhere. A physiological function of the recombination losses could be limitation of the lifetime of the reactive donor-side tyrosine radical (Y(.) (Z)) in the case of low-pH blockage of water oxidation.

Photosynthetic dioxygen formation studied by time-resolved delayed fluorescence measurements--method, rationale, and results on the activation energy of dioxygen formation

The analysis of the time-resolved delayed fluorescence (DF) measurements represents an important tool to study quantitatively light-induced electron transfer as well as associated processes, e.g. proton movements, at the donor side of photosystem II (PSII). This method can provide, inter alia, insights in the functionally important inner-protein proton movements, which are hardly detectable by conventional spectroscopic approaches. The underlying rationale and experimental details of the method are described. The delayed emission of chlorophyll fluorescence of highly active PSII membrane particles was measured in the time domain from 10 mus to 60 ms after each flash of a train of nanosecond laser pulses. Focusing on the oxygen-formation step induced by the third flash, we find that the recently reported formation of an S4-intermediate prior to the onset of O-O bond formation [M. Haumann, P. Liebisch, C. Müller, M. Barra, M. Grabolle, H. Dau, Science 310, 1019-1021, 2006] is a multiphasic process, as anticipated for proton movements from the manganese complex of PSII to the aqueous bulk phase. The S4-formation involves three or more likely sequential steps; a tri-exponential fit yields time constants of 14, 65, and 200 mus (at 20 degrees C, pH 6.4). We determine that S4-formation is characterized by a sizable difference in Gibbs free energy of more than 90 meV (20 degrees C, pH 6.4). In the second part of the study, the temperature dependence (-2.7 to 27.5 degrees C) of the rate constant of dioxygen formation (600/s at 20 degrees C) was investigated by analysis of DF transients. If the activation energy is assumed to be temperature-independent, a value of 230 meV is determined. There are weak indications for a biphasicity in the Arrhenius plot, but clear-cut evidence for a temperature-dependent switch between two activation energies, which would point to the existence of two distinct rate-limiting steps, is not obtained.

Dynamics of generating transients of delayed fluorescence induction signal and photosynthetic antennas: A possible relationship: Mathematical modeling approach

A mathematical model was developed for resolved temporal transients of experimentally recorded delayed fluorescence (DF) induction signal. During an intermittent light regime, antennas of the photosynthetic apparatus were treated as targets, repeatedly hit by potentially absorbable photons within a series of consecutive light flashes. Formulas were derived for the number of antennas, cumulatively hit by a specific number of photons, as function of the flash serial number (time). Model parameters included: number of absorbable photons in one flash, antenna sizes and numbers. A series of induction curves were analyzed, obtained from a ZeamaysL. leaf segment and differing in the previous dark period (td). Each curve, consisting of the two most prominent DF transients (C and D), was fitted with several model types, differing in the number of absorbed photons. For both transients, the best fitting result was achieved when DF induction was linked to the second absorbed photon. As expected, model parameters related to antenna sizes showed weaker dependence on td than those referring to antenna numbers. With restrictions applied in this model, the two DF induction transients may be related to two classes of photosynthetic antennas. Their different sizes may have a predominant influence on the efficiency of photon absorption, and possibly time-dependent appearance of DF transients.

Advances on photosystem II investigation by measurement of delayed chlorophyll fluorescence by a phosphoroscopic methods

A method for data acquisition based on recording of light signal from a conventional phophoroscope fluorometer with high-speed digitalization is proposed to extract more information from a delayed chlorophyll a fluorescence (DF) signal. During the signal processing from all points registered by the fluorometer, we obtain simultaneously a large number of induction curves of DF decaying at different time ranges. In addition, it is possible to register a series of dark relaxation kinetics of DF, recorded at different moments during the induction period or at different temperatures. This allows the evaluation of the contribution of DF kinetic components during the induction period or at different temperatures and the comparison between DF signals registered with different phophoroscopes. With the phosphoroscope system used in this study, we have shown that the contribution of the millisecond components (with lifetimes 0.6 and 2-4 ms) predominates during the first second of the induction period. After 1 s of illumination, the amplitudes of the 0.6 ms and 2-4 ms components and of the slower one (with lifetime more than 10 ms) become approximately equal. The change in lifetime of the different components during the induction and during gradual heating is also observed. It is shown that all registered DF kinetic components have different temperature dependences.

Chlorophyll luminescence as an indicator of stress-induced damage to the photosynthetic apparatus. Effects of heat-stress in isolated chloroplasts

A brief review is given of investigations on stres-induced alterations of ms-to s-luminescence yield of chlorophyll in plants. Three different approaches are considered: phytoluminography, luminescence-temperature curves, and luminescence induction curves. The remainder of this article presents new results of the effect of heat stress on luminescence induction curves of isolated chloroplasts. Three parameters with widely different heat resistances were resolved from induction curves. A fast valinomycin sensitive transient, L'i, with a 50% inhibition temperature of 33 to 34°C was correlated with the magnitude of the light-induced membrane potential after heat pretreatment. A slower nigericin sensitive transient, L'm, with a 50% inhibition temperature of 39 to 40°C was mainly correlated with the light-induced proton gradient. An uncoupler resistant part of the induction curve, L0, was enhanced by heat stress (half maximum after pretreatment at 46°C) and was correlated with the degree of inhibition of oxygen evolution. Since L0 was also raised by other treatments impairing the oxygen evolving enzyme system, and since this rise was inhibited by DCMU and hydroxylamine, this type of luminescence was ascribed to the intrinsic backreaction. We conclude that luminescence induction curves can serve as an useful indicator of the intactness of the membrane potential, the proton gradient, and the oxygen evolving enzyme system in isolated chloroplasts after heat stress.

O2-dependent electron flow, membrane energization and the mechanism of non-photochemical quenching of chlorophyll fluorescence

Recent progress in chlorophyll fluorescence research is reviewed, with emphasis on separation of photochemical and non-photochemical quenching coefficients (qP and qN) by the 'saturation pulse method'. This is part of an introductory talk at the Wageningen Meeting on 'The use of chlorophyll fluorescence and other non-invasive techniques in plant stress physiology'. The sequence of events is investigated which leads to down-regulation of PS II quantum yield in vivo, expressed in formation of qN. The role of O2-dependent electron flow for ΔpH- and qN-formation is emphasized. Previous conclusions on the rate of 'pseudocyclic' transport are re-evaluated in view of high ascorbate peroxidase activity observed in intact chloroplasts. It is proposed that the combined Mehler-Peroxidase reaction is responsible for most of the qN developed when CO2-assimilation is limited. Dithiothreitol is shown to inhibit part of qN-formation as well as peroxidase-induced electron flow. As to the actual mechanism of non-photochemical quenching, it is demonstrated that quenching is favored by treatments which slow down reactions at the PS II donor side. The same treatments are shown to stimulate charge recombination, as measured via 50 μs luminescence. It is suggested that also in vivo internal thylakoid acidification leads to stimulation of charge recombination, although on a more rapid time scale. A unifying model is proposed, incorporating reaction center and antenna quenching, with primary control of ΔpH at the PS II reaction center, involving radical pair spin transition and charge recombination to the triplet state in a first quenching step. In a second step, triplet excitation is trapped by zeaxanthin (if present) which in its triplet excited state causes additional quenching of singlet excited chlorophyll.

Modification of oxygen evolving center by Tris-washing

Tris-washing inhibits the O2-evolving center of chloroplasts and their particles specifically and reversibly, and it was applied to many investigations on O2-evolving center and PS II reaction center. In this review are introduced the various photosynthetic investigations in which Tris-washing was applied and are also discussed briefly on the site and the mechanism of Tris-inactivation, properties of P680 and Z, characteristic change in fluorescence and delayed light emission, and reactivation of O2-evolving center by DCPIP.H2-treatment and photo-reactivation of Tris-washed chloroplasts and their particles.

Temperature dependence of delayed chlorophyll fluorescence in intact leaves of higher plants. A rapid method for detecting the phase transition of thylakoid membrane lipids

The temperature dependence of the yield of in vivo prompt and delayed chlorophyll fluorescence was investigated in maize and barley leaves. In the chilling-sensitive maize, delayed fluorescence at steady-state level showed a maximum near the temperature at which thylakoid membrane lipids undergo a phase transition as revealed by differential scanning calorimetry measurements. In the chilling-resistant barley, no phase transition was detected above 0°C and the delayed light emission varied in a monotonic fashion. It was shown that measurements of delayed luminescence intensity in vivo can provide a rapid and sensitive method for detecting the phase change of membrane lipids in intact leaves of chilling-sensitive plant species such as tomato, cotton, cucumber, castor bean or avocado. In contrast, the use of steady-state prompt chlorophyll fluorescence as an indicator of membrane fluidity change was not successful.

Temperature dependence of delayed ehlorophyll fluorescence in intact leaves of higher plants. A rapid method for detecting the phase transition of thylakoid membrane lipids

The temperature dependence of the yield of in vivo prompt and delayed chiorophyll fluorescence was investigated in maize and barley leaves. In the chilling-sensitive maize, delayed fluorescence at steady-state level showed a maximum near the temperature at which thylakoid membrane lipids undergo a phase transition as revealed by differential scanning calorimetry measurements. In the chilling-resistant barley, no phase transition was detected above 0°C and the delayed light emission varied in a monotonic fashion. It was shown that measurements of delayed luminescence intensity in vivo can provide a rapid and sensitive method for detecting the phase change of membrane lipids in intact leaves of chilling-sensitive plant species such as tomato, cotton, cucumber, castor bean or avocado. In contrast, the use of steady-state prompt chlorophyll fluorescence as an indicator of membrane fluidity change was not successful.

Effects of hydroxylamine and silicomolybdate on the decay in delayed light emission in the 6-100 μs range after a single 10 ns flash in pea thylakoids

Measurements are reported on μs delayed light emission, following a single 10 ns excitation flash, in Alaska pea thylakoids treated with hydroxylamine (NH2OH) or with silicomolybdate. 1. In thylakoids treated with 2 mM NH2OH in the light, or in the dark, the quantum yield of delayed light emission is considerably enhanced. A 10 μs lifetime component of delayed light emission is not significantly changed, whereas a 50-70 μs lifetime component is increased. MnCl2 and diphenylcarbazide are unable to reverse the above effects of NH2OH treatment. Thus Mn(2+) and diphenylcarbazide must not donate electrons directly to reaction center II but on the oxygen-evolution side of the NH2OH block. 2. When the closed form of photosystem II reaction centers (P680Q(-)), where P680 is the reaction center chlorophyll and Q is a 'stable' electron acceptor, is generated by preillumination of NH2OH-treated thylakoids with diuron present, the μs delayed light emission is inhibited, but a low level residual delayed light emission remains. Possible origins of this emission are discussed. It is believed that the best explanation for residual DLE is the existence of another acceptor besides Q that partakes in charge separation and rapid dissipative recombination when the reaction center is in the P680Q(-) state. 3. The quantum yield of delayed light emission from 'closed' reaction centers (P680 (+)Q(-)) that have all charge stabilization reactions (i.e., flow of electrons to P680 (+) and out of Q(-)) blocked by NH2OH treatment and addition of diuron is 1.1×10(-3) for components measured in a range from 6 to 400 μs and extrapolated to zero time. 4. The addition of silicomolybdate, which accepts electron from Q(-), causes delayed light emission in the μs range to be greatly inhibited.

Influence of temperature on photosystem II electron transfer reactions

The influence of temperature on the rate of reduction of P-680+, the primary donor of Photosystem II, has been studied in the range 5-294 K, in chloroplasts and subchloroplasts particles. P-680 was oxidized by a short laser flash. Its oxidation state was followed by the absorption level at 820 nm, and its reduction attributed to two mechanisms: electron donation from electron donor D1 and electron return from the primary plastoquinone (back-reaction). Between 294 and approx. 200 K, the rate of the back-reaction, on a logarithmic scale, is a linear function of the reciprocal of the absolute temperature, corresponding to an activation energy between 3.3 and 3.7 kcal . mol-1, in all of the materials examined (chloroplasts treated at low pH or with Tris; particles prepared with digitonin). Between approx. 200 K and 5 K the rate of the back-reaction is temperature independent, with t 1/2 = 1.6 ms. In untreated chloroplasts we measured a t 1/2 of 1.7 ms for back-reaction at 77 and 5 K. The rate of electron donation from the donor D1 has been measured in dark-adapted Tris-treated chloroplasts, in the range 294-260 K. This rate is strongly affected by temperature. An activation energy of 11 kcal . mol-1 was determined for this reaction. In subchloroplast particles prepared with Triton X-100 the signals due to P-680+ were contaminated by absorption changes due to the triplet state of chlorophyll a. This triplet state has been examined with pure chlorophyll a in Triton X-100. An Arrhenius plot of its rate of decay shows a temperature-dependent region (292-220 K) with an activation energy of 9 kcal . mol-1, and a temperature-independent region (below 200 K) with t 1/2 = 1.1 ms.

A study of dark luminescence in Chlorella. Background luminescence, 3-(3,4-dichlorophenyl)-1,1-dimethylurea-triggered luminescence and hydrogen peroxide chemiluminescence

Dark luminescence, defined as the ability of completely relaxed (dark-adapted) photosynthetic systems to emit light, has been studied in Chlorella. Three main effects have been demonstrated. 3-(-3,4-Dichlorophenyl)-1,1-dimethylurea elicits a weak emission LD of very long lifetime (several minutes); it is believed to result from a negative shift of redox potential of the secondary System II electron acceptor B producing in some centers a state Q- (reduced primary acceptor), as postulated by Velthuys and Amesz ((1974 Biochim. Biophys. Acta 333, 85--94), which can recombine with an oxidizing equivalent in a state S2 present in very small amount. As in photoinduced luminescence, this recombination excites chlorophyll which then emits light. A much stronger emission LH is observed after injection of H2O2. Both signals are modified or suppressed by treatments specific of the oxygen emission system, such as: thermal denaturation at 50 degrees C, NH2OH, etc. In addition, a weak, permanent background luminescence L0 has been observed; like LD and and LH, it is a System II property and requires the integrity of the oxygen-evolving system. It is believed to reflect a very slow back flow of electrons from an endogeneous reductant pool to oxygen through part of the photosynthetic chain. Using flash preillumination, it is demonstrated that H2O2 is able to oxidize S0 into S2, the latter giving rise to LH; H2O2 does not act on S1 (or much less). The reactive site of H2O2 seems to be the same as the binding site of NH2OH. Evidence is given that the strong LH signal in particular reveals a stable, low pH of the intrathylakoid phase in Chlorella.

Effects of pH on reactions on the donor side of photosystem II

The effects of pH on the increase of fluorescence yield measured in the microsecond range, and on the microsecond delayed fluorescence have been studied in dark adapted chloroplasts as a function of flash number. (1) At pH 7, the amplitude of the fast-phase of the microsecond fluorescence yield rise oscillated as a function of flash number with period 4 and with maxima on flashes 1 and 5, and minima on flashes 3 and 7. The damped oscillations were apparent over the range between 6 and 8, although the absolute amplitude of the fast phase was diminished at the lower end of the range. At pH 4, there was no fast phase in the rise and, at pH 9, an enhanced fast-phase occurred only for the first flash. (2) The decay of microsecond delayed fluorescence was described by the sum of exponentials with half-times of 10--15 mus and 40--50 mus. Over the pH range 6- less than 8, the extrapolated initial amplitude and the proportion of the change due to the faster component showed oscillations which were opposite in phase to those observed for the prompt fluorescence yield rise; the slower component showed weaker oscillations of the same phase. At pH 4, there were no oscillations and the slow phase predominated. At pH 9, the delayed fluorescence intensity was diminished on the first flash, and high on subsequent flashes. (3) The results are interpreted in terms of a model in which protons are released during all transitions of the S-states with the exception of S1 leads to S2, and in which ther are two sites of inhibition on the donor side of the photo-system at extreme pH values. At pH 4, electron donation to P+ occurs with a half-time approx. 135 mus, either by a back reaction from Q-, or from D; electron transport is interrupted between Z1 and P. At pH 9, electron transport is inhibited between Z1 and Z2; rapid re-reduction of P+ by Z1 occurs after 1 flash, and on subsequent flashes electrons from D, an alternative donor reduce P+. The location of the positive charge on states S2 and S3 is discussed.

A rapid vectorial back reaction at the reaction centers of photosystem II in tris-washed chloroplasts induced by repetitive flash excitation

In Tris-washed chloroplasts, completely lacking the oxygen-evolving capacity, absorption changes in the range of 420--560 nm induced by repetitive flash excitation have been measured in the presence and absence of electron donors. It was found: (1) At 520 nm flash-induced absorption changes are observed, which predominantly decay via a 100--200-mus exponential kinetics corresponding to that of the back reaction between the primary electron donor and acceptor of Photosystem II (Haveman, J. and Mathis, P. (1976) Biochim. Biophys. Acta 440, 346--355; Renger, G. and Wolff, Ch. (1976) Biochim. Biophys. Acta 423, 610--614). In the presence of hydroquinone/ascorbate as donor couple the amplitude is nearly doubled and the decay becomes significantly slowed down. (2) The difference spectrum of the absorption changes obtained in the presence of hydroquinone/ascorbate, which are sensitive to ionophores, is nearly identical with that of normal chloroplasts in the range of 460--560 nm (Emrich, H.M., Junge, W. and Witt, H.T. (1969) Z. Naturforsch. 24b, 114--1146). In the absence of hydroquinone/ascorbate the difference spectrum of the absorption changes, characterized by a 100--200-mus decay kinetics, differs in the range of 460--500 nm and by a hump in the range of 530--560 nm. The hump is shown to be attributable to the socalled C550 absorption change, which reflects the turnover of the primary acceptor of Photosystem II (van Gorkom, H.J.(1976) Thesis, Leiden), while the deviations in the range of 460--500 nm are understandable as to be due to the overlapping absorption changes of chlorphyll alpha II+. The problems arising with the latter explanation are discussed. (3) The electron transfer due to the rapid turnover at Photosystem II, which can be induced by flash groups with a short dark time between the flashes, is not able to energize the ATPase and to drive photophosphorylation. On the basis of the present results it is inferred, that in Tris-washed chloroplasts under repetitive flash excitation a rapid transmembrane vectorial electron shuttle takes place between the primary acceptor (X320) and donor (Chl alpha II) of Photosystem II, which is not able to energize the photophosphorylation. Furthermore, the data are shown to confirm the localization of X320 and Chl alpha II within the thylakoid membrane at the outer and inner side, respectively.

Primary and secondary electron donors in photosystem II of chloroplasts. Rates of electron transfer and location in the membrane

Absorption changes at 820 or 515 nm after a short laser flash were studied comparatively in untreated chloroplasts and in chloroplasts in which oxygen evolution is inhibited. In chloroplasts pre-treated with Tris, the primary donor of Photosystem II (P-680) is oxidized by the flash it is re-reduced in a biphasic manner with half-times of 6 microseconds (major phase) and 22 microseconds. After the second flash, the 6 microseconds phase is nearly absent and P-680+ decays with half-times of 130 microseconds (major phase) and 22 microseconds. Exogenous electron donors (MnCl2 or reduced phenylenediamine) have no direct influence on the kinetics of P-680+. In untreated chloroplasts the 6 and 22 microseconds phases are of very small amplitude, either at the 1st, 2nd or 3rd flash given after dark-adaptation. They are observed, however, after incubation with 10 mM hydroxylamine. These results are interpreted in terms of multiple pathways for the reduction of P-680+: a rapid reduction (less than 1 microseconds) by the physiological donor D1; a slower reduction (6 and 22 microseconds) by donor D'1, operative when O2 evolution is inhibited; a back-reaction (130 microseconds) when D'1 is oxidized by the pre-illumination in inhibited chloroplasts. In Tris-treated chloroplasts the donor system to P-680+ has the capacity to deliver only one electron. The absorption change at 515 nm (electrochromic absorption shift) has been measured in parallel. It is shown that the change linked to Photosystem II activity has nearly the same magnitude in untreated chloroplasts or in chloroplasts treated with hydroxylamine or with Tris (first and subsequent flashes). Thus we conclude that all the donors (P-680, D1, D'1) are located at the internal side of the thylakoid membrane.

Kinetics of reduction of the oxidized primary electron donor of photosystem II in spinach chloroplasts and in Chlorella cells in the microsecond and nanosecond time ranges following flash excitation

Absorption changes (deltaA) at 820 nm, following laser flash excitation of spinach chloroplasts and Chlorella cells, were studied in order to obtain information on the reduction time of the photooxidized primary donor of Photosystem II at physiological temperatures. In the microsecond time range the difference spectrum of deltaA between 750 and 900 nm represents a peak at 820 nm, attributable to a radical-cation of chlorophyll a. In untreated dark-adapted material the signal can be attributed solely to P+-700; it decays in a polyphasic manner with half-times of 17 microseconds, 210 microseconds and over 1 ms. The oxidized primary donor of Photosystem II (P+II) is not detected with a time resolution of 3 microseconds. After treatment with 3--10 mM hydroxylamine, which inhibits the donor side of Photosystem II, P+II is observed and decays biphasically (a major phase with t1/2=20--40 microseconds, and a minor phase with t1/2 congruent to 200 microseconds), probably by reduction by an accessory electron donor. In the nanosecond range, which was made accessible by a new fast-response flash photometer operating at 820 nm, it was found the P+II is reduced with a half-time of 25--45 ns in untreated dark-adapted chloroplasts. It is assumed that the normal secondary electron donor is responsible for this fast reduction.

The rise in chlorophyll a fluorescence yield and decay in delayed light emission in tris-washed chloroplasts in the 6-100 microseconds time range after an excitation flash

Parallel measurements of the rise in chlorophyll a fluorescence yield and delayed light emission decay, after a 10 ns saturating excitation flash, have been made in tris (hydroxymethyl)aminomethane-washed chloroplasts. Various electron donor systems (Mn2+; ascorbate; reduced phenylenediamine and benzidine) were used in conjuction with different preillumination regimes to alter [P+-680], the oxidized form of the Photosystem II reaction center chlorophyll a. Conditions giving rise to high [p+ -680] resulted in only a small rise in fluorescence yield, an inhibition of a 6 microseconds component of delayed light emission. These results confirm the hypothesis that P+-680 acts as a quencher of fluorescence and that delayed light emission in the microsecond time range is due to the back reaction of P+-680 and Q-. (Q is the first "stable" electron acceptor of Photosystem II.) Two preillumination flashes are required before the full effect of Tris washing is observed in the delayed light emission decay and fluorescence yield rise; this suggests that a capacity to hold two charges exists between the Tris block and P+-680. Tris washing has no direct effect on the movement of electrons from Z (the first electron donor to P+-680. Finally, Mn2+ donates electrons to P+-680 via Z.

Temperature dependence on the delayed fluorescence of chlorophyll a in blue-green algae

1. The delayed fluorescence of chlorophyll a was measured with a phosphoroscope by changing the temperature in a range of room temperatures in intact cells of blue-green algae, Anacystis nidulans, two strains of Anabaena variabilis and Plectonema boryanum, and other kinds of algae, Cyanidium caldarium and Chlorella pyrenoidosa. The induction of delayed fluorescence remarkably depended on the temperature of measurment. Nevertheless, the induction pattern was characterized by three levels of intensity; the initial rise level at the onset of excitation light, the maximum level after a period of excitation and the steady-state level after 10 min of excitation. 2. In A. nidulans and a strain of A. variabilis grown at various temperatures, close relationship was found between the phase transition of membrane lipids and the initial rise and the steady-state levels of delayed fluorescence. The initial rise level showed the maximum at the temperature of phase transition between the liquid crystalline and the mixed solid-liquid crystalline states, The steady-state levels showed a remarkable change from a high in the liquid crystalline state to a low level in the mixed solid-liquid crystalline state. 3. The millisecond decay kinetics of the delayed fluorescence measured at the steady-state level in A. nidulans grown at 38 degrees C consisted of two components with different decay rates. The half-decay time of the fast component was about 0.17 ms and was constant throughout the temperature range of measurement. The half decay time of slow component ranged from 0.6 to 1.5 ms, depending on the temperature of measurment.

A one microsecond component of chlorophyll luminescence suggesting a primary acceptor of system II of photosynthesis different from Q

The kinetics of the luminescence of chlorophyll a in Chlorella vulgaris were studied in the time range from 0.2 mus to 20 mus after a short saturating flash (t 1/2 = 25 ns) under various pretreatment including anaerobiosis, flashes, continuous illumination and various additions. A 1 mus luminescence component probably originating from System II was found of which the relative amplitude was maximum under anaerobic conditions for reaction centers in the state SPQ- before the flash, about one third for centers in the state S+PQ- or SPQ before the flash, and about one tenth for centers in the state S+PQ before the flash. S is the secondary donor complex with zero change; S+ is the secondary donor complex with 1 to 3 positive charges; P, the primary donor, is the photoactive chlorophyll a, P-680, of reaction center 2; Q- is the reduced acceptor of System II, Q. Under aerobic conditions, where an endogenous quencher presumably was active, the luminescence was reduced by a factor two. The 1 mus decay of the luminescence is probably caused by the disappearance of P+ formed in the laser flash according to the reaction ZP+ leads to Z+ in which Z is the molecule which donates an electron to P+ and which is part of S. After addition of hydroxylamine, the 1 mus luminescence component changed with the incubation time exponentially (tau = 27 s) into a 30 mus component; during the same time, the variable fluorescence yield, measured 9 mus after the laser flash, decreased by a factor 2 with the same time constant. Hereafter in a second much slower phase the fluorescence yield decreased as an exponential function of the incubation time to about the dark value; meanwhile the 30 mus luminescence increased about 50% with the same time constant (tau = 7 min). Heat treatment abolished both luminescence components. The 1 mus luminescence component saturated at about the same energy as the System II fluorescence yield 60 mus after the laser flash and as the slower luminescence components. From the observation that the amplutide is maximum if the laser flash is given when the fluorescence yield is high after prolonged anaerobic conditions (state SQ-), we conclude that the 1 mus luminescence is probably caused by the reaction PWQ- + hv leads to P*WQ- leads to P+W-Q- leads to P*WQ- leads to PWQ- + hv in which W is an acceptor different from Q. The presence of S+ reduced the luminescence amplitude to about one third. Two models are discussed, one with W as an intermediate between P and Q and another, which gives the best interpretation, with W on a side path.

Flash-induced absorption changes of the primary donor of photosystem II at 820 nm in chloroplasts inhibited by low pH or tris-treatment

A comparative study is made, at 15 degrees C, of flash-induced absorption changes around 820 nm (attributed to the primary donors of Photosystems I and II) and 705 nm (Photosystem I only), in normal chloroplasts and in chloroplasts where O2 evolution was inhibited by low pH or by Tris-treatment. At pH 7.5, with untreated chloroplasts, the absorption changes around 820 nm are shown to be due to P-700 alone. Any contribution of the primary donor of Photosystem II should be in times shorter than 60 mus. When chloroplasts are inhibited at the donor side of Photosystem II by low pH, an additional absorption change at 820 nm appears with an amplitude which, at pH 4.0, is slightly higher than the signal due to oxidized P-700. This additional signal is attributed to the primary donor of Photosystem II. It decays (t 1/2 about 180 mus) mainly by back reaction with the primary acceptor and partly by reduction by another electron donor. Acid-washed chloroplasts resuspended at pH 7.5 still present the signal due to Photosystem II (t 1/2 about 120 mus). This shows that the acid inhibition of the first secondary donor of Photosystem II is irreversible. In Tris-treated chloroplasts, absorption changes at 820 nm due to the primary donor of Photosystem II are also observed, but to a lesser extent and only after some charge accumulation at the donor side. They decay with a half-time of 120 mus.