PHOTOPHYSICS OF THE FLUORESCENT Ca2+ INDICATOR QUIN-2

The photophysics of the complex forming reaction between Quin-2 and Ca²⁺ were investigated using steady-state and time-resolved fluorescence measurements. The fluorescence decay traces were analyzed with global compartmental analysis yielding the following values for the rate constants at room temperature in aqueous solution with EGTA as Ca²⁺ buffer: k₀₁= 8.6 times 10⁸ s^?1, k₂₁= 1 times 10¹¹M^?1 s^?1, k₀₂= 8.8 times 10⁷ s^?1, k₁₂= 4 times 10⁴ s^?1. k₀₁ and k₀₂ denote the respective deactivation rate constants of the Ca²⁺ free and bound forms of Quin-2 in the excited state. The constant k₂₁ represents the second-order rate constant of binding of Ca²⁺ and Quin-2 in the excited state while k₁₂ is the first-order rate constant of dissociation of the excited Ca²⁺:Quin-2 complex. From the estimated values of k₁₂ and k₂₁ the dissociation constant K_d* in the excited state was calculated. It was found that pK_d* (6.4) is slightly smaller than pK_d (7.2). There was no interference of the excited-state complex forming reaction with the determination of K_d. Intracellular Ca²⁺ concentrations can thus accurately be determined from fluorometric measurements using Quin-2 as Ca²⁺ indicator.     

CORRELATION BETWEEN THE BINDING OF FORMATE AND DECREASED RATES OF CHARGE TRANSFER THROUGH THE PHOTOSYSTEM II QUINONES

Abstract— The binding parameters of bicarbonate to the thylakoid membrane at different formate concentrations have been established [Stemler and Murphy (1983) Photochem. Phorobiol. 38, 701–707]. Based on these parameters, predictions could be made concerning the effects of bicarbonate and formate on photosynthetic electron flow. In this work these effects of various concentrations of bicarbonate and formate are measured and compared to predictions from the binding study. Electron flow is measured between Q_A and Q_B (the primary and secondary quinone acceptors) and Q_B and the plastoquinone pool. Also, these same concentration effects are determined for silicomolybdate supported oxygen evolution. It is found that the results of the bicarbonate binding study are in good agreement with the concentration dependence determined for the quinone reactions, as well as the silicomolybdate reaction. The bicarbonate concentrations required for half-maximal effects are approximately 100 μM, 300 μM and 1.3 mM in the presence of 0, 20 mM and 100 mM formate, respectively. It is concluded that a hierarchy of possible electron flow rates exist. The slowest rates occur when formate is bound. A substantially higher rate occurs when neither formate nor bicarbonate (< 2 μM) are present, but only chloride is present. The highest rates of electron flow occur when bicarbonate is bound. The Q_A- Q_B→ Qa Qb^? Qa^? Qb^2– PQ → Q_a Q_b- PQ^2–, and the silicomolybdate reactions all have the same concentration dependence on formate and bicarbonate. From this it is concluded that a single binding site for formate and bicarbonate affect all of these reactions. The possibility that multiple sites exist with approximately equal affinities for bicarbonate cannot be excluded.     

LIGHT-INDUCED FORMATION OF O-2˙ OXYGEN RADICALS IN SYSTEMS CONTAINING CHLOROPHYLL

It has been shown recently that photosystem 1 particles, photosystem 1 lipid vesicles and chlorophyll-a lipid vesicles show identical photochemical reactions in the presence of oxygen e.g. H⁺-and O₂-uptake (Van Ginkel, 1979). Therefore, spin-trapping experiments were done to identify the oxygen radicals formed. The spintrap phenyltertiarybutylnitrone (PBN) failed to yield information about oxygen radicals. With the spintrap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO), however, we obtained a mixed spectrum of O^-_2˙ and OH·-adducts generated in chloroplasts, photosystem 1 particles or chlorophyll-a lipid vesicles. These data indicate that chlorophyll-a in an artificial membrane can also catalyze O^-_2˙-formation. Chlorophyll-a lipid vesicles catalyze light-induced formation of the Tiron-semiquinone free radical, which has been proposed as a specific O^-_2˙-probe (Greenstock and Miller, 1975). However, OH· scavengers strongly reduce the formation of this radical, whereas superoxide dismutase does not. Pulse-radiolysis measurements showed that the rate constant for the reaction of Tiron with OH· is 8.2 · 10⁹M^-1 s^-1, which is considerably higher than the published Tiron/O^-_2˙ rate constants. Therefore, Tiron is a better spin probe for OH· than for O^-_2˙. We suggest that light-induced H⁺-and O^-_2˙-uptake in membranes containing chlorophyll-a in the presence of ascorbate is caused mainly by the very rapid reaction of OH· with ascorbate.     

EPR and Spectrophotometric Studies of Free Radicals (O2°−, °OH, BPD-MA°−) and Singlet Oxygen (1O2) Generated by Irradiation of Benzoporphyrin Derivative Monoacid Ring A

Abstract— Benzoporphyrin derivative monoacid ring A (BPD-MA), a chlorin-type molecule, is a new photosensitizer currently in phase II clinical trials for the treatment by pho-todynamic therapy of cancerous lesions, psoriasis and pathologic neovascularization. The photochemistry (type I and/or II) of BPD-MA has been studied in homogeneous solution and in aqueous dispersions of unilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) using electron paramagnetic resonance and spectrophotometric methods. When oxygen-saturated solutions of BPD-MA were illuminated with 690 nm light, singlet oxygen (¹O₂), superoxide anion radical (O₂^?), hydroxyl radical (OH) and hydrogen peroxide (H₂O₂) were formed. The BPD-MA generates ¹O₂ with a quantum yield of ca 0.81 in ethanolic solution. The quantum yield does not change upon incorporation of BPD-MA into liposomes of DPPC. The superoxide anion radical was generated by the BPD-MA anion radical (BPD-MA^?) via electron transfer to oxygen, and this process was significantly enhanced by the presence of electron donors. The rate of production of 0₂ was also dependent on the concentration of BPD-MA used (3-100 μM). The quantum yield of O₂^?was found to be 0.011 and 0.025 in aqueous solution and DPPC liposomes, respectively. Moreover, O₂^_upon dis-proportionation can generate H₂O₂ and ultimately the highly reactive OH via the Fenton reaction. In anaerobic homogeneous solution, BPD-MA^?was predominantly photoproduced via the self-electron transfer between the excited- and ground-state species. The presence of an electron donor significantly promotes the reduced form of BPD-MA. These findings suggest that the photodynamic action of BPD-MA may proceed via both type I and type II mechanisms.     

Comparative Study of Ru(bpz)32+Ru(bipy)32+and Ru(bpz)2CI2 as Photosensitizers of DNA Cleavage and Adduct Formation

Abstract— The efficiency of ruthenium complexes for photosensitizing DNA damage depends on the oxidizing character of their ligands. Here we report on the difference in behavior of tris(2.2'-bipyrazyl)ruthenium(II) (Ru[bpz]₃²+), tris(2,2′-bipyridyl)ruthenium(II) (Ru[bipy]₃²+) and cis-dichlorobis(2,2′-bipyrazyl)ruthenium(II) (Ru[bpz]₂Cl₂). Upon irradiation at 436 nm, Ru(bpz)₃²+was far less stable than Ru(bipy)₃²+. Ru(bpz)₃²+in phosphate buffer containing NaCl undergoes a photoanation reaction leading to the formation of Ru(bpz)₂Cl₂, as previously reported also in organic media. In the presence of phage φX174 DNA, Ru(bpz)₃²+photosensitized the formation of single strand breaks with an efficiency that was, at the beginning of irradiation, similar to that of Ru(bipy)₃²+. After 8 min of irradiation, the cleavage efficiency of Ru(bpz)₃²+reached a plateau that may correspond to its photode-composition. For the same conditions, Ru(bpz)₂Cl₂ did not induce DNA breakage. Scavenging experiments showed that, in the presence of oxygen, DNA cleavage induced by Ru(bpz)₃²+partly resulted from the formation of singlet oxygen and hydroxyl radical while in the absence of oxygen an additionnal mechanism involving electron transfer between the excited state of the ruthenium complex and DNA is proposed. The ICP measurement showed that Ru(bpz)₃²+and Ru(bpz)₂Cl₂ gave rise to covalent binding onto DNA in contrast with Ru(bipy)₃²+, which did not bind to DNA under the experimental conditions. The results are discussed with regard to the potential use of these photosensitizers in phototherapy.     

Decomposition of Copper Formate Clusters: Insight into Elementary Steps of Calcination and Carbon Dioxide Activation

The decomposition of copper formate clusters is investigated in the gas phase by infrared multiple photon dissociation of Cu(II)_n(HCO₂)_2n+1⁻, n≤8. In combination with quantum chemical calculations and reactivity measurements using oxygen, elementary steps of the decomposition of copper formate are characterized, which play a key role during calcination as well as for the function of copper hydride based catalysts. The decomposition of larger clusters (n > 2) takes place exclusively by the sequential loss of neutral copper formate units Cu(II)(HCO₂)₂ or Cu(II)₂(HCO₂)₄, leading to clusters with n=1 or n=2. Only for these small clusters, redox reactions are observed as discussed in detail previously, including the formation of formic acid or loss of hydrogen atoms, leading to a variety of Cu(I) complexes. The stoichiometric monovalent copper formate clusters Cu(I)_m(HCO₂)_m+1⁻, (m=1,2) decompose exclusively by decarboxylation, leading towards copper hydrides in oxidation state +I. Copper oxide centers are obtained via reactions of molecular oxygen with copper hydride centers, species containing carbon dioxide radical anions as ligands or a Cu(0) center. However, stoichiometric copper(I) and copper(II) formate Cu(I)(HCO₂)₂⁻ and Cu(II)(HCO₂)₃⁻, respectively, is unreactive towards oxygen.     

INTERACTIONS OF COBALT(II) TETRASULFOPHTHALOCYANINE WITH NITRITE IN THE PRESENCE OF NITRATE AND PERCHLORATE IONS

Abstract

Spectroscopic changes observed on addition of nitrite to solutions of cobalt(II) tetra-sulfophthalocyanine ([Co(II)TSPc]^4-) in the presence of N^? ₃ or ClO^? ₄ are reported. There is spectroscopic evidence for the oxidation of [Co(II)TSPc]^4- to a [Co(III)TSPc]^3- species in the presence of nitrite ions. Equilibrium and kinetic studies for the interaction between [Co(II)TSPc]^4- and NO^? ₂ are reported. The rate was found to be first order in both [Co(II)TSPc]^4- and NO^? ₂. The rate constant for the forward reaction, k _f=1.6 × 10^?4 dm³mol^?1s^?1 was determined at 20°C for the interaction between nitrite ions and [Co(II)TSPc]^4- in the presence of NO₃ ^? or ClO₄ ^? ions.     

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.     

RUTHENIUM RED INHIBITION OF OXYGEN EVOLUTION AND SPECIFIC RELEASE OF THE EXTRINSIC 16 kDa POLYPEPTIDE IN A PHOTOSYSTEM II PREPARATION

The inhibitory effect of the dye ruthenium red was studied in photosystem II-enriched submembrane fractions. A number of distinct types of interaction were found, which differed in their concentration range and required incubation time. Ruthenium red instantaneously quenches the initial chlorophyll a fluorescence level (F₀) and the maximum fluorescence level (F_m) by enhancing radiationless deactivation in the chlorophyll light harvesting complex. Associated with this quenching of fluorescence is an instantaneous decrease in the quantum yield of oxygen evolution. Ruthenium red also inhibited the light saturated rate of oxygen evolution and the variable fluorescence, monitored 80 µs after a saturating excitation-flash. These inhibitions increased with incubation time and became greater than 50% within 5 min. Although ruthenium red was known to affect Ca²⁺ or Cl^? sites specifically, the inhibitory action was more pronounced than simple Ca²⁺ or Cl^? depletion. Incubation with ruthenium red for 5 min blocks the Z P680⁺ → Z⁺ P680 charge transfer reaction. Upon mixing with the photosystem II preparation, ruthenium red induced specific release of the extrinsic 16 kDa polypeptide associated with water-splitting without release of Mn. It is proposed that the inhibitor produces an ionic imbalance which alters the configuration of the donor side of photosystem II.     

Blue Light Requirement for HC03 Uptake and Its Action Spectrum in Monoraphidium braunii

The uptake and assimilation of HCO₃ by the green unicellular alga Monoraphidium braunii can be monitored by the alkalinization of the external medium or by the O₂ evolution associated with the uptake and reduction of this anion. The activation of HCO₃ uptake in this microalga required the irradiation of the cell suspensions with low photon fluence rates of short wavelength radiation. Thus, when the cells were irradiated with strong red light in the presence of HCO₃, very little alkalinization of the external medium or O₂ evolution could be observed. The O₂ evolution rates measured under red light could be due to the assimilation of the CO₂ derived from the HCO₃ present in the medium. The blue light-dependent O₂ evolution rates were not diminished by a periplasmic carbonic anhydrase inhibitor, suggesting that HCO₃ -dependent O₂ evolution was due to the photoactivation of a selective HCO₃ uptake system at the plasma membrane. The action spectrum for HCO₃- uptake in M. braunii was very similar to those reported for NO₃- and CI^- suggested that a flavoprotein may be the photoreceptor for this response.     

Kinetics and mechanism of the catalytic dehydration of HCO3− by zinc(II) complexes of tripod ligands

The kinetics of the catalyzed dehydration of HCO₃^? by zinc(II) containing tripod complexes has been studied at 25°C using the stopped‐flow technique. The direction of reaction curve was changed in aqueous solution when the pH of the solution was greater than 7.5. The pH‐profile of rates of the dehydration reactions indicates that only the aqua complex catalyzes the dehydration of HCO₃^? via a ligand substitution process. The second‐order rate constants for the dehydration of HCO₃^? catalyzed by complexes Zn₃L1, Zn₃L2, Zn₃L3, and Zn₃L4 are 0.96, 2.53, 12.05, and 6.99 mol^?1 dm³ s^?1 respectively. At the same time, the pK_a values 7.60, 7.16, 7.51, and 7.42 for the deprotonation of the Zn(II)‐bound water in the four catalysts were obtained, which are consistent with those that resulted from pH titrations, i.e. 7.47, 7.25, 7.52, and 7.38 respectively. The mechanism is proposed and the results are compared with other model complexes of carbonic anhydrase. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 197–203, 2004     

Micelle-Catalyzed Interaction Between [Ni(II)-Gly-Gly]+ and Ninhydrin

The effect of cationic micelles of cetyltrimethyl ammonium bromide (CTAB) on the observed pseudo-first-order rate constant for the interaction of nickel dipeptide complex [Ni(II)-Gly-Gly]⁺ with ninhydrin has been studied spectrophotometrically. At constant temperature and pH, increase in the [CTAB] from 0.0 to 60.0 × 10^?3 mol dm^?3 caused nearly three-fold increase of the rate constant. The micellar catalysis is explained in terms of the pseudophase model. From the observed kinetic data, binding constants of micelle–[Ni(II)-Gly-Gly]⁺ (K _S), and micelle–ninhydrin (K _N) are evaluated, respectively, to be 5.3 mol^?1 dm³ and 84.0 mol^?1 dm³. The role of added inorganic (NaCl, NaBr, Na₂SO₄) and organic salts (NaBenz, NaSal) on the reaction rate has also been examined.     

CHLOROPHYLL a FLUORESCENCE OF GONYAULAX POLYEDRA GROWN ON A LIGHT-DARK CYCLE AND AFTER TRANSFER TO CONSTANT LIGHT

Abstract— The chlorophyll a fluorescence properties of Gonyaulax polyedra cells before and after transfer from a lightdark cycle (LD) to constant dim light (LL) were investigated. The latter display a faster fluorescence transient from the level ‘I’ (intermediary peak) to ‘D’ (dip) to ‘P’ (peak) than the former (3 s as compared to 10 s), and a different pattern of decline in fluorescence from ‘I’ to ‘D’ and from ‘P’ to the steady state level with no clearly separable second wave of slow fluorescence change, referred to as ‘s' (quasi steady state)→‘M’ (maximum) →‘T’ (terminal steady state). The above differences are constant features of cells in LD and LL, and are not dependent on the time of day. They are interpreted as evidence for a greater ratio of photosystem II/photosystem I activity in cells in LL. After an initial photoadaptive response following transfer from LD to LL, the cell absorbance at room temperature and fluorescence emission spectra at 77 K for cells in LL and LD are comparable. The major emission peak is at 685–688 nm (from an antenna Chl a 680, perhaps Chl a-c complex), but, unlike higher plants and other algae, the emission bands at 696–698 nm (from Chl a_II complex, Chl a 685, close to reaction center II) and 710–720 nm (from Chl a₁, complexes, Chl a 695, close to reaction center I) are very minor and could be observed only in the fluorescence emission difference spectra of LL minus LD cells and in the ratio spectra of DCMU-treated to non-treated cells. Comparison of emission spectra of cells in LL and LD suggested that, in LL, there is a slightly greater net excitation energy transfer from the light-harvesting peridinin-Chl a (Chl a 670) complex, fluorescing at 675 nm, to the other antenna chlorophyll a complex fluorescing at 685–688 nm, and from the Chl a., complex to the reaction center II. Comparison of excitation spectra of fluorescence of LL and LD cells, in the presence of DCMU, confirmed that cells in LL transfer energy more extensively from the peridinin-Chl a complex to other Chl a complexes than do cells in LD.     

PHOTOSYNTHETIC ACTIVITY AND CHLOROPHYLL ABSORPTION SPECTRA OF FRACTIONS FROM THE ALGA,DUNALIELLA*

Abstract— Dunaliella chloroplasts were fractionated according to C. Arntzen et al, Biochim. Biophys. Acta 256 , 85–107, 1972. The initial French-press treatment and differential centrifugation produced Fraction 1 (Fr 1) enriched in photosystem I activity and a heavier Fraction 2 (Fr 2). When Fr 2 was treated with digitonin followed by either gradient or differential centrifugation, two more fractions were recovered: Fr 1 g with a photosystem 1 activity similar to that of Fr 1, and Fr 2 g with very low photosystem II activity. Photosystem II activity was considerably lower in these Dunaliella chloroplasts and fractions than in spinach particles measured under the same conditions, but the relative activities between the fractions were similar to those for spinach. Fr 2 always had greater photosystem II activity than Fr 1, but the digitonin fractions were low and similar in photosystem II activity. Photosystem II activity was measured as the reduction of 2, 6–dichlorophenol indophenol (DCIP) with H₂O, diphenylcarbazide (DPC) or Mn²⁺ as electron donor. The results indicated that exogenous manganous ion competed with H₂O as an electron donor to photosystem II in broken chloroplasts initially, but after 10–15 s of illumination, the Mn³⁺ formed began to reoxidize DCIP and a cyclic reaction ensued. DPC and Mn²⁺ appeared to react at different sites. Computer-assisted curve analysis of the absorption spectrum of each fraction revealed four major component curves representing the absorbing forms of chlorophyll a at 663, 670, 679 and 684 nm seen in numerous other in vivo chlorophyll spectra (C. S. French et al., Plant Physiol. 49 , 421–429, 1972). However, Fr 2g had approx. 20 percent more of Ca663 and Ca670 and 10% more absorption by chl b than Fr 1 which correlated with the difference in photosystem II activity. On the long wavelength side, Fr 2 g had no Ca694 and almost no photosystem I activity. The results are not sufficient to answer the question of whether the photosystem I particle obtained from the original homogenate is significantly similar to or different from the corresponding fraction obtained from Fr 2 with digitonin.     

Synthesis,structural characterization and DNA-binding studies of a new cobalt (II) complex: (HAcr)<Subscript>4</Subscript>[Co(Sal)<Subscript>3</Subscript>]

A new cobalt (II) coordination compound was synthesized using proton transfer mechanism. The reaction between CoCl₂·2H₂O, Salicylic acid (H₂Sal) and acridine (Acr) gave a new coordination compound formulated as (HAcr)₄[Co(Sal)₃], which was characterized by elemental analysis, NMR, IR and UV/Vis spectroscopies. The interaction of this complex with DNA has been investigated in vitro using UV absorption, fluorescence spectroscopy, viscosity measurements and gel electrophoresis methods. The intrinsic binding constant has been estimated to be 5.8 × 10⁵ M^?1 using UV absorption. The interaction of DNA–Co (II) complex caused quenching in fluorescence. The binding constant, the number of binding site and Stern–Volmer quenching constant have been calculated to be 7.7 × 10⁴ M^?1, 1.143 and 1.5 × 10⁴ Lmol^?1, respectively. The increase in the viscosity of DNA with increasing the concentration of the Co (II) complex and the observations of other experiments suggest that the cobalt (II) complex binds to DNA by partial intercalation binding mode. Furthermore, the interaction of DNA–Co (II) complex was confirmed using gel electrophoresis studies. Moreover, molecular docking technique predicted partial intercalation binding mode for the complex.     

The interaction of polystyryl radical with Cu(DMF)2Cl2

Kinetics of 2,2′-azobisisobutyronitrile initiated polymerization of styrene in N,N-dimethylformamide (DMF) were investigated in the presence of dichloro bis(N,N-dimethylformamide)copper(II) complex. The complex was prepared in situ by mixing tetrakis(N,N-dimethylformamide)copper(II) perchlorate with LiCl in the molar ratio of 1:2. The equilibrium constant for

was calculated by the limiting logarithmic method as 1.07 × 10¹⁰ l² mol^?2. The velocity constant at 60 for the interaction of polystyryl radical with Cu(DMF)₂Cl₂ is 2.16 × 10⁴ l. mol^?1 sec^?1.     

Redox Reaction of the Pd0 Complex Bearing the Trost Ligand with meso‐Cycloalkene‐1,4‐biscarbonates Leading to a Diamidato PdII Complex and 1,3‐Cycloalkadienes: Enantioselective Desymmetrization Versus Catalyst Deactivation

The Pd⁰ complex 1 that bears the Trost ligand 2 undergoes a facile redox reaction with 1,4‐biscarbonates 5 b – d and rac‐ 22 under formation of the diamidato–Pd^II complex 7 and the corresponding 1,3‐cycloalkadienes 8 b – d . The redox deactivation of complex 1 was the dominating pathway in the reaction of 5 b – d with HCO₃^? at room temperature. However, at 0 °C the six‐membered biscarbonate 5 b , catalytic amounts of complex 1 , and HCO₃^? mainly reacted in an allylic alkylation, which led to a highly selective desymmetrization of the substrate and gave alcohol 6 b with ≥99 % ee in 66 % yield. An increase of the catalyst loading in the reaction of 5 b with 1 and HCO₃^? afforded the bicyclic carbonate 12 b (96 % ee, 92 %). Formation of carbonate 12 b involves two consecutive inter‐ and intramolecular substitution reactions of the π‐allyl–Pd^II complexes 16 b and 18 b , respectively, with O‐nucleophiles and presumably proceeds through the hydrogen carbonate 17 b as key intermediate. The intermediate formation of 17 b is also indicated by the conversion of alcohol rac‐ 6 b to carbonate 12 b upon treatment with HCO₃^? and 1 . The Pd⁰‐catalyzed desymmetrization of 5 b with formation of 12 b and its hydrolysis allow an efficient enantioselective synthesis of diol 13 b . The reaction of the seven‐membered biscarbonate 5 c with ent‐ 1 and HCO₃^? afforded carbonate ent‐ 12 c (99 % ee, 39 %). The Pd⁰ complex 1 is stable in solution and suffers no intramolecular redox reaction with formation of complex 7 and dihydrogen as recently claimed for the similar Pd⁰ complex 9 . Instead, complex 1 is rapidly oxidized by dioxygen to give the stable Pd^II complex 7 . Thus, formation of the Pd^II complex 10 from 9 was most likely due to an oxidation by dioxygen. Oxidative workup (air) of the reaction mixture stemming from the desymmetrization of 5 c catalyzed by 1 gave the Pd^II complex 7 in high yield besides carbonate 12 c .     

UV/Vis Spectroscopy of Copper Formate Clusters: Insight into Metal-Ligand Photochemistry

The electronic structure and photochemistry of copper formate clusters, Cu^I₂(HCO₂)₃⁻ and Cu^II_n(HCO₂)_2n+1⁻, n≤8, are investigated in the gas phase by using UV/Vis spectroscopy in combination with quantum chemical calculations. A clear difference in the spectra of clusters with Cu^I and Cu^II copper ions is observed. For the Cu^I species, transitions between copper d and s/p orbitals are recorded. For stoichiometric Cu^II formate clusters, the spectra are dominated by copper d–d transitions and charge-transfer excitations from formate to the vacant copper d orbital. Calculations reveal the existence of several energetically low-lying isomers, and the energetic position of the electronic transitions depends strongly on the specific isomer. The oxidation state of the copper centers governs the photochemistry. In Cu^II(HCO₂)₃⁻, fast internal conversion into the electronic ground state is observed, leading to statistical dissociation; for charge-transfer excitations, specific excited-state reaction channels are observed in addition, such as formyloxyl radical loss. In Cu^I₂(HCO₂)₃⁻, the system relaxes to a local minimum on an excited-state potential-energy surface and might undergo fluorescence or reach a conical intersection to the ground state; in both cases, this provides substantial energy for statistical decomposition. Alternatively, a Cu^II(HCO₂)₃Cu⁰⁻ biradical structure is formed in the excited state, which gives rise to the photochemical loss of a neutral copper atom.     

Reaction of trans-[Co(en)2(SO3)(H2O)]+ with imidazole and containing ligands

trans-[Co(en)₂(SO₃)(H₂O)]⁺ reacts with imidazole (ImH) and imidazole containing ligands (L) to form trans-[Co(en)₂(SO₃)L]⁺ in the pH range 6.0–9.0. The complex seems to react both in the hydroxy and in the aquo form. The rate constant for the reaction of imidazole with the aquo form is 6.0±0.7 and 4±1M^?1s^?1 for the reaction with the hydroxy form at 25°C. The apparent equilibrium constant for formation of the imidazole complex at pH 7 is consistent with the value of 3 x 10² measured previously. Appreciable amounts of complex form only in the pH 6–9 range. Above pH 9 NMR spectra show that even the immediate products are different. In aged solutions at all pHs other products form.     

EFFECTS OF SHORT PULSES OF BLUE LIGHT ON THE ALKALINIZATION ASSOCIATED WITH THE UPTAKE OF NO3− AND CL− BY THE GREEN ALGA MONORAPHIDIUM BRAUNII AND RELATED ACTION SPECTRA

In Monoraphidium braunii, uptake of NO₃^?, NO₂^? and Cl^? is associated with proton transport and triggered by blue light (BL). Only 10 s after cells able to reduce NO₃^? to NH₄⁺ were irradiated with continuous, low-fluence BL in the presence of NO₃^?, an alkalinization of the medium began and only became interrupted by switching off the BL with a 60–90 s time lag. With 30 s BL pulses, the NO₃^?-dependent alkalinization lasted 3–5 min until it stopped. When the cells were exposed to continuous BL in the presence of Cl^?, the alkalinization also started within 10 s but lasted only 3 min. After that, the pH remained constant and decreased when the BL was switched off. With 30 s BL pulses, the Cl^?-dependent alkalinization lasted 3 min and then decreased to its initial value. The NO₃^?-dependent alkalinization shown by cells unable to reduce NO₃^? to NH₄⁺ was similar to that observed in the presence of Cl^?. These alkalinization rates fit the Bunsen-Roscoe reciprocity law. With 2 s pulses of high-fluence BL, the delay time of the NO₃ - or Cl^?-dependent alkalinizations was only 2 s, one of the fastest BL responses reported so far. The action spectra for Cl^? and NO₃^? uptakes proved to be very similar and matched the absorption spectra of flavins, including the 267 nm peak.