STOICHIOMETRY and KINETICS OF PROTON RELEASE UPON PHOTOSYNTHETIC WATER OXIDATION

Abstract— We studied the magnitude and the rise kinetics of proton release into the interior of thylakoids by flash spectrophotometty with neutral red as pH indicator. Excitation of dark-adapted thylakoids by a series of between 4 and 11 flashes produced a complex pattern of proton release into the thylakoid lumen. Proton release upon each flash was time resolved.
A slow component of proton release oscillated weakly in magnitude with period of two as function of flash number. It exhibited a half-rise time of approximately 20 ms from the very first flash on, and it was abolished by inhibitors of plastohydroquinone oxidation. This component was attributed to the oxidation of plastohydroquinone by PS I via the Cytb₆/f complex.
Additionally, rapid and multiphasic proton release was observed with half-rise times of less than 2 ms which exhibited a pronounced and damped oscillation with period of four as function of flash number. This rapid proton release was attributed to water oxidation. A detailed kinetic analysis suggested that proton release occurred with the following stoichiometry and with the following half-rise times during the transitions S₁ S_i+1 of water oxidation: 1 H⁺(250 μs, S₀→₁): 0 H⁺(S₁→ S₂):1 H⁺(200 μs, S₂→S₃):2 H⁺(1.2 ms, S₃→ S₄→ S₀) . Proton release and proton rebinding upon oxidation and reduction of the intermediate electron carrier Z, respectively, may have influenced the kinetics of the respective proton yields but not the stoichiometric pattern.     

COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2 EVOLUTION-II. DAMPING OF FLASH YIELD OSCILLATION,DEACTIVATION*

Abstract— A quantitative analysis is made of a linear 4-step model for photosynthetic O₂ evolution in which each photochemical trapping center or an associated enzyme cycles through 5 oxidation states (S₀, S₁, S₂, S₃, S₄). The overall reaction is: S₀→ S₄, S₄→ S₀+ O₂, where k_d= rate of dark reaction. Based on data obtained with isolated chloroplasts, four aspects were considered: (1) The two perturbations which damp the oscillation of the O₂ flash yield in a flash sequence given after a dark period-(a) a failure rate (α) of the trapping centers in the photochemical conversion (‘misses’) and (b) double effective excitations in a fraction (8) of the centers which are in the S₀ and S₁ states (‘double hits’). Best fit with the experimental data is obtained for α= 0.1, β=0.05. (2) The kinetics and the mechanism of deactivation—the loss of + charges (O₂ precursors) in the dark. The momentary distribution of the four oxidation states (S₀-_-S₃) in a sample can be computed from the O₂ yields of four consecutive flashes with corrections for a and β. The time course of the various states in the dark following specific preilluminations reveals that deactivation proceeds in single equivalent steps: S₃→S₂, S₂,→S₁. S₁, itself stable in the dark, is the end product of deactivation. The ground state S₀ cannot be formed by deactivation in the dark but is only formed during illumination. (3) The various ratios [S₀]/[S₁] which can be observed in a sample after deactivation following different preilluminations with flashes or continuous light. (4) The transients of the O₂ evolution rate in weak continuous light as observed after deactivation with or without flash preillumination. In all instances satisfactory agreement is found between the observations and the predictions of the model.     

Photocatalytic Water Oxidation by a Mixed‐Valent MnIII3MnIVO3 Manganese Oxo Core that Mimics the Natural Oxygen‐Evolving Center

The functional core of oxygenic photosynthesis is in charge of catalytic water oxidation by a multi‐redox Mn^III/Mn^IV manifold that evolves through five electronic states (S_i , where i=0–4). The synthetic model system of this catalytic cycle and of its S₀→S₄ intermediates is the expected turning point for artificial photosynthesis. The tetramanganese‐substituted tungstosilicate [Mn^III₃Mn^IVO₃(CH₃COO)₃(A‐α‐SiW₉O₃₄)]^6? (Mn₄POM) offers an unprecedented mimicry of the natural system in its reduced S₀ state; it features a hybrid organic–inorganic coordination sphere and is anchored on a polyoxotungstate. Evidence for its photosynthetic properties when combined with [Ru(bpy)₃]²⁺ and S₂O₈^2? is obtained by nanosecond laser flash photolysis; its S₀→S₁ transition within milliseconds and multiple‐hole‐accumulating properties were studied. Photocatalytic oxygen evolution is achieved in a buffered medium (pH 5) with a quantum efficiency of 1.7 %.     

A kinetic study of the homogeneous oxidation of hydroxylamine by manganese(III)-bis(salicylaldimine) complexes

Summary The kinetics of the oxidation of hydroxylamine by manganese(III)-bis (salicylaldimine) complexes have been studied over the 5.2–8.4 pH range. The reaction is first order in both hydroxylamine and oxidant, and inversely proportional to [H⁺]. The [complex]: [hydroxylamine] stoichiometric ratio is 11 in both acidic and neutral media, and 21 in an alkaline medium. The second-order rate constant increased in the sequence: [Mn^III(L²)OH₂]-ClO₄·2H₂O > [Mn^III(L¹)OH₂]ClO₄ > [Mn^IIIL¹)OAc]-H₂O. The reactivity of unprotonated hydroxylamine is much higher than that of the protonated form. The reaction rate decreased significantly with addition of chloride ions. A plausible mechanism is proposed.     

Temperature dependence of DC electrical conductivity of kaolin

The samples from kaolin Sedlec were investigated by the help of DTA, TG, and temperature dependences of DC conductivity using Pt wire electrodes and linear heating up to 1,050 °C. After drying, the samples contained ~1.5 mass% of the physically bound water. DTA and TG reflected generally known facts about a release of the physically bound water, dehydroxylation, and metakaolinite → Si–Al spinel transformation. The results of electrical measurements showed the electric current passed over the maximum at 60 °C. The self-ionization of water results in the process H₂O → H⁺ + OH^? in the water layers on the crystal surfaces; consequently, OH^? and H⁺ are the main charge carriers in the low-temperature region. The water molecules simultaneously evaporate from the sample which decreases the number of the charge carriers. When the physically bound water evaporates, the current is carried mostly by K⁺ and Na⁺ ions. During dehydroxylation, the hydroxyls OH^? split into H⁺ and O^2?. The ions H⁺ jump to the neighboring OH^? groups creating the water molecules. The ions O^2?remain bounded to the newly created metakaolinite lattice. Therefore, mobile protons contribute to the electric current. At the same time, this contribution gradually decreases because of the escape of H₂O from the sample. The sharp current peak and DTA peak at 970 °C imply relatively fast metakaolinite → Si–Al spinel transformation. This DC current peak results from the shift of Al³⁺ and O^2? ions into new positions.     

Kinetics and mechanism of the reduction of monochloramine by hydroxylamine and hydroxylammonium ion

The kinetics and mechanism by which monochloramine is reduced by hydroxylamine in aqueous solution over the pH range of 5–8 are reported. The reaction proceeds via two different mechanisms depending upon whether the hydroxylamine is protonated or unprotonated. When the hydroxylamine is protonated, the reaction stoichiometry is 1:1. The reaction stoichiometry becomes 3:1 (hydroxylamine:monochloramine) when the hydroxylamine is unprotonated. The principle products under both conditions are Cl^–, NH⁺₄, and N₂O. The rate law is given by ?[d[NH₂Cl]/dt] = k₊[NH₃OH⁺][NH₂Cl] + k₀[NH₂OH][NH₂Cl]. At an ionic strength of 1.2 M, at 25°C, and under pseudo‐first‐order conditions, k₊= (1.03 ± 0.06) ×10³ L · mol^?1 · s^?1 and k₀=91 ± 15 L · mol^?1 · s^?1. Isotopic studies demonstrate that both nitrogen atoms in the N₂O come from the NH₂OH/NH₃OH⁺. Activation parameters for the reaction determined at pH 5.1 and 8.0 at an ionic strength of 1.2 M were found to be ΔH? = 36 ± 3 kJ · mol^–1 and Δ S? = ?66 ± 9 J · K^?1 · mol^?1, and Δ H? = 12 ± 2 kJ · mol^?1 and Δ S? = ?168 ± 6 J · K^?1 · mol^?1, respectively, and confirm that the transition states are significantly different for the two reaction pathways. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 124–135, 2006     

Reduction of iodine by hydroxylamine within the pH range 0.7–3.5

The reduction of iodine by hydroxylamine within the [H⁺] range 3×10⁻¹–3×10⁻⁴ mol.L⁻¹ was first studied until completion of the reaction. In most cases, the concentration of iodine decreased monotonically. However, within a narrow range of reagent concentrations ([NH₃OH⁺]₀/[I₂]₀ ratio below 15, [H⁺] around 0.1 mol.L⁻¹, and ionic strength around 0.1 mol.L⁻¹), the [I₂] and [I₃⁻] vs. time curves showed 2 and 3 extrema, respectively. This peculiar phenomenon is discussed using a 4 reaction scheme (I₂+I⁻⇔︁I₃⁻, 2 I₂+NH₃OH⁺+H₂O→HNO₂+4 I⁻+5 H⁺, NH₃OH⁺+HNO₂→N₂O+2 H₂O+H⁺, and 2 HNO₂+2 I⁻+2 H⁺→2 NO+I₂+2 H₂O). In a flow reactor, sustained oscillations in redox potential were recorded with an extremely long period (around 24 h). The kinetics of the reaction was then investigated in the starting conditions. The proposed rate equation points out a reinforcement of the inhibition by hydrogen ions when [H⁺] is above 4×10⁻² mol.L⁻¹ at 25°C. A mechanism based on ion-transfer reactions is postulated. It involves both NH₂OH and NH₃OH⁺ as the reducing reactive species. The additional rate suppression by H⁺ at low pH would be connected to the existence of H₂OI⁺ in the reactive medium. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 785–797, 1998     

Preparative syntheses of bis(4-tert-butylphenyl)aminoxyl

Bis(4-tert-butylphenyl)aminoxyl was obtained in 80 and 95% yield by oxidation of the corresponding amine and hydroxylamine with H₂O₂/WO ₄ ^2? in methanol at 65°C. The oxidation of bis(4-tert-butylphenyl)hydroxylamine to bis(4-tert-butylphenyl)aminoxyl was catalyzed by Cu⁺ and Ag⁺ ions which also catalyzed disproportionation of the former to bis(4-tert-butylphenyl)amine and bis(4-tert-butylphenyl)aminoxyl. Mechanisms of the catalytic oxidation of the amine and hydroxylamine and disproportionation of the latter were proposed.     

Zundel‐like and Eigen‐like Hydrated Protons on a Platinum Surface

The nature of hydrated protons is an important topic in the fundamental study of electrode processes in acidic environment. For example, it is not yet clear whether hydrated protons are formed in the solution or on the electrode surface in the hydrogen evolution reaction on a Pt electrode. Using mass spectrometry and infrared spectroscopy, we show that hydrogen atoms are converted into hydrated protons directly on a Pt(111) surface coadsorbed with hydrogen and water in ultrahigh vacuum. The hydrated protons are preferentially stabilized as multiply hydrated species (H₅O₂⁺ and H₇O₃⁺) rather than as hydronium (H₃O⁺) ions. These surface‐bound hydrated protons may play an important role in the interconversion between adsorbed hydrogen atoms and solvated protons in solution.     

Stabilities in Polar Nonaqueous Solvents and Transfer Activity Coefficients from Water to Nonaqueous Solvents of 19-Crown-6-Alkali Metal Ion Complexes

Formation constants of 1 : 1 19-crown-6(19C6) complexes with alkali metal ions weredetermined conductometrically at 25 °Cin acetonitrile(AN), propylene carbonate (PC), methanol, DMF, andDMSO. 19C6 always forms the most stable complex withK⁺. The selectivity order of 19C6 forheavy alkali metal ions isK⁺ > Rb⁺ > Cs⁺.The selectivity for Na⁺ varies withthe solvent; that for Li⁺ is the second lowest(AN, DMSO) or the lowest (PC). Transfer activity coefficients(^SH _{2 O}) of19C6 from water to the nonaqueous solvents (S) weremeasured at 25 °C. The contributions of a methylenegroup and an ether oxygen atom to thelog ^SH _{2 O}value of a crown ether wereobtained. The ^SH _{2 O}values of the 19C6–alkali metal ion complexes(^SH _{2 O} (ML⁺)) werecalculated, M⁺ and L denoting an alkali metal ionand a crown ether, respectively. For AN, PC, andCH₃OH, although the M⁺ ion is more stronglysolvated by water than by AN, PC, or CH₃OH, thelog ^SH _{2 O} (ML⁺) islarger than the correspondinglog ^SH _{2 O} (L)expect for the case of M⁺ = Li⁺.The higher lipophilicity of the19C6 complex ion is attributed to an enforcement ofthe hydrogen-bonded structure of water for the complexion caused by the greatly decreased hydrogen bondingbetween ether oxygen atoms and water uponcomplexation. For DMF and DMSO, thelog ^SH _{2 O} (ML⁺) is also greater thanthe correspondinglog ^SH _{2 O} (L).It was concluded from thisfinding that the unexpectedly lowest stability of the19C6 complex ion in water is due to the hydrogenbonding between 19C6 and water. The stabilities and thelog ^SH _{2 O}of 19C6–alkali metal ion complexes were compared with those of 18C6complexes.     

Detailed electron spectrometric study of the ionization of H2 by He* (21 S, 23 S)

The energy spectra of electrons released in thermal energy (≈ 50 meV) ionizing collisions of He*(2¹ S, 2³ S) with H₂ have been measured with high resolution and low background. Based on a detailed data analysis, we report accurate H ₂ ⁺ (v′) vibrational populationsP(v′) for both He*(2¹ S)+H₂(v′=0–10) and He*(2³ S)+H₂(v′=0–15) and the spectral shapeS(ε) for the individual vibrational peaks. The vibrational populationsP(v′) are quite similar to the Franck-Condon factorsf _v ′0 for unperturbed H₂(v″=0)→H ₂ ⁺ (v′) transitions, but, more in detail, the ratiosP(v′)/f _v ′0 show a characteristically differentv′-dependence for He*(2³ S), He*(2¹ S), and HeI_α(58.4 nm) ionization. The vibrational level separations in the He*(2¹ S, 2³ S)+H₂ spectra agree with those in the HeI photoelectron spectrum to within 1–2 meV. The spectral shapesS(ε) are characteristically different for He*(2¹ S)+H₂ and He*(2³ S)+H₂, reflecting the respective differences in the entrance channel potentials, as determined previously in ab initio calculations and from scattering experiments.     

State of Molecules and Ions in the Structural Channels of Synthetic Beryl with an Ammonium Impurity

The contents of the structural channels of beryl, grown hydrothermally from an ammonium-containing solution, were investigated by IR and EPR spectroscopy. Using IR spectroscopy we found that water molecules, ammonium ions, and a small number of HCl molecules enter the structural channels of beryl in the course of mineral growth. In these beryls, the ammonium ions play the role of alkali cations. The ammonium ions are as rigidly fixed in the lattice as are water molecules; they are eliminated by calcination at high temperatures close to the decomposition temperature. On exposure to radiation at 77 K, the paramagnetic NH ₃ ⁺ and H⁰ radicals are stabilized in the structural channels of beryl. In addition to the known H⁰ radical, other states of atomic hydrogen, interacting with medium protons, are observed as well. For one of the additional radicals, H_b, we suggest the model of atomic hydrogen stabilized at the center of a silicon-oxygen ring with two water molecules in adjacent cavities.     

Redox Processes in Water,Initiated by Electric Discharge over Its Surface

The yields of oxidation, reduction in a flash corona discharge between a solid cathode andthe water surface are compared. As the cathode was used a system of five aluminum electrodes. As the gasmedium were tested air, oxygen, and nitrogen. The models of processes in the discharge suggest formation of atomic hydrogen in water vapor: H₂O H + OH. However, the reduction yield is equal to the Faraday value irrespective of the gas composition. In the presence of oxygen, oxidation prevails. The yield of Fe^{2 +} oxidation in oxygen is about 190 reaction events per electron passed in the circuit; in air it is lower by a factor of 2, and in nitrogen the yield is equal to the Faraday value.     

Structure and Composition of the 200 K‐Superconducting Phase of H2S at Ultrahigh Pressure: The Perovskite (SH−)(H3S+)

At ultrahigh pressure (>110 GPa), H₂S is converted into a metallic phase that becomes superconducting with a record T_c of approximately 200 K. It has been proposed that the superconducting phase is body‐centered cubic H₃S (Im m, a=3.089 Å) resulting from the decomposition reaction 3 H₂S→2 H₃S+S. The analogy between H₂S and H₂O led us to a very different conclusion. The well‐known dissociation of water into H₃O⁺ and OH^? increases by orders of magnitude under pressure. H₂S is anticipated to behave similarly under pressure, with the dissociation process 2 H₂S→H₃S⁺+SH^? leading to the perovskite structure (SH^?)(H₃S⁺). This phase consists of corner‐sharing SH₆ octahedra with SH^? ions at each A site (the centers of the S₈ cubes). DFT calculations show that the perovskite (SH^?)(H₃S⁺) is thermodynamically more stable than the Im m structure of H₃S, and suggest that the A site hydrogen atoms are most likely fluxional even at T_c .     

The Ideal Ionic Liquid Salt Bridge for Direct Determination of Gibbs Energies of Transfer of Single Ions,Part II: Evaluation of the Role of Ion Solvation and Ion Mobilities

An important intermediate goal to evaluate our concept for the assumption‐free determination of single‐ion Gibbs transfer energies Δ_trG°(i, S₁→S₂) is presented. We executed the crucial steps a) and b) of the methodology, described in Part I of this treatise, exemplarily for Ag⁺ and Cl^‐ with S₁ being water and S₂ being acetonitrile. The experiments showed that virtually all parts of the liquid junction potentials (LJPs) at both ends of a salt bridge cancel, if the bridge electrolyte is an “ideal” ionic liquid, that is, one with nearly identical diffusion of anion and cation. This ideality holds for [N₂₂₂₅]⁺[NTf₂]^‐ in the pure IL, but also in water and acetonitrile solution. Electromotive force measurements of solvation cells between S₁ and S₂ demonstrated Nernstian behavior for Ag⁺ concentration cells and constant like cell potentials for solutions with five tested Ag⁺ counterions.     

The Ideal Ionic Liquid Salt Bridge for Direct Determination of Gibbs Energies of Transfer of Single Ions,Part II: Evaluation of the Role of Ion Solvation and Ion Mobilities

An important intermediate goal to evaluate our concept for the assumption‐free determination of single‐ion Gibbs transfer energies Δ_trG°(i, S₁→S₂) is presented. We executed the crucial steps a) and b) of the methodology, described in Part I of this treatise, exemplarily for Ag⁺ and Cl^‐ with S₁ being water and S₂ being acetonitrile. The experiments showed that virtually all parts of the liquid junction potentials (LJPs) at both ends of a salt bridge cancel, if the bridge electrolyte is an “ideal” ionic liquid, that is, one with nearly identical diffusion of anion and cation. This ideality holds for [N₂₂₂₅]⁺[NTf₂]^‐ in the pure IL, but also in water and acetonitrile solution. Electromotive force measurements of solvation cells between S₁ and S₂ demonstrated Nernstian behavior for Ag⁺ concentration cells and constant like cell potentials for solutions with five tested Ag⁺ counterions.     

Effects of C3-aromatic heterocycles on 1,3,5-triaryl-2-pyrazoline sulfonium salt photoacid generators as light-emitting diode-sensitive cationic photoinitiators

Four 1,5-diphenyl-3-aromatic heterocyclyl-2-pyrazoline-based sulfonium salt photoacid generators (PAGs) with different aromatic heterocycles substituted on C3 atom and dimethyl sulfonium group on C5 atom were synthesized. These PAGs were highly photosensitive in the 365–425 nm light-emitting diode region, and the intramolecular charge transfer from the pyrazoline ring to sulfonium salts induced efficient photolysis and high Φ_H⁺. The heterocycles as well as their substituted positions significantly influenced the energy of the S₂ orbital, which was determined by the electrochemical and absorption properties of the PAGs. The raising of the S₂ orbital energy enlarged the energy gap of S₀–S₂ and S₁–S₂, resulting in blue shift of the absorption spectra and increase in the quantum yield of photoacid generation (Φ_H⁺), respectively. When the energy of excited electrons was higher than that of the S₂ orbital, the transition from S₀ to S₂ (π–π*) occurred before the C-S cleavage on S₁ and the PAGs showed high Φ_H⁺ values (0.52–0.72). The transition from S₀ to S₁ (π–σ*) occurred when the energy of electrons is lower than that of the S₂ orbital, and the PAGs showed low Φ_H⁺ value. The photopolymerization kinetics demonstrated that these PAGs were highly efficient cationic photoinitiators.     

SIMPLE EXPLANATION OF THE MISSES IN THE COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2 EVOLUTION

Abstract —In the model of Forbush et at. (1971) the observed damping of the flash yield sequence of photosynthetic O₂ evolution was related to a certain percentage of ‘misses’ (α; i.e. centers not converted). The possibility of a miss was supposed to be equal for all states S_0.1,2,3. We propose a new model and a new recurrence law that gives better quantitative agreement with the O₂ yield oscillations observed in Chlorella during a sequence of flashes. We find a better fit with all experimental results by assuming very unequal misses; the misses occur nearly exclusively on S₂ (and also sometimes on S₃). In the simpler case of only one miss on one state, half of S₂ exists as an inactive form S₂₊^- because it is in apparent equilibrium with pool A. The active form of S₂ is converted to S₃ in a flash and the unchanged inactive form S₂₊^- explains the miss: S ₁ hv→S₂₊^-=S₂hv→S₃ (S₂₊^- is a transition state between S₁ to S₂ associated with Q^-). In the dark, the apparent equilibrium constant K_A between pool A and Q (i.e. S₀, S₁ in the dark) is very large; this explains why there is no miss on these states. In light, the experimental value of K_A between pool A and Q (i.e. S₂, S₃ in the light) is 1, and this explains why the misses are large for states S₂, S₃; i.e., S₂₊^-/S2- 1 and sometimes S₃₊^-/S₃?0–1. This new model predicts that the total number of active states ΣS_i=S₀+S₁+S₂+S₃ is an oscillating function of the flash number. This sum 2S, is also the number of trapping centers for excitons. As fluorescence is proportional to excitons that are not trapped, our model explains why the fluorescence oscillates as a function of the flash number. We find also that the initial rates of O₂ evolution after (n - 1) flashes vs the 02 yield of the n^th flash are not exactly on a straight line, which also favors our model.     

Unlocking the Hydrolytic Mechanism of GH92 α-1,2-Mannosidases: Computation Inspires the use of C-Glycosides as Michaelis Complex Mimics

The conformational changes in a sugar moiety along the hydrolytic pathway are key to understand the mechanism of glycoside hydrolases (GHs) and to design new inhibitors. The two predominant itineraries for mannosidases go via ^OS₂→B_2,5→¹S₅ and ³S₁→³H₄→¹C₄. For the CAZy family 92, the conformational itinerary was unknown. Published complexes of Bacteroides thetaiotaomicron GH92 catalyst with a S-glycoside and mannoimidazole indicate a ⁴C₁→⁴H₅/¹S₅→¹S₅ mechanism. However, as observed with the GH125 family, S-glycosides may not act always as good mimics of GH's natural substrate. Here we present a cooperative study between computations and experiments where our results predict the E₅→B_2,5/¹S₅→¹S₅ pathway for GH92 enzymes. Furthermore, we demonstrate the Michaelis complex mimicry of a new kind of C-disaccharides, whose biochemical applicability was still a chimera.     

Specific Photographic and Luminescent Properties of Sulfur + Gold–Sensitized Silver Halide Emulsions

It was found that the introduction of univalent gold ions at the initial step of sulfur sensitization could lead to a dramatic fall in the light sensitivity (S) and a considerable increase in the intensity of low-temperature ( = 77 K) luminescence (I) of silver sulfide clusters produced by sensitization. An increase in the hold time was accompanied by an increase in S and a decrease in I. The fall in S is associated with the oxidation of the silver moiety in (Ag₂S)_pAg⁺ _k or (Ag₂S)_qAg⁰ _n (q > p) clusters, which are light sensitivity centers. AgBr(I) emulsions subjected to sulfur + gold-sensitization exhibited a flash of IR-excited green luminescence from paired iodine centers. The appearance of this flash is due to the generation of deep electron traps by sulfur–gold sensitization.