ANALYSIS OF MICROSECOND FLUORESCENCE YIELD AND DELAYED LIGHT EMISSION CHANGES AFTER A SINGLE FLASH IN PEA CHLOROPLASTS: EFFECTS OF MONO- AND DIVALENT CATIONS

Abstract. New results are presented on the effects of mono- and divalent cations on concurrent changes in the microsecond yields and kinetics of chlorophyll a fluorescence and delayed light emission, and the light saturation curve for the latter at 100 μs, following a 10 ns flash at 337 nm. (1) The fluorescence yield increases exponentially from 3 to 30 μs (lifetime, τ, 6.4 ± 0.6/μs), and decays biphasically between 50 and 800μs. (2) The delayed light emission decays biphasically with two exponential phases: fast phase, T= 7–10μs, and slow phase, T= 33–40μs. (3) The light saturation curve for 100μs delayed light emission is satisfactorily represented by a one-hit Poisson saturation curve. (4) Addition of 5 mM NaCl to salt-depleted chloroplasts decreases (by as much as 40%) the yields of μs fluorescence and delayed light emission, and the subsequent addition of 5mM MgCl₂ increases the yields (≤2 × over samples with only NaCl). (5) The fluorescence yield rise and delayed light emission decay kinetics are independent of low concentrations of cations. The lifetime of the fast phase of fluorescence decay changes from ?90μs to ?160μs, when Na⁺ or Na⁺+ Mg²⁺ are added. Based on a detailed analysis presented in this paper, the following conclusions regarding the effects of low concentrations (few mM) of mono-and divalent cations in sucrose-washed chloroplasts at room temperature are made: (a) Na⁺ decreases (?6%) and Mg²⁺ increases (? 20% compared with the Na⁺ sample) the sensitization of photosystem II photochemistry: this effect is small, but significant. (b) Na⁺ increases and Mg²⁺ decreases the efficiency for radiationless transitions in singlet excited Chl a in the antenna and closed reaction center of PS II; this includes non-radiative energy transfer to PS I, intramolecular intersystem crossing and internal conversion. The ratio of the sum of the rate constants for radiationless transitions to that for fluorescence increases by ? 2-fold upon the addition of Na⁺, and is completely reversed by the addition of Mg²⁺. (c) The rate constant for the re-oxidation of Q^- decreases (about 50%) in the presence of Na⁺ or Na⁺+ Mg²⁺. These conclusions imply that cations produce multiple changes in the primary photoprocesses of PS II at physiological temperatures. It is proposed that these changes are mutually independent and can co-exist.     

DELAYED-LIGHT STUDIES ON PHOTOSYNTHETIC ENERGY CONVERSION-IV. EFFECT OF TRIS POISONING, AND OF ITS REVERSAL BY HYDROGEN DONORS, ON THE MILLISECOND EMISSION FROM CHLOROPLASTS*

Abstract— Destruction of the oxygen-evolving activity of chloroplasts by treatment with 0.8 M Tris-HC1 results in an extremely rapid dark decay of millisecond delayed light. Addition of electron acceptors such K₃Fe(CN)₆ or NADP⁺ does not change the decay characteristics of this msec delayed light. Artificial electron donors such as DPC partially restore the msec delayed emission to the slowly decaying situation which is found in control chloroplasts. Addition of electron acceptors to this photochemically competent system results in more rapid decay and in an increase of emission at 1 msec, as in control chloroplasts. We suggest on the basis of the delayed light data that Tris treatment induces a rapid side reaction which uselessly dissipates the oxidizing and reducing power which is stored by Photoreaction II. Artificial electron donors allow the Tris-poisoned photoreaction to store energy long enough for utilization of the energy by normal photosynthetic reactions, as shown by the flattening of the delayed light curves. In the restored system the normal reactions of electron transport are thus able to compete with the Tris-induced side reaction. This interpretation is supported by the finding that the restored system requires higher exciting intensities for saturation of NADP⁺ reduction than the control system.     

STUDIES OF SYSTEM II PHOTOCENTERS BY COMPARATIVE MEASUREMENTS OF LUMINESCENCE,FLUORESCENCE, AND OXYGEN EMISSION*

Abstract— New results are presented on the emission of oxygen by algae and chloroplasts illuminated by a sequence of short saturating flashes. These results favor the four-state hypothesis of Kok and co-workers, in which formation of oxygen requires the accumulation of four oxidants produced by four successive photoreactions. Deactivation of the more oxidized precursor states in the dark is studied under different conditions of preillumination. Our results suggest that both a one step and a two step mechanism of deactivation exist. In order to understand the biological significance of Kok's parameter α—the fraction of photochemical centers unable to react on each flash (“misses”)-we study reoxidation of acceptor Q after one flash by fluorescence techniques. It appears that a fraction of Q^- is reoxidized by a back reaction which cancels the effect of the preilluminating flash and is probably responsible for the misses. The results of some luminescence experiments are also reported. These experiments demonstrate that delayed emission of light is associated with the deactivation of states S₂ and S₃. It is possible that excitons produced by deactivation can be reabsorbed by active photochemical centers, which can modify considerably the deactivation process.     

Driving the photoluminescent and structural properties of X2-Y2SiO5 by varying the dopant Dy3+ concentration towards cool WLED applications

Dy³⁺ doped Y₂SiO₅ nanophosphors were synthesized by solution combustion technique using Calotropis gigantean milk latex and NaCl as fuel and flux respectively. Powder X-ray diffraction (PXRD) confirmed the formation of monoclinic X₂-phase Y₂SiO₅ belonging to the phase group C₂/c. Fourier transform infrared spectroscopy (FTIR) shows characteristic metal–oxygen (Y–O) vibration band at 721 cm⁻¹. Transmission electron microscopic (TEM) and Scanning electron microscopic (SEM) morphological feature exhibits non-uniform almost spherical shaped nanosized particles. The photoluminescence (PL) emission peaks, recorded at 388 nm, showed radiative emissions at 483, 575 and 636 nm respectively. Judd–Ofelt (JO) analysis was carried out to estimate the radiative (A_R) properties, radiative life time (τ_R), branching ratio (β_R) and stimulated emission crossection (σλ_p). The CIE and CCT was estimated using McCamy empirical formula. In the beginning, the CIE co-ordinate values were lying in the light blue region. However, with increase in Dy³⁺ concentration the values shifted towards white region. CCT value was found to be ∼6984 K. Therefore, Y₂SiO₅:Dy³⁺ (9 mol%) can be used for cool day light and WLED applications.     

DELAYED-LIGHT STUDIES ON PHOTOSYNTHETIC ENERGY CONVERSION-III. EFFECT OF 3-(3,4-DICHLOROPHENYL)-1,1 -DIMETHYL UREA ON THE MILLISECOND EMISSION FROM CHLOROPLASTS PERFORMING PHOTOREDUCTION OF FERRICYANIDE*

Abstract— How does a plant convert electronic excitation of chlorophyll into stable chemical potential? The time scales of fluorescence (10^--⁹ sec) and steady-state enzymatic turnover (10–² sec) indicate that energy storage must be involved. Millisecond delayed singlet emission from chlorophyll allows measurement of metastable energy storage at Photoreaction II. Activation of noncyclic electron transport results in more rapid decay and in increase of emission at 10^--³ sec, both effects being inhibited by the poison DCMU. These results can be explained by at least three different models of the reaction center: the oxidized chlorophyll model, the chlorophyll triplet model, and the two-quantum electron-hole model.     

Photoinitiated free radical polymerization of vinyl compounds in aqueous solution

A new method for the photochemical initiation of polymerization of vinyl compounds in aqueous solution is described. The photochemically active species is an ion pair complex of the formula Fe³⁺X⁻(X⁻ = OH⁻, CI⁻, N⁻₃, etc.). The light absorption by the ion pair leads to an electron transfer causing reduction of the cation and oxidation of the anion to an atom or free radical X. The latter leads to the initiation of polymerization in accordance with X + CH₂CHR→XCH₂ CHR . The kinetics of the reaction were studied by the measurement of: (a) ferrous ion formed (colorimetrically), (b) monomer disappearance (by titration and by weighting the polymer), (c) the chain length of the polymer (in the case of methyl methacrylate). The dependence of the quantum yield on the light intensity, light absorption fraction, and the concentration of vinyl monomer and ferrous ion added initially was investigated. A complete mechanism, both with regard to the formation of free radicals and the polymerization reaction, was put forward involving: (1) light absorption, (2) a primary dark back reaction, (3) dissociation of the primary product, (4) a secondary dark back reaction, (5) initiation of polymerization by free radicals, (6) propagation of polymerization, and (7) termination by recombination of active polymer endings. The mechanism was verified by the experimental results and some constant ratios were estimated quantitatively.     

Multivariate statistical treatment of solvent effects in the photoreduction of CoIII(En)2(Br)(RC6H4NH2)2+ complexes in aqua-organic solvent media

Ultraviolet photolysis (λ = 254 nm) studies were carried out for a series of cobalt(III) complexes, Co^III(En)₂(Br)(RC₆H₄NH₂)²⁺, where R = m-OMe, p-F, H, m-Me, p-Me, p-OEt, and p-OMe, in various compositions of water-methanol/1,4-dioxane mixtures (0, 5, 10, 15, 20, 25, and 30 vol % organic cosolvent) at two different temperatures (287 and 300 K). The ligand-to-metal charge-transfer excited state produced in the excitation of the complex initially generated a solvent-caged {Co^II; ligand radical} pair, which eventually undergoes recombination/separation into products. The quantum yield sharply increased from a mixture containing a lower mole fraction of organic cosolvent (x _org) to higher one. In other words, when x _org in the mixture increases, a steady increase in the quantum yield is observed. The quantum yield of Co^III(En)₂(Br)(RC₆H₄NH₂)²⁺ in various solvent mixtures is found to exhibit a linear (logΦ_Co(II) − 1/ε_r) dependence. This is consistent with solvation or solvent cage effect, which may be nonspecific, specific, or both. In order to throw light on these effects, a phenomenological model of solvent effects was applied. Therefore, the quantum yield values have been correlated statistically with some success containing different solvent parameters. The solvent parameters considered in this work are Grunwald-Winstein’s Y, Krygowski-Fawcett’s E _T ^N and DN ^N along with Kamlet-Taft’s α, β, π* parameters. The regression model proposed is Y _S = Y ₀ + Σ _{i = 1} ⁿ a _i X _i , where Y_S is the solvent-dependent property (here logΦ_Co(II)) in a given solvent; Y ₀ is the statistical quantity corresponding to the value of property in the reference solvent; X ₁, X ₂, X ₃ … are explanatory variables, the solvent parameters, which can explain the various solvation effects on reactants/{Co^II; ligand radical} pair, and a ₁, a ₂, a ₃… etc., are the regression coefficients. The coefficient values can be quantified to measure the relative importance of solvent effects on the physicochemical quantity, that is, the photoreduction yields in the present investigation. The text was submitted by the author in English.     

CINETIQUE DES ECHANGES D'OXYGENE ET DE LA FLUORESCENCE DES CHLOROPLASTES ISOLES

Abstract Oxygen evolution and fluorescence have been studied with isolated chloroplasts illuminated, in the absence of Hill reagents, by flashes or continuous light. As in whole cells, at least two substances are involved in the primary process leading to the oxygen evolution. The first, called E, probably is the photochemical “complex” of System II. After a long period of darkness, E is not active. It is activated in two steps. Step one is a photochemical reaction, induced by a quantum of light absorbed by pigment-system II, which results in the production of E in a reduced state. Step two is a dark oxidation of the reduced E by the second substance, A. The oxidized E can then enter the normal photochemical cycle of system II. Reduced E might alternatively be oxidized by oxygen, this reaction being responsible for a very rapid and brief light-induced oxygen uptake. Substance A is measured by the oxygen burst and is present in the chloroplasts at the approximate ratio of 1 molecule for 70 molecules of total chlorophyll while E is at the ratio of about 1/2800. This gives a E over A value of 1/35 which is much smaller than the one found in whoe cells (ca. 1/10). This independent behavior of E and A suggests that chloroplast extraction destroys some photochemical centers without having a direct impact on A, which might diffuse from one center to another. Besides the brief light-induced oxygen uptake above mentioned, there is another one which is related to System I functioning. The kinetics of the oxygen evolution and of the fluorescence have been compared. During the activation process of the oxygen evolving ability, rate of oxygen evolution and fluorescence yield increase in a parallel way. After the maximum velocity of the oxygen burst is reached (i.e. after activation), the fluorescence yield keeps growing up until the steady-state is attained (with an intermediary plateau), whereas the rate of oxygen emission slows down. The time-course curves of fluorescence obtained with inactivated or activated chloroplasts are essentially different in that the initial yield is higher in the latter case.     

Photoluminescence of Eu-doped triple phosphate Ca8MgR(PO4)7 (R=La, Gd, Y)

Eu³⁺-doped triple phosphate Ca₈MgR(PO₄)₇ (R=La, Gd, Y) was synthesized by the general high-temperature solid-state reaction. Excitation and emission spectra as well as luminescence decay were used to characterize the phosphors. Photoluminescence excitation and emission spectra showed that the phosphor could be efficiently excited by UV-vis light from 260 to 450 nm to give bright red emission assigned to the transition (⁵D₀→⁷F₂) at 612 nm. The richness of the red color has been verified by determining their color coordinates (X, Y) from the CIE standard.     

ON THE ORIGIN OF GLOW PEAKS IN EUGLENA CELLS, SPINACH CHLOROPLASTS AND SUBCHLOROPLAST FRAGMENTS ENRICHED IN SYSTEM I OR II

Abstract— The origin of glow peaks (thermoluminescence) was investigated in isolated spinach chloroplasts and Euglena cells by pretreatment with various concentrations of 3-(3,4 dichlorophenyl)-1,1-dimethylurea (DCMU)?, different light intensities, and after mild heating at various temperatures. Experiments are also reported on subchloroplast fractions enriched in pigment systems I (PSI) or II (PSII) (prepared under conditions to reduce destruction of membranes by excessive detergent contact). These results provide the following, most likely, suggestion for the origin of glow peaks: (1) Z peak originates in metastable states; it is insensitive to DCMU, temperature (320–328 K), and appears only when other peaks are saturated (10 Wm^-2). (2) Peak I involves the use of a reducing entity A (plastoquinone) beyond Q (the primary electron acceptor of pigment system II, PSII), or, of a high “S” state (charge accumulator) of oxygen evolving system; its intensity is dramatically reduced by low concentrations (1 μM) of DCMU, and, there is more of it in PSII than in PSI particles. (3) Peak II is due to reaction of Q^- with the “S” states of the oxygen evolving system; its intensity increases upon the addition of low concentrations of DCMU, at the expense of peak I; it is most sensitive to mild heating, and there is more of it in PSII than in PSI particles. (4) Peak III was not studied here as it was not resolved in most of our preparations. (5) Peak IV is from both pigment system I and II; it is sensitive to heating (>50°C), is somewhat sensitive to DCMU, and is present in both PSI and PSII particles. (6) Peak V is from PSI; it is least sensitive to mild heating, and it is enriched in PSI particles. The present studies have extended our knowledge regarding the origin of glow peaks in spinach chloroplasts and Euglena cells; in particular, the involvement of the charge accumulating “S” states of oxygen evolution (for peaks I and II) and of system I (for peak V) are emphasized in this paper.     

Quantum-Chemical Estimation of the Stability and Reactivity of Diphosphonium Salts

For a series of diphosphonium salts containing two positively charged covalently bonded phosphorus atoms, X_{/sub> n} Y_3-n P⁺P⁺X _n Y_3-n (X = alkyl substituent, Y = amino group, n = 0-3), the stability, reactivity, and P-P bond strength were evaluated by various physicochemical methods. The P-P bond energy is appreciably influenced by both steric factors and donor properties of the substituents. The calculations confirmed that transformations of diphosphonium salts can involve cleavage of both P-P and P-N (or P-C) bonds.     

Treatment of excess thermodynamic quantities for liquid mixtures

Thermodynamic data on mixtures of liquids are usually expressed as excess quantitiesY ^EX and analyzed with the Redlich-Kister equation. This approach can in some cases be misleading and hide strong interactions at low concentrations. In agreement with the original statement of Redlich and Kister, it is therefore better to plot such data asY ^EX/X ₂(1-X ₂). This quantity, which is directly related to the apparent molar quantities of both components, is often a more useful quantity and has just as much a thermodynamic significance asY ^EX.     

Ultrafast spectroscopy studies on the mechanism of electron transfer and energy conversion in the isolated pseudo ginseng, water hyacinth and spinach chloroplasts

The spectroscopy characteristics and the fluorescence lifetime for the chloroplasts isolated from the pseudo ginseng, water hyacinth and spinach plant leaves have been studied by absorption spectra, low temperature steady-state fluorescence spectroscopy and single photon counting measurement under the same conditions and by the same methods. The similarity of the absorption spectra for the chloroplasts at room temperature suggests that different plants can efficiently absorb light of the same wavelength. The fluorescence decays in PS II measured at the natural QA state for the chloroplasts have been fitted by a three-exponential kinetic model. The three fluorescence lifetimes are 30, 274 and 805 ps for the pseudo ginseng chloroplast; 138, 521 and 1494 ps for the water hyacinth chloroplast; 197, 465 and 1459 ps for the spinach chloroplast, respectively. The slow lifetime fluorescence component is assigned to a collection of associated light harvesting Chl a/b proteins, the fast lifetime component to the react     

PHOTOCHEMISTRY OF THE REACTION CENTERS OF SYSTEM II UNDER REPETITIVE FLASH GROUP EXCITATION IN ISOLATED CHLOROPLASTS

Abstract— Absorption changes induced in isolated chloroplasts by excitation with repetitive flash groups have been measured at 690 nm, indicating the photochemical turnover of chlorophyll-a_II (Chl-αn), and at 480 nm and 513 nm respectively, reflecting via electrochromic effect the formation of a transmembrane electric field. The data are compared with measurements of oxygen evolution. In chloroplasts with practically fully intact oxygen evolving capacity it was found: 1. The initial amplitude of the 690 nm absorption change induced by the second flash as a function of the time t_v between the first and second flash of a group increases with a half life of about 35 µs. On the other hand, the average oxygen yield due to the second flash as a function of the time t_v rises with a half life of about 600 µs (and a kinetics in the ms-range of minor extent), confirming the data of Vater et al. (1968). 2. Under far red background illumination, where contributions due to PS I in the µs-range can be excluded, the difference spectrum in the red of the absorption changes induced by the first flash corresponds with that of the absorption changes induced by the second flash fired 200 µ after the first flash. 3. The pattern of the absorption changes at 690 nm induced by repetitive double flash groups at t_v= 200 µs does not markedly change in normal chloroplasts by the presence of DBMIB?. Similar 690 nm absorption changes occur in trypsin treated chloroplasts, independent of the presence of DCMU. 4. The fast regeneration in the µs-range of Chl-a_n is also observed in the third flash of a triple flash group at a time t_v= 200 µs between the flashes of the group. 5. The initial amplitudes of the absorption changes with a decay kinetics slower than 100 µs induced by the second flash at 480 nm and 513 nm, respectively, as a function of the time t, between the first and second flash of a group, are characterized by a recovery half-time of about 600 µs, confirming earlier measurements at 520 nm (Witt and Zickler, 1974). On the basis of these results it is inferred that there does exist a photoreaction of Chl-α_n., with an electron acceptor, referred to as X_a, other than the ‘primary’ plastoquinone acceptor X320, if X320 persists in its reduced state. Under conditions of X320 being in the reduced state, this photochemical reaction was shown to be highly dissipative with respect to charging up the watersplitting enzyme system Y. Furthermore, this Chl-a_n-photoreaction with X_a does not lead to a vectorial transmembrane charge separation, which is stable for more than a few microseconds. Different models for the functional and structural organization of PS II are discussed.     

BASF 13.338 INDUCED CHANGES IN STRUCTURE and FUNCTION OF THE PHOTOSYNTHETIC APPARATUS OF WHEAT SEEDLINGS

Abstract— Growing wheat seedlings in the presence of BASF 13.338 [4-chloro-5-dimethylamino-2-phenyl-3(2H)pyridazinone], a PS II inhibitor of the pyridazinone group, brought about notable changes in the structure and functioning of photosynthetic apparatus. In BASF 13.338 treated plants, there was a decrease in the ratio of Chi a/Chl b, an increase in xanthophyll/carotene ratio and an increase in the content of Cyt b 559 (HP + LP). Chl/p700 ratio increased when measured with the isolated chloroplasts but not with the isolated PS I particles of the treated plants. The SDS-PAGE pattern of chloroplast preparations showed an increase in the CPII/CP I ratio. The F685/F740 ratio in the emission spectrum of chloroplasts at -196°C increased. The difference absorption spectrum of chloroplasts between the control and the treated plants showed a relative increase of a chlorophyll component with a peak absorption at 676 nm and a relative decrease of a chlorophyll component with a peak absorption at 692 nm for the treated plants. The excitation spectra of these chloroplast preparations were similar. Chloroplasts from the treated plants exhibited a greater degree of grana stacking as measured by the chlorophyll content in the 10 K pellet. The rate of electron transfer through photosystem II at saturating light intensity in chloroplast thylakoids isolated from the treated plants increased (by 50%) optimally at treatment of 125 μM BASF 13.338 as compared to the control. This increase was accompanied by an increase in (a) I₅₀ value of DCMU inhibition of photosystem II electron transfer; (b) the relative quantum yield of photosystem II electron transfer; (c) the magnitude of C550 absorbance change; and (d) the rate of carotenoid photobleaching. These observations were interpreted in terms of preferential synthesis of photosystem II in the treated plants. The rate of electron transfer through photosystems I and through the whole chain (H₂O → methyl viologen) also increased, due to an additional effect of BASF 13.338, namely, an increase in the rate of electron transfer through the rate limiting step (between plastoquinol and cytochrome f). This was linked to an enhanced level of functional cytochrome f. The increase in the overall rate of electron transfer occurred in spite of a decrease in the content of photosystem I relative to photosystem II. Treatment with higher concentrations (> 125 μM) of BASF 13.338 caused a further increase in the level of cytochrome f, but the rate of electron transfer was no greater than in the control. This was due to an inhibition of electron transfer at several sites in the chain.     

Thermoluminescence and temperature effects on delayed light emission (corrected for changes in quantum yield of fluorescence) in DCMU-treated algae

Abstract—

• 1 The simultaneous measurements of delayed light emission (DLE) and chlorophyll (Chl) fluorescence yield in DCMU§ treated Chlorella were made in the time range of 1 to 10 sec at various temperatures from 0 to 50°C. Similar measurements were made for DCMU treated thermophilic strain of Synechococcus in the temperature range of 0 to 75°C.
• 2 Using the basic assumption that DLE is produced by the back reaction of primary photoproducts of system II, and that two such reactions are required for it, a linear relationship between J^-1/2 (where J is energy per unit time available for DLE) and time after illumination was derived. This second-order relationship was confirmed experimentally at several temperatures (2°, 5°, 10° and 15°C). From these analyses, reaction rate decay constants, at specific temperatures, were calculated.
• 3 An Arrhenius plot was made for these calculated rate constants. Its slope (8–10 kcal/mole) agreed well with previous reports; however, it had a region of zero slope which occurred at the physiological temperature of the organisms used.
• 4 Thermoluminescence or temperature jump delayed light emission (TDLE) was measured using various temperature conditions and it was found that not only the magnitude of the temperature jump (ΔT), but the initial and final temperatures of the sample were important. For example, a temperature jump of 8°C from 2 to 10°C gave much higher TDLE than from 12 to 20°C.
• 5 Many properties e.g., magnitude, temperature dependence and time independence of TDLE could be explained by the DLE decay data (corrected for changes in fluorescence yield) and the kinetic analysis.
• 6 It is suggested that, in addition to the back reaction of Z⁺ (the primary oxidized photoproduct of system II) with Q^- (the primary reduced photoproduct of system II), a reducing entity, beyond the sites of DCMU and antimycin a action, is somehow involved in the production of slow DLE.

     

From Shape Similarity to Shape Complementarity: Toward a Docking Theory

Formal relations between similarity and docking are analyzed, and a general docking theory is proposed for colored mixtures of multivariate distributions. X and Y being two colored mixtures with given marginal distributions, their shape complementarity coefficient is defined as the lower bound of the variance of (X–Y)· (X-Y), taken over the set of joint distributions of X and Y. The docking is performed via minimization of the shape complementarity coefficient for all translations and rotations of the mixtures. The properties of the docking criterion are derived, and are shown to satisfy the practical requirements encountered in molecular shape analysis.     

Photodynamic oxidation of iodide ion and aromatic amino acids by eosin

Abstract— The photosensitized oxidation of I^-, tyrosine, and tryptophan by aqueous eosin has been investigated with a photostationary technique, in which the system is irradiated with sinusoidally modulated light and phase sensitive detection is used to measure the spectra of short-lived intermediates and their rate constants. It has been shown that the principal oxidizing agent for I^- is semi-oxidized eosin (X) produced by the reduction of oxygen by triplet state eosin. The same mechanism obtains when femcyanide ion is substitued for oxygen. A second oxidizing agent is effective in oxygenated solutions at high I^- concentrations, attributed to O₂^-. The X mechanism controls when tyrosine or tryptophan are irradiated at low substrate concentrations in the presence of oxygen or femcyanide. However, at high substrate concentrations, or in the absence of the electron accepting agents, the mechanism switches to the ‘dye-substrate’ process where triplet state eosin oxidizes the aromatic substrate and is reduced to the semiquinone. The conversion quantum yields agree with the predictions based on the measured quenching and reaction rate constants.     

Optimization of process variables and corrosion properties of a multi layer silica sol gel coating on AZ91D using the Box–Behnken design

Anti-corrosion silica coating was prepared via the sol–gel method for AZ91D magnesium alloy using tetraethoxysilane and methyltriethoxysilane as precursors. Silica coating was deposited on fluorinated magnesium alloy substrates by dip coating. The surface morphology of the silica coating was characterized by scanning electron microscope (SEM). The corrosion properties were studied by electrochemical impedance measurements and polarisation technique in 3.5 wt% Sodium chloride solution. The results showed an improvement in the corrosion performance from these coatings. A three-factor, three-level design of experiment (DOE) with response surface methodology including a Box–Behnken design was run to evaluate the main and interaction effects of several independent formulation variables, which included precursor ratios MTES/TEOS (X₁), sintering temperature (X₃) and sol dilution (X₂) which measured the volume of the diluted sol divided by the initial volume of sol. The dependent variables included the corrosion current derived from the polarisation curve (i_cor = Y₁) and the coating resistance derived from the Nyquist curve (R_coat = Y₂). Optimizations were predicted to yield Y₁ and Y₂ values of 1.57018E–7A cm⁻² and 14279 Ω cm², when X₁, X₂, and X₃ were 3.36, 1.52 and 222, respectively.     

TEMPERATURE DEPENDENCE OF DELAYED LIGHT EMISSION IN THE 6 to 340 MICROSECOND RANGE AFTER A SINGLE FLASH IN CHLOROPLASTS

Abstract. The delayed light emission decay rate (up to 120 μs) and the rise in chlorophyll a fluorescence yield (from 3 to 35 μs) in isolated chloroplasts from several species, following a saturating 10 ns flash, are temperature independent in the 0–35°C range. However, delayed light in the 120–340 μs range is temperature dependent. Arrhenius plots of the exponential decay constants are: (a) linear for lettuce and pea chloroplasts but discontinuous for bush bean (12–17°C) and spinach (12–20°C) chloroplasts; (b) unaffected by 3-(3,4 dichlorophenyl)-1,1-dimethylurea (inhibitor of electron flow), gramicidin D (which eliminates light-induced membrane potential) and glutaraldehyde fixation (which stops gross structural changes).
The discontinuities, noted above for bush bean and spinach chloroplasts, are correlated with abrupt changes in (a) the thylakoid membrane lipid fluidity (monitored by EPR spectra of 12 nixtroxide stearate, 12NS) and (b) the fluidity of extracted lipids (monitored by differential calorimetry and EPR spectra of 12 NS). However, no such discontinuity was observed in (a) chlorophyll a fluorescence intensity of thylakoids and (b) fluorescence of tryptophan residues of delipidated chloroplasts.
Microsecond delayed light is linearly dependent on light intensity at flash intensities as low as one quantum per 2 times 10⁴ chlorophyll molecules. We suggest that this delayed light could originate from a one quantum process in agreement with the hypothesis that recombination of primary charges leads to this light emission. A working hypothesis for the energy levels of Photosystem II components is proposed involving a charge stabilization step on the primary acceptor side, which is in a lipid environment.
Finally, the redox potential of P680 (the reaction center for chlorophyll of system II) is calculated to be close to 1.0–1.3 V.