Delayed fluorescence from Rhodopseudomonas sphaeroides reaction centers. Enthalpy and free energy changes accompanying electron transfer from P-870 to quinones

Delayed fluorescence from isolated reaction centers of Rhodopseudomonas sphaeroides was measured to study the energetics of electron transfer from the bacteriochlorophyll complex (P-870, or P) to the primary and secondary quinones (Q_A and Q_B). The analysis was based on the assumption that electron transfer between P and Q reaches equilibrium quickly after flash excitation, and stays in equilibrium during the lifetime of the P⁺Q⁻ radical pair. Delayed fluorescence of 1Q reaction centers (reaction centers that contain only Q_A) has a lifetime of about 0.1 s, which corresponds to the decay of P⁺Q⁻_A. 2Q reaction centers (which contain both Q_A and Q_B) have a much weaker delayed fluorescence, with a lifetime that corresponds to that of P⁺Q⁻_B (about 1 s). In the presence of o-phenanthroline, the delayed fluorescence of 2Q reaction centers becomes similar in intensity and decay kinetics to that of 1Q reaction centers. From comparisons of the intensities of the delayed fluorescence from P⁺Q⁻_A and P⁺Q⁻_B, the standard free energy difference between P⁺Q⁻_A and P⁺Q⁻_B is calculated to be 78 ± 8 meV. From a comparison of the intensity of the delayed fluorescence with that of prompt fluorescence, we calculate that P⁺Q⁻_A is 0.86 ± 0.02 eV below the excited singlet state of P in free energy, or about 0.52 eV above the ground state PQ_A. The temperature dependence of the delayed fluorescence indicates that P⁺Q⁻_A is about 0.75 eV below the excited singlet state in enthalpy, or about 0.63 eV above the ground state.     

Thermoluminescence as a probe of photosystem II. The redox and protonation states of the secondary acceptor quinone and the O2-evolving enzyme

The thermoluminescence band observed in chloroplasts after flash excitation at ambient temperatures has recently been identified as being due to recombination of the electron on the semiquinone form of the secondary plastoquinone acceptor, Q_B, with positive charges on the oxygen-evolving enzyme, S₂ and S₃ (Rutherford, A.W., Crofts, A.R. and Inoue, Y. (1982) Biochim. Biophys. Acta 682, 457–465). Further investigation of this thermoluminescence confirms this assignment and provides information on the function of PS II. The following data are reported: (1) Washing of chloroplasts with ferricyanide lowers the concentration of Q_B⁻ in the dark and predictable changes in the extent of the thermoluminescence band are observed. (2) The thermoluminescence intensity arising from S₂Q_B⁻ is approximately one half of that arising from S₃Q_B⁻. (3) Preflash treatment followed by dark adaptation results in changes in the intensity of the thermoluminescence band recorded after a series of flashes. These changes can be explained according to the above assignments for the origin of the thermoluminescence and if Q_B⁻ provides an important source of deactivating electrons for the S states. Computer simulations of the preflash data are reported using the above assumptions. Previously unexplained data already in the literature (Läufer, A. and Inoue, Y. (1980) Photobiochem. Photobiophys. 1, 339–346) can be satisfactorily explained and are simulated using the above assumptions. (4) Lowering the pH to pH 5.5 results in a shift of the S₂Q_B⁻ thermoluminescence band to higher temperatures while that arising from S₃Q_B⁻ does not shift. This effect is interpreted as indicating that Q_B⁻ is protonated and the S₂ to S₃ reaction involves deprotonation while the S₁ to S₂ reaction does not.     

Evidence for slow turnover in a fraction of Photosystem II complexes in thylakoid membranes

We used two different techniques to measure the recovery time of Photosystem II following the transfer of a single electron from P-680 to Q_A in thylakoid membranes isolated from spinach. Electron transfer in Photosystem II reaction centers was probed first by spectroscopic measurements of the electrochromic shift at 518 nm due to charge separation within the reaction centers. Using two short actinic flashes separated by a variable time interval we determined the time required after the first flash for the electrochromic shift at 518 nm to recover to the full extent on the second flash. In the second technique the redox state of Q_A at variable times after a saturating flash was monitored by measurement of the fluorescence induction in the absence of an inhibitor and in the presence of ferricyanide. The objective was to determine the time required after the actinic flash for the fluorescence induction to recover to the value observed after a 60 s dark period. Measurements were done under conditions in which (1) the electron donor for Photosystem II was water and the acceptor was the endogenous plastoquinone pool, and (2) Q₄₀₀, the Fe²⁺ near Q_A, remained reduced and therefore was not a participant in the flash-induced electron-transfer reactions. The electrochromic shift at 518 nm and the fluorescence induction revealed a prominent biphasic recovery time for Photosystem II reaction centers. The majority of the Photosystem II reaction centers recovered in less than 50 ms. However, approx. one-third of the Photosystem II reaction centers required a half-time of 2–3 s to recover. Our interpretation of these data is that Photosystem II reaction centers consist of at least two distinct populations. One population, typically 68% of the total amount of Photosystem II as determined by the electrochromic shift, has a steady-state turnover rate for the electron-transfer reaction from water to the plastoquinone pool of approx. 250 e⁻ / s, sufficiently rapid to account for measured rates of steady-state electron transport. The other population, typically 32%, has a turnover rate of approx. 0.2 e⁻ / s. Since this turnover rate is over 1000-times slower than normally active Photosystem II complexes, we conclude that the slowly turning over Photosystem II complexes are inconsequential in contributing to energy transduction. The slowly turning over Photosystem II complexes are able to transfer an electron from P-680 to Q_A rapidly, but the reoxidation of Q⁻_A is slow (t1/2 = 2 s). The fluorescence induction measurements lead us to conclude that there is significant overlap between the slowly turning over fraction of Photosystem II complexes and PS II_β reaction centers. One corollary of this conclusion is that electron transfer from P-680 to Q_A in PS II_β reaction centers results in charge separation across the membrane and gives rise to an electrochromic shift.     

Inhibition of the reoxidation of the secondary electron acceptor of Photosystem II by bicarbonate depletion

In bicarbonate-depleted chloroplasts, the chlorophyll a fluorescence decayed with a halftime of about 150 ms after the third flash, and appreciably faster after the first and second flash of a series of flashes given after a dark period. After the fourth to twentieth flashes, the decay was also slow. After addition of bicarbonate, the decay was fast after all the flashes of the sequence. This indicates that the bicarbonate depletion inhibits the reoxidation of the secondary acceptor R²⁻ by the plastoquinone pool; R is the secondary electron acceptor of pigment system II, as it accepts electrons from the reduced form of the primary electron acceptor (Q⁻). This conclusion is consistent with the measurements of the DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea)-induced chlorophyll a fluorescence after a series of flashes in the presence and the absence of bicarbonate, if it is assumed that DCMU not only causes reduction of Q if added in the state QR⁻, but also if added in the state QR²⁻.     

High pH effect on S-state turnover in chloroplasts studied by thermoluminescence. Short-time alkaline incubation reversibly inhibits S3-to-S4 transition

The influence of high pH on the functioning of the oxygen-evolving system was studied with isolated thylakoids by measuring flash oxygen yield in parallel with thermoluminescence B band which originates in the recombination between the positive charges on S₂ and S₃, the oxidized states of the water-oxidizing enzyme, and the negative charges on Q⁻_B, the semireduced form of the secondary quinone acceptor of Photosystem II. It was found that a mild alkaline incubation of thylakoids (3 min at pH = 8.8–9.1 in darkness) largely inhibits O₂ evolution, while much less the B-band amplitude. The flash-induced period-four oscillation of the B band was abolished at high pH, showing normal oscillatory response only after the 1st and 2nd flashes, but no more oscillation after the 3rd flash. These observations indicated an inhibition of S₃-to-S₄ transition by high pH and were correlated primarily with the liberation of the 33 kDa peripheral protein followed by release of functional Mn. The above phenomena were largely reversed when the pH was returned to neutral. A possible mechanism of high pH inhibition of oxygen-evolving system is discussed.     

Oxygen photoreduction and variable fluorescence during a dark-to-light transition in Chlorella pyrenoidosa

When dark-adapted (5 min in the dark) Chlorella cells were deposited on a bare platinum electrode, treated with DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) and illuminated, O₂ was consumed after a lag time of about 250 ms. The comparison of the O₂ consumption kinetics with the fluorescence O-I-D-P-S transition (the fast change in chlorophyll fluorescence which occurs after the onset of illumination of dark-adapted algae and is over within 2 s) observed in untreated algae indicates that no O₂ is consumed during the fluorescence rise and that O₂ uptake is initiated approximately when the maximum level of fluorescence P is reached. Mass spectrometry measurements of O₂ exchange (using ¹⁸O₂) were performed during dark to light transition with DCMU-untreated Chlorella cells. Under these conditions, O₂ reduction began after a lag time (about 200–400 ms) and stopped after about 5 s of illumination. The above experiments clearly show that the reduction of O₂ starts nearly at the same time that the fluorescence P-S decline. On the other hand, we show that the reduction of CO₂ does not interfere in the fluorescence O-I-D-P-S transient. We found the same apparent affinity for O₂ (about 57 μM) for both the fluorescence P-S decline and the reduction of O₂. At least three consecutive short (2 μs) saturating flashes were required to affect the fluorescence transient significantly and also to induce a significant uptake of O₂. Moreover, parabenzoquinone, an artificial Photosystem I electron acceptor, inhibited both the fluorescence D-P rise and the 250 ms lag time observed in the reduction of O₂. We conclude from the above results that in the early stages of the illumination of dark-adapted algae, some Photosystem I electron acceptors are in an inactive form. In this form, the electron transport chain is unable to reduce either O₂ or CO₂. This would lead to the accumulation of electrons on the Photosystem II acceptors (principally Q⁻_A and the plastoquinone pool) and therefore explains the fluorescence D-P rise. The light activation, probably achieved through the reduction of at least two electron acceptors, first allows the reduction of O₂, and therefore explains the P-S fluorescence decline. By accepting electrons before the site of regulation and mediating rapid O₂ reduction, parabenzoquinone avoids the accumulation of electrons and therefore inhibits the D-P fluorescence rise.     

Fluorescence induction of Photosystem II membranes shows the steps till reduction and protonation of the quinone pool

Chlorophyll fluorescence induction (Chl-F) was investigated in Photosystem II (PSII)-enriched membranes, which predominantly include active (Q_B reducing) PSII reaction centres (RCs) and lack Photosystem I (PSI). The Chl-F curve of these preparations show a polyphasic rise from F₀, the minimal fluorescence, to F_P, the maximal fluorescence, with several intermediate transitions. Analyses of these transitions revealed three exponential rise components with lifetimes of 18 ms, 400 ms and 800 ms. The 18 ms component was assigned to the photoaccumulation of reduced Q_A. The two slowest components, of 400 ms and 800 ms, were assigned to Q_B reduction (Q_B⁻ and Q_B⁼) and further Q_B⁼ protonation (till Q_BH₂), respectively. These assignments were based on the observation of specific quenching of the phases by DCMU or by different oxidized, reduced and protonated quinones. The work is done in low light conditions which are saturating to avoid photoinhibition or PSII inactivation effects. The results suggest that the Chl-F curve observed in PSII-enriched membranes can be attributed to the sequential steps till the photoaccumulation (reduction and protonation) of plastoquinone (PQ) by PSII. These results are in good agreement with the molecular models that show a correspondence between Chl-F and PQ reduction steps, like the models that propose and explain the O-J-I-P transients.     

The dimerization of protochlorophyll pigments in non-polar solvents

The infrared, visible and nuclear magnetic resonance spectra of protochlorophyll a and vinylprotochlorophyll a in dry non-polar solvents (carbon tetrachloride, chloroform, cyclohexane) are presented and interpreted in terms of dimer interaction.

The infrared spectra in the 1600–1800 cm⁻¹ region clearly show the existence of a coordination interaction between the C-9 ketone oxygen function of one molecule and the central magnesium atom of another molecule. Infrared spectra in the OH stretching region (3200–3800 cm⁻¹) provide a valuable test of the water content in the samples.

The analysis of the absorption and circular dichroism spectra of protochlorophyll a and vinylprotochlorophyll a in carbon tetrachloride demonstrates the existence of a monomer-dimer equilibrium in the concentration range from 10⁻⁶ to 5 · 10⁻⁴ M. The dimerization constants are (6±2) · 10⁵ 1 · M⁻¹ for protochlorophyll a and (4.5±2) · 10⁵ 1 · M⁻¹ for vinylprotochlorophyll a at 20 °C. The deconvolution of visible spectra in the red region has been performed in order to obtain quantitative information on the dimer structure. Two models involving a parallel or a perpendicular arrangement of the associated molecules are considered.

From ¹H NMR spectra, it appears that the region of overlap occurs near ring V, in agreement with the interpretation of the infrared spectra.     

Binding of the inhibitor NH3 to the oxygen-evolving apparatus of spinach chloroplasts

Experiments are described on flash-induced luminescence of isolated spinach chloroplasts after addition of NH₄Cl. The results indicate a binding of NH₃, presumably in competition with water, in the oxidation states S₂ and S₃, i.e. the states reached upon illumination of dark-adapted material with one and two flashes, respectively. In the initial state S₁, no binding of NH₃ occurs. In state S₂ the binding of ammonia is rapid (half-time about 0.5 s) and rapidly reversible; in state S₃ the binding is slower (half-time about 10 s) and slowly reversible. NH₃ bound to S₄ prevents the oxidation of water. NH₃ bound to S₂ decreases the rate of the back reaction of reduced primary acceptor (Q⁻), indicating a charge stabilization, i.e. a decrease in the redox potential of S₂ due to interaction with ammonia. In Tris-washed chloroplasts, the stability of the positive charge generated in a flash is much smaller than in normal chloroplasts and not increased by NH₃. On the basis of these observations it is postulated that, in the absence of NH₃, states S₂ and S₃ are stabilized by manganese-coordinated, bound water.     

Electron donation in Photosystem II

The electron transfer resulting from illumination and dark storage of PS II has been studied using EPR signals from several electron carriers. The recombination of D⁺ (Signal II) and Q⁻_A formed by illumination occurred during dark storage at 77 K and was used to deplete reaction centres of D⁺. The donor D was then shown to be oxidized in the dark by the S₂ state of the oxygen-evolving complex. A slow change which occurred during dark storage of PS II samples was detected using the power saturation characteristics of D. We interpret this effect on D to be an indirect result of a rearrangement of the manganese complex during long-term dark adaptation. A role for D in the stability, protection and perhaps initial manganese binding of the oxygen-evolving complex is suggested.     

Photo-accumulation of the P+QB- radical pair state in purple bacterial reaction centres that lack the QA ubiquinone

Photo-excitation of membrane-bound Rhodobacter sphaeroides reaction centres containing the mutation Ala M260 to Trp (AM260W) resulted in the accumulation of a radical pair state involving the photo-oxidised primary electron donor (P). This state had a lifetime of hundreds of milliseconds and its formation was inhibited by stigmatellin. The absence of the Q_A ubiquinone in the AM260W reaction centre suggests that this long-lived radical pair state is P⁺Q_B⁻, although the exact reduction/protonation state of the Q_B quinone remains to be confirmed. The blockage of active branch (A-branch) electron transfer by the AM260W mutation implies that this P⁺Q_B⁻ state is formed by electron transfer along the so-called inactive branch (B-branch) of reaction centre cofactors. We discuss how further mutations may affect the yield of the P⁺Q_B⁻ state, including a double alanine mutation (EL212A/DL213A) that probably has a direct effect on the efficiency of the low yield electron transfer step from the anion of the B-branch bacteriopheophytin (H_B⁻) to the Q_B ubiquinone.     

The deactivation of Photosystem II in Chlorella as a second-order process

The kinetics of deactivation of the S₃ state in Chlorella have been observed under a variety of conditions. The S₃ state appears to decline in a dark period coming after a sequence of 30 saturating flashes in a second-order reaction, the rate constant of which is 0.132/[S*₃] s⁻¹ and which involves an electron donor, D₁, of concentration 1.25[S*₃] where [S*₃] is the concentration of the S₃ state when the oxygen yield of the light flashes is constant. If a 1 min period of 650 nm illumination is employed after the sequence of flashes, the subsequent S₃ state deactivation kinetics are more complex. There is an initial phase of S₃ state deactivation, accounting for about 35% of the original S₃ state, which is complete in less than 100 ms. The remaining 65% of the S₃ state appears to deactivate in a second-order reaction, the rate constant of which is 1.36/[S*₃] s⁻¹ and which involves an electron donor of initial concentration 0.58[S*₃]. If a 1 min period of 710 nm illumination comes after the 30 flashes, at least 98% of the S₃ state deactivates according to first-order kinetics. It is shown that this can be explained using a second-order model if there is an electron donor present of which the concentration is large compared with [S*₃]. However, S₃ state deactivation observed after 5 min of dark and two saturating flashes can be described neither by a first-order model nor a second-order model. Deactivation of the S₂ state after a 5 min dark period and one saturating flash follows second-order kinetics with a rate constant of 0.2/[S*₃] s⁻¹ and appears to involve an electron donor of initial concentration 1.3[S*₃]. Arguments are presented which tend to rule out the primary electron acceptor to Photosystem II as being any of the electron donors but it appears quite possible that the large plastoquinone pool is involved.     

Effect of low temperature (−30 to −60°C) on the reoxidation of the Photosystem II primary electron acceptor in the presence and absence of 3(3,4-dichlorophenyl)-1,1-dimethylurea

The fluorescence yield has been measured on spinach chloroplasts at low temperature (−30 to −60°C) for various dark times following a short saturating flash. A decrease in the fluorescence yield linked to the reoxidation of the Photosystem II electron acceptor Q is still observed at −60°C. Two reactions participate in this reoxidation: a back reaction or charge recombination and the transfer of an electron from Q⁻ to Pool A. The relative competition between these two reactions at low temperature depends upon the oxidation state of the donor side of the Photosystem II center:

1. (1) In dark-adapted chloroplasts (i.e. in States S₀+S₁ according to Kok, B., Forbush, B. and McGloin, M. (1970) Photochem. Photobiol. 11, 457–475), Q, reduced by a flash at low temperature, is reoxidized by a secondary acceptor and the positive charge is stabilized on the Photosystem II donor Z. Although this reaction is strongly temperature dependent, it still occurs very slowly at −60°C.

2. (2) When chloroplasts are placed in the S₂+S₃ states by a two-flash preillumination at room temperature, the reoxidation of Q⁻ after a flash at low temperature is mainly due to a temperature-independent back reaction which occurs with non-exponential kinetics.

3. (3) Long continuous illumination of a frozen sample at −30°C causes 6–7 reducing equivalents to be transferred to the pool. Thus, a sufficient number of oxidizing equivalents should have been generated to produce at least one O₂ molecule.

4. (4) A study of the back reaction in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) shows the superposition of two distinct non-exponential reactions one temperature dependent, the other temperature independent.

A low-temperature-sensitive intermediate state between S2 and S3 in photosynthetic water oxidation deduced by means of thermoluminescence measurements

The temperature dependence of S-state transitions in Photosystem II was measured by means of thermoluminescence using two different protocols for low-temperature flash excitation: protocol A, “last flash at low temperature”, and protocol B, “all flashes at low temperature”. Comparison of the temperature-dependence curves obtained by these two protocols revealed a marked difference particular for the three-flash experiments. The difference was attributed to the formation of a low-temperature sensitive precursor state between S₂ and S₃. The state is formed by two flash illumination given at −5 to −50°C, spontaneously transforms to normal S₃ on dark warming, and is not converted to S₀ by the 3rd flash. The precursor state was tentatively assigned to an S₃ in which H⁺ release is not completed.     

Quantum yield and rate of formation of the carotenoid triplet state in photosynthetic structures

The formation of the triplet state of carotenoids (detected by an absorption peak at 515 nm) and the photo-oxidation of the primary donor of Photosystem II, P-680 (detected by an absorption increase at 820 nm) have been measured by flash absorption spectroscopy in chloroplasts in which the oxygen evolution was inhibited by treatment with Tris. The amount of each transient form has been followed versus excitation flash intensity (at 590 or 694 nm). At low excitation energy the quantum yield of triplet formation (with the Photosystem II reaction center in the state Q⁻) is about 30% that of P-680 photo-oxidation. The yield of carotenoid triplet formation is higher in the state Q⁻ than in the state Q, in nearly the same proportion as chlorophyll a fluorescence. It is concluded that, for excited chlorophyll a, the relative rates of intersystem crossing to the triplet state and of fluorescence emission are the same in vivo as in organic solvent. At high flash intensity the signal of P-680⁺ completely saturates, whereas that of carotenoid triplet continues to increase.

The rate of triplet-triplet energy transfer from chlorophyll a to carotenoids has been derived from the rise time of the absorption change at 515 nm, in chloroplasts and in several light-harvesting pigment-protein complexes. In all cases the rate is very high, around 8 · 10⁷ s⁻¹ at 294 K. It is about 2–3 times slower at 5 K. The transitory formation of chlorophyll triplet has been verified in two pigment-protein complexes, at 5 K.     

Structures and magnetism of rubidium and cesium salts of dimeric oxovanadium(IV) tartrate(4−) complexes

Complexes of type A₄[VO(tart)]₂·nH₂O, where A = Rb or Cs and tart =d,l-tartrate(4−) (n = 2) or d,d-tartrate(4−) (n = 2 for Rb and n = 3 for Cs), were prepared from an aqueous mixture of V₂O₅, AOH and H₄tart. These complexes were studied by single-crystal X-ray diffraction methods: Rb₄[VO(d,l-tart)]₂·2H₂O, space group P1 with a = 8.156(1),b = 8.246(1),c = 8.719(1)Å, = 66.09(1)°, β = 65.07(1)°, γ = 82.40(1)°,Z = 2, 1917 observed reflections, and final R_w = 0.035; Cs₄[VO(d,l-tart)]₂·2H₂O, space group P₂₁/c with a = 9.350(1),b = 13.728(2),c = 8.479(1)Å, β = 106.77(1)°,Z = 4, 2235 observed reflections, and final R_w = 0.054; Rb₄[VO(d,d-tart)]₂·2H₂O, space group P₄₁₂₂ with a = 8.072(1),c = 32.006(3)Å,Z = 8, 1014 observed reflections and final R_w = 0.038; Cs₄[VO(d,d-tart)]₂·3H₂O, space group P₁₂₂ with a = 8.184(1),c = 33.680(5)Å,Z = 8, 1310 observed reflections, and final R_w = 0.063. Bulk magnetic susceptibility data (1.5–300 K) for these compounds and A₄[VOl,l-tart)]₂·nH₂O (A = Rb, Cs) were obtained on polycrystalline samples. These data were analyzed in terms of a Van Vleck exchange coupled S = 1/2 model which was modified to include an interdimer exchange parameters Θ. Analysis of the low-temperature (1.5–20 K) susceptibility data gave 2J = +1.30 cm⁻¹ and Θ = −1.86 K for Rb₄[VO(d,l-tart)]₂·2H₂O, 2J = +1.16 cm⁻¹ and Θ = −1.69 K for Cs₄[VO(d,l-tart)]₂·2H₂O, 2J = +1.90 cm⁻¹ and Θ = −0.82 K for Rb₄[VO(d,d-tart)]₂·2H₂O, 2J = +2.04 cm⁻¹ and Θ = −0.80 K for Rb₄[VO(l,l-tart)]₂·2H₂O, 2J = +1.52 cm⁻¹ and Θ = −0.25 K for Cs₄[VO(d,d-tart)]₂·3H₂O, and 2J = +1.64 cm⁻¹ and Θ = −0.31 K for Cs₄[VO(l,l-tart)]₂·3H₂O. These results suggest the magnitudes of intradimer (ferromagnetic and interdimer (antiferromagnetic) exchange interactions are similar in these complexes, as observed for the analogous Na salts.     

Réoxydation du quencher de fluorescence “Q” en présence de 3-(3,4-dichlorophényl)-1,1-diméthylurée

Reoxidation of the fluorescence quencher “Q” in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea

Reoxidation of the fluorescence quencher Q in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea shows the following properties:

It is sensitive to very low concentrations of hydroxylamine (a few μM).

It corresponds to a back reaction between Q⁻ and the primary oxidant Z⁺ formed in the light. A part of this back reaction gives rise to luminescence emission.

Within the range we studied the kinetic of reoxidation is second order with regards to Q⁻.     

A functional site of Ca in the oxygen-evolving photosystem II preparation from Synechococcus sp.

Treatment of the oxygen-evolving photosystem II preparation from the thermophilic cyanobacterium Synechococcus sp. with EDTA inhibited electron flow from Z to P680 and consequently induced a back electron flow from Q⁻_a to P680⁺. The inhibition was reversed fully by Ca²⁺and partially by Mn²⁺ and Mg²⁺ when EDTA-treated preparations had been incubated with respective divalent metal cations for several minutes, whereas diphenylcarbazide had no effect on the recombination between q⁻_a and P680⁺ in EDTA-treated preparations. It is concluded that Ca²⁺ is essential for electron transport from Z to P680.

Oxygen evolution Electron transport Photosystem II Ca²⁺ Thermophilic cyanobacterium     

Mixed anion lanthanide(III) crown ether complexes: crystal structures of [LaCl2(NO3)(12-crown-4)]2, [La(NO3)(OH2)4-(12-crown-4)]Cl2·CH3CN and [LaCl2(NO3)(18-crown-6)]

Reaction of LaCl₃·7H₂O containing small amounts of La(NO₃)₃·7H₂O as an impurity with 12-crown-4 or 18-crown-6 in 3:1 CH₃CN:CH₃OH resulted in the isolation of the mixed anion complexes [LaCl₂(NO₃)(12-crown-4)]₂, [La(NO₃)(OH₂)₄(12-crown-4)]Cl₂·CH₃CN and [LaCl₂(NO₃)(18-crown-6)]. The nine-coordinate dimer, [LaCl₂(NO₃)(12-crown-4)]₂, has all of the anions in the inner coordination sphere and La³⁺ has a capped square antiprismatic geometry. It crystallizes in the orthorhombic space group Pbca with (at −150 °C) a = 12.938(6), B = 15.704(3), C = 13.962(2) Å, and D_calc = 2.08 g cm⁻³ for Z = 4. The second complex isolated from the same reaction, [La(NO₃)(OH₂)₄(12-crown-4)]Cl₂·CH₃CN, has the bidentate nitrate anion in the inner coordination sphere but the two chloride anions are in a hydrogen bonded outer sphere. This complex is ten-coordinate 4A,6B-expanded dodecahedral and crystallizes in the monoclinic space group P2₁ with (at 20 °C) A = 7.651(2), B = 11.704(7), C = 11.608(4) Å, β = 95.11(2)°, and D_calc = 1.80 g cm⁻³ for Z = 2. The 18-crown-6 complex, [LaCl₂(NO₃)(18-crown-6)], has all inner sphere anions and has ten-coordinate 4A,6B-expanded dodecahedral La³⁺ centers. It crystallizes in the orthorhombic space group Pbca with (at 20 °C) a = 14.122(7), B = 13.563(5), C = 19.311(9) Å, and D_calc = 1.89 g cm⁻³ for Z = 8.     

Identification of cytochromes o and a3 as functional terminal oxidases in the thermophilic bacterium, PS3

Low-temperature photodissociation spectra of membranes from the thermophile PS3 reveal cytochromes o and a₃. The latter reacts with O₂ at −103°C to give a light-insensitive compound(s), but the initial stages of O₂ binding to cytochrome o could not be studied under these conditions. Photochemical action spectra identify cytochromes a₃ and o, but not a CO-binding c-type cytochrome, as functional terminal oxidases in this bacterium.