Metal ion-coordinating properties of imidazole and derivatives in aqueous solution: interrelation between complex stability and ligand basicity

The stability constants of the 1:1 complexes formed between Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ or Cd²⁺ (M²⁺) and the simple, sterically unhindered imidazole-type ligands, imidazole, 1-methylimidazole, 5-chloro-1-methylimidazole, N-(2,3,5,6-tetrafluorophenyl)imidazole or 4′-(imidazol-1-yl)acetophenone (L), were determined by potentiometric pH titrations in aqueous solution (25°C; I = 0.5 M, NaNO₃). The construction of log K_ML^M versus pK_HL^H plots results in straight lines; the equations for the least-squares lines are calculated and listed. These data allow calculation of the expected stability constant for a complex of any imidazole-type ligand, provided its pK_HL^H value (in the pK_a range 4–8) is known. For the stabilities of Fe²⁺ complexes with imidazole-type ligands an estimation procedure is provided. It is shown further that the complex formation between 1-methylbenzimidazole (MBI) and Mn²⁺, Ni²⁺, Cu²⁺ or Zn²⁺ is s sterically hindered, i.e. the data points for these M(MBI)²⁺ complexes do not fall on the straight lines defined by the imidazole-type ligands.     

Improvement of the binding capacity of metal cations by sugar-beet pulp. 2. Binding of divalent metal cations by modified sugar-beet pulp

Binding of some divalent cations (Ca²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Zn²⁺) in aqueous solution by saponified and cross-linked sugar-beet pulp was investigated. Saponification doubled the cation-exchange capacity, while cross-linking decreased specific surface area and hydration properties to low and stable values independent of pH and ionic strength conditions. The sorption isotherms indicated a high metal-binding capacity which increased with sorbent concentration, and followed a clear order of selectivity: Cu²⁺˜Pb²⁺ Zn²⁺˜Cd²⁺ > Ni²⁺ > Ca²⁺. The sorption data were better represented by the Langmuir isotherm than by the Freundlich one, suggesting that the monolayer sorption, mainly due to ion-exchange, would not be disturbed by lateral interactions between cations sorbed with similar sorption energies. The same order of selectivity could be drawn from the Langmuir parameters, sorption equilibrium constants (K_L) and maximum binding capacities (Me_Amax). Whatever the cation, K_L decreased with increasing sorbent concentration, while Me_bmax increased. Higher quantities of Cu²⁺ and Pb²⁺ than predicted by the one divalent cation to two carboxyl functions ratio were bound. This was attributed to the partial contribution to the sorption phenomenon of hydroxyl functions close to ionic sites, explaining the higher affinity of such cations for substrates. Cross-linked pulp exhibited higher metalbinding capacity per volume unit than the raw pulp.     

Thermostability of β-xylosidase from Aspergillus sydowii MG49

Heating of Aspergillus β-xylosidase at 85°C ± 1°C and pH 5.5–6.0 (optimum for activity), causes irreversible, covalent thermoinactivation of the enzyme, involving oxidation of the thiol groups that are required for catalysis. Exogenous addition of cysteine, DTT, GSH and mercaptoethanol stabilizes the enzyme by extending its half-life. A similar effect is also exhibited by bivalent cations like Mg²⁺, Mn²⁺, Co²⁺, Ca²⁺and Zn²⁺ while, on the other hand Cu²⁺ accelerates thermoinactivation. Chemical modification of crude β-xylosidase with cross-linking agents like glutaraldehyde or covalent immobilization to a nonspecific protein like gelatin and BSA also enhances enzyme thermostability. These results suggest that addition of thiols and bivalent metal ions to a crude β-xylosidase preparation or immobilization/chemical modification enhances its thermal stability, thus preventing loss of catalytic activity at elevated temperatures.     

Characterisation and selectivity of divalent metal ions binding by citrus and sugar-beet pectins

A scale of selectivity for the binding of calcium and some heavy metal ions by citrus and sugar-beet pectins was set up by pH-measurements. The same order of selectivity was found for the two pectins, decreasing as follows: Cu²⁺ Pb²⁺ Zn²⁺ > Cd²⁺ Ni²⁺ ≥ Ca²⁺. Binding isotherms for Ca²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Zn²⁺ ions have shown a greater binding level when the ionic strength decreased and when the pectin concentration increased in the presence of 0.1 M NaNO₃. By comparing binding isotherms, the same order of selectivity was found as by pH-studies. Scatchard plots and Hill index evaluation showed for all ions and all pectins anticooperative interactions in water. In the presence of 0.1 M NaNO₃, citrus pectins displayed cooperative interactions for all metal ions. In contrast, for sugar-beet pectins, cooperative interactions only occured with Cu²⁺ and Pb²⁺. With Ca²⁺, Ni²⁺ and Zn²⁺ sugar-beet pectins displayed Scatchard plots which could not be distinguished from an anticooperative binding. This difference of behaviour could be related to the presence of acetyl groups decreasing the affinity of Me²⁺ for sugar-beet pectins.     

Influence of co-cations on biosorption of lead and zinc – a comparative evaluation in binary and multimetal systems

The influence of co-cations (cadmium, copper, cobalt and nickel) on lead and zinc biosorption by Streptoverticillium cinnamoneum and Penicillium chrysogenum in binary and multimetal systems was evaluated. The metal sorption capacity of S. cinnamoneum was higher than P. chrysogenum for all the metals tested. Both the biomasses exhibited preferential uptake of lead in a multimetal situation. Even though mutual inhibition was seen for all binary systems containing zinc, systems containing lead exhibited unequal inhibition. The extent of metal sorption was dependent on metal chemistry, affinity for binding sites and the type of metal binding. In multimetal systems, S. cinnamoneum and P. chrysogenum exhibited preferential sorption orders: Pb²⁺ > Zn²⁺=Cu²⁺ > Cd²⁺ > Ni²⁺ > Co²⁺ and Pb²⁺ > Cu²⁺ > Zn²⁺ > Cd²⁺ > Ni²⁺ > Co²⁺. The order of metal biosorption in a multimetal system could be predicted well on the basis of Langmuir parameters evaluated in binary metal systems.     

Activities and relative affinities of divalent metals in unmodified and phosphorothioate-substituted hammerhead ribozymes

The roles of metals in the phosphodiester bond cleavage reaction performed by the hammerhead ribozyme are under investigation. In this study, the apparent affinities and the abilities of several different metals to support ribozyme activity are reported. The relative affinities of divalent cations for the hammerhead ribozyme are determined by measuring their ability to release bound Mn²⁺. The EPR-detected Mn²⁺ competition studies give an order of apparent affinity of Mn²⁺ Co²⁺ Zn²⁺>Cd²⁺Mg²⁺. This ordering generally follows the trend of maximum rates of cleavage determined at pH 7.0, 0.1 M NaCl, and saturating metal concentrations, of Mn²⁺>Co²⁺>Cd²⁺>Mg²⁺. The maximum rate is observed for Mn²⁺ under these conditions and may be related to the high affinity, low pK_a and low ΔH_hyd of this ion. Substitution of phosphorothioates 5′ to each of the nine adenosines in the enzyme strand yields a change in the Mn²⁺ binding properties of the hammerhead complex. In the phosphorothioate-substituted hammerhead complex, eight to nine Mn²⁺ bind in two types of classes: ‘type 1’ (n=1±0.3, K_d=1.1±1 μM) and weaker ‘type 2’ (n=7.7±0.3, K_d=125±27 μM). The multiple phosphorothioate substitutions result in the loss of two to three of the higher affinity sites observed in the unmodified ribozyme. Metal competition studies with the phosphorothioate-substituted ribozyme indicate that the relative affinities of the metals are Cd²⁺>Zn²⁺>Co²⁺, Mg²⁺ with the number of Mn²⁺ displaced and apparent affinity of the thiophilic Cd²⁺ most affected by the phosphorothioate substitutions.     

Effects of divalent metal ions on chlorophyll a fluorescence in isolated spinach chloroplasts

The effects of divalent metal ions on the yields of chlorophyll a fluorescence were investigated in isolated spinach chloroplasts at room and liquid nitrogen temperatures. Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺ and Mn²⁺ increased the yields of fluorescence emission at 684 and 695 nm from pigment system II and decreased that at 735 nm from pigment system I. Al³⁺ showed similar but less significant effects on the fluorescence yields. Zn²⁺ and Cd²⁺ showed no significant effect on the fluorescence yields at concentrations lower than 5 mM.

In accordance with the results of our previous study concerning the effects of Mg²⁺ on the excitation transfer in the chloroplasts, it was concluded that ions of alkaline earths and manganese suppress the excitation transfer from bulk chlorophylla of pigment system II to that of pigment system I.     

Purification and chemical characterization of thermostable amylases produced by Bacillus stearothermophilus

The amylases produced by a Bacillus stearothermophilus were purified through a series of four steps. Two separable enzyme fractions having starch hydrolysing activity were eluted from a DEAE-cellulose column by NaCl gradient elution. The homogeneity of the purified enzymes was checked on polyacrylamide gel electrophoresis. The product formation studies indicated that fraction I was an -amylase whereas fraction II was a β-amylase. The molecular weights were determined to be 48 000 and 57 000 and the carbohydrate moiety was found to be 13.2 and 0.8% for - and β-amylase, respectively. The protein digest of these enzymes indicated a total number of 15 amino acids with aspartic and glutamic acid showing the highest value. The purified amylase showed maximal activity at 80°C and pH 6.9. Fe³⁺, Cd²⁺, Pb²⁺, Hg²⁺, Ni²⁺ and Ag¹⁺ were potent inhibitors whereas Zn²⁺, Mg²⁺, Mn²⁺ and Al³⁺ were mild inhibitors. Ca²⁺, Ba²⁺, Sr²⁺ and K⁺ stimulated amylase activity in the order of Ca²⁺ > Ba²⁺ > Sr²⁺ > K⁺. PCMB, EDTA and sodium iodoacetate were inhibitory whereas glutathione (GSH) and cysteine afforded protection of enzyme activity. EDTA showed dose-dependent noncompetitive inhibition of both - as well as β-amylase activities. EDTA inhibition was reversed by the addition of Ca²⁺ and PCMB inhibition by the addition of glutathione (reduced). The K_m for - and β-amylases were found to be 1.05 and 1.25 mg starch per ml, respectively.     

Factors affecting the protease activity of venom from jellyfish Rhopilema esculentum Kishinouye

In this paper, the effects of some chemical and physical factors such as temperature, pH values, glycerol, and divalent metal cations on the protease activity of venom from jellyfish, Rhopilema esculentum Kishinouye, were assayed. Protease activity was dependent on temperature and pH values. Zn²⁺, Mg²⁺, and Mn²⁺ in sodium phosphate buffer (0.02 M, pH 8.0) could increase protease activity. Mn²⁺ had the best effects among the three metal cations and the effect was about 20 times of that of Zn²⁺ or Mg²⁺ and its maximal protease activity was 2.3 × 10⁵ U/mL. EDTA could increase protease activity. PMSF had hardly affected protease activity. O-Phenanthroline and glycerol played an important part in inhibiting protease activity and their maximal inhibiting rates were 87.5% and 82.1%, respectively.     

The synthesis of enzyme-bound ATP by the F1-ATPase from the thermophilic bacterium PS3 in the presence of organic solvents

Synthesis of enzyme-bound ATP was demonstrated with purified TF₁ (F₁-ATPase from thermophilic bacterium PS3) from medium inorganic phosphate (P_i) and enzyme-bound ADP in the presence of organic solvents such as dioxane, ethanol, dimethylformamide, methanol, acetone, acetonitrile or ethyleneglycol. The optimal concentrations of dimethylformamide, ethanol or methanol were 50%, 30% and 40% and the half-maximal concentrations of P_i were 13 mM, 20 mM and 18 mM, respectively. Thus it is evident that the effect of dimethylsulfoxide on TF₁ to form enzyme-bound ATP [8] is not due to a specific interaction between dimethylsulfoxide and the enzyme, but to a decrease in polarity of the medium. In the presence of methanol, the dependence of ATP synthesis on various divalent metal ions was compared to that for the ATP-hydrolyzing activity and the ATP-driven proton-translocating activity of TF₁. While Mn²⁺, Co²⁺, Zn²⁺ and Cd²⁺ are as effective as Mg²⁺ for the ATP-hydrolyzing activity of TF₁, Zn²⁺ and Cd²⁺ are either less or not effective for proton translocation and for ATP synthesis. This result appears to be consistent with the idea that the TF₁-ATP complex formed in organic solvents represents one of the intermediates in the reaction sequences of ATP synthesis by H⁺-ATPase using the proton gradient.     

Properties and function of clostridial membrane ATPase

ATPase (ATP phosphohydrolase, EC 3.6.1.3) was detected in the membrane fraction of the strict anaerobic bacterium, Clostridium pasteurianum. About 70% of the total activity was found in the particulate fraction. The enzyme was Mg²⁺ dependent; Co²⁺ and Mn²⁺ but not Ca²⁺ could replace Mg²⁺ to some extent; the activation by Mg²⁺ was slightly antagonized by Ca²⁺. Even in the presence of Mg²⁺, Na⁺ or K⁺ had no stimulatory effect. The ATPase reaction was effectively inhibited by one of its products, ADP, and only slightly by the other product, inorganic phosphate. Of the nucleoside triphosphates tested ATP was hydrolyzed with highest affinity ([S]_{0.5 V} = 1.3 mM) and maximal activity (120 U/g). The ATPase activity could be nearly completely solubilized by treatment of the membranes with 2 M LiCl in the absence of Mg²⁺. Solubilization, however, led to instability of the enzyme.

The clostridial solubilized and membrane-bound ATPase showed different properties similar to the “allotopic” properties of mitochondrial and other bacterial ATPases. The membrane-bound ATPase in contrast to the soluble ATPase was sensitive to the ATPase inhibitor dicyclohexylcarbodiimide (DCCD). DCCD, at 10^-4 M, led to 80% inhibition of the membrane-bound enzyme; oligomycin, ouabain, or NaN₃ had no effect. The membrane-bound ATPase could not be stimulated by trypsin pretreatment.

Since none of the mono- or divalent cations had any truly stimulatory effect, and since a pH gradient (interior alkaline), which was sensitive to the ATPase inhibitor DCCD, was maintained during growth of C. pasteurianum, it was concluded that the function of the clostridial ATPase was the same as that of the rather similar mitochondrial enzyme, namely H⁺ translocation. A H⁺-translocating, ATP-consuming ATPase appears to be intrinsic equipment of all prokaryotic cells and as such to be phylogenetically very old; in the course of evolution the enzyme might have been developed to a H⁺-(re)translocating, ATP-forming ATPase as probably realized in aerobic bacteria, mitochondria and chloroplasts.     

Stability constants of metal complexes of phosphonoacetic acid

The antiviral drug, phosphonoacetic acid (PAA), forms stable complexes with Mg²⁺, Ca²⁺, Cu²⁺ and Zn²⁺. Stability constants of these complexes were determined in aqueous solution (0.15 M in KNO₃, 37°) by potentiometric titration. Mixed ligand complex formation of Cu²⁺ and Zn²⁺ with PAA and glycinate ion, and with PAA and histidinate ion, was studied. In a theoretical model for blood plasma, PAA affects the distribution of Mg²⁺ and, to a lesser extent, Ca²⁺.     

Spectrophotometric and kinetic studies of the binding of Ni, Co, and Mg to poly(dG-dC) · poly(dG-dC). Determination of the stoichiometry of the Ni-induced B → Z transition

Data are reported for the binding of Ni²⁺, Co²⁺, and Mg²⁺ to the B-form of double-stranded poly(dG-dC) at ionic strength conditions I = 0.001 M, 0.01 M, and 0.1 M. The apparent binding constants for Ni²⁺ and Co²⁺ are about the same and are 2- to 3-fold higher than those for Mg²⁺. Kinetic studies indicate that Mg²⁺ binds to the polynucleotide mainly (or solely) as a mobile cloud (electrostatically, outer-sphere), whereas the transition metal ions undergo site binding (inner-sphere coordination) with poly(dG-dC). The kinetic data suggest that an Ni²⁺ ion coordinates to more than one binding site at the polynucleotide, presumably to G-N₇ and a phosphate group.

At low ionic strength conditions the addition of Ni²⁺ induces a B → Z conformational transition in poly(dG-dC). As demonstrated by UV absorption and CD spectroscopy, the transition occurs at I = 0.001 M already when 3 × 10⁻⁵ – 7 × 10⁻⁵ M of Ni²⁺ are added to 8 × 10⁻⁵ M (in monomeric units) of poly(dG-dC), and at I = 0.01 M between 2.5 × 10⁻⁴ and 4.5 × 10⁻⁴ M of Ni²⁺. Using murexide as an indicator of the concentration of free Ni²⁺ ions, the amount of Ni²⁺ which is bound to the polynucleotide could be determined. At I = 0.001 M it was established that the B → Z transition begins when 1 Ni²⁺ is bound coordinatively per four base pairs, and the transition is complete when 1 Ni²⁺ is bound coordinatively per three base pairs. It is this coordinated Ni²⁺ which induces the B → Z transition.     

Light-dependent changes of the Mg concentration in the stroma in relation to the Mg dependency of CO2 fixation in intact chloroplasts

(1) Light-dependent changes of the Mg²⁺ content of thylakoid membranes were measured at pH 8.0 and compared with earlier measurements at pH 6.6. In a NaCl and KCl medium, the light-dependent decrease in the Mg²⁺ content of the thylakoid membranes at pH 8.0 is found to be 23 nmol Mg²⁺ per mg chlorophyll, whereas in a sorbitol medium it is 83 nmol Mg₂₊ per mg chlorophyll.

(2) A light dependent increase in the Mg²⁺ content of the stroma was detected when chloroplasts were subjected to osmotic shock, amounting to 26 nmol/mg chlorophyll. Furthermore, a rapid and reversible light-dependent efflux of Mg²⁺ has been observed in intact chloroplasts when the divalent cation ionophore A 23 187 was added, indicating a light-dependent transfer of about 60 nmol of Mg²⁺ per mg chlorophyll from the thylakoid membranes to the stroma.

(3) CO₂ fixation, but not phosphoglycerate reduction, could be completely inhibited when A 23 187 was added to intact chloroplasts in the absence of external Mg²⁺. If Mg²⁺ was then added to the medium, CO₂ fixation was restored. Half of the maximal restoration was achieved with about 0.2 mM Mg²⁺, which is calculated to reflect a Mg²⁺ concentration in the stroma of 1.2 mM. The further addition of Ca²⁺ strongly inhibits CO₂ fixation.

(4) The results suggest that illumination of intact chloroplasts causes an increase in the Mg²⁺ concentration of 1–3 mM in the stroma. Compared to the total Mg²⁺ content of chloroplasts, this increase is very low, but it appears to be high enough to have a possible function in the light regulation of CO₂ fixation.     

改性粉煤灰对废水中镉的吸附作用

以工业固废粉煤灰(FA)为原料,将其与NaOH和Ca(OH)₂混合后,在250 ℃下焙烧1.5 h,制得改性粉煤灰(MFA).利用扫描电镜(SEM)、能量色散X射线光谱仪(EDS)、X-射线衍射仪(XRD)、全自动比表面积分析仪(BET)和傅里叶红外光谱仪(FTIR)等技术分析MFA的理化性质.BET分析表明,MFA的比表面积比FA增大了20.6倍.SEM分析表明,FA表面玻璃相溶解,表面粗糙,具有多孔结构.FTIR分析表明,-OH官能团在Cd²⁺的吸附过程中具有重要作用.静态吸附试验研究表明,当MFA的投加量为0.2 g、Cd²⁺初始浓度为100 mg·L^-1、溶液pH为7、25 ℃下吸附90 min时,Cd²⁺的去除率达到97.3%;共存阳离子(K⁺、Na⁺、Mg²⁺和Ca²⁺)均会抑制MFA对Cd²⁺的吸附,其中Ca²⁺的抑制作用最强.Langmuir等温吸附模型和准二级吸附动力学方程能较好地描述Cd²⁺在MFA上的吸附行为,且最大吸附量为55.77 mg·g^-1.热力学研究表明,MFA对Cd²⁺的吸附是一个自发的吸热过程.MFA吸附能力优于FA,在废水处理方面有一定的应用前景.     

Cation-induced thermal stability of an alkaline protease from a Bacillus sp.

An alkaline protease produced from a Bacillus sp. was stimulated by the metal ions Ca²⁺, Mg²⁺ and Mn²⁺; with Ca²⁺ having the maximum effect. The thermal stability of the enzyme was also enhanced to varying degrees in the presence of these ions.     

Ion transport in heart mitochondria. XIII. The effect of ethylenediaminetetraacetate on monovalent ion uptake

1. Ethylenediaminetetraacetate (EDTA) markedly activates the accumulation of Na⁺ and Li⁺ and the swelling which accompanies the ion uptake by isolated heart mitochondria. This activation is reflected in the removal of limited amounts of endogenous Mg²⁺ and extensive loss of K⁺. The removal of these cations requires the presence of Na⁺, a source of energy, and a permeant anion as well as EDTA. The effects of EDTA on the activation of Na⁺ uptake and cation removal are duplicated by chelators with a high affinity for Mg²⁺, but not by ethyleneglycol-bis-(β-aminoethylether)-N, N′-tetraacetic acid. Mg²⁺ at concentrations 5 to 6 times less than EDTA prevents both activation of Na⁺ uptake and cation removal.

2. EDTA does not appear to be bound by heart mitochondria. At neutral pH the chelator penetrates into the mitochondrial water volume to the same extent as sucrose and mannitol. At pH 8.1 where the removal of mitochondrial Mg²⁺ by EDTA is more effective, EDTA penetrates virtually the entire water volume. This penetration requires the presence of a source of energy, a transported cation such as Na⁺, and a permeant anion. It appears possible that the oscillations in ion uptake and swelling observed in the presence of EDTA at pH 8.1 may be related to the presence of the chelator in the interior compartment under these conditions.     

不同炭化温度制备的蚕丝被废弃物生物炭对重金属Cd2+的吸附性能

以蚕丝被废弃物为原料,在300、500和700 ℃高温缺氧条件下热解炭化制备成3种生物炭(BC300、BC500和BC700).利用扫描电镜(SEM)、傅里叶红外光谱仪(FT-IR)、X-射线衍射仪(XRD)、比表面积分析仪等对其理化性质进行表征,并研究了不同温度下制备的生物炭对溶液中Cd²⁺的吸附特性.结果表明: 随着炭化温度上升,BET比表面积、pH、灰分均增大,生物炭表面形态结构越来越不规则.XRD结果显示:不同温度下获得的生物炭中均含有一定量的方解石,FT-IR光谱图上的峰主要为-OH和方解石典型的吸收峰;pH对生物炭吸附Cd²⁺的影响不大;Langmuir方程能更好地拟合3种生物炭对Cd²⁺的吸附等温过程,其最大吸附量分别为25.61、52.41和91.07 mg·g^-1.3种生物炭对Cd²⁺吸附过程均更符合准二级动力学方程,且BC700对Cd²⁺的吸附效果最佳.进一步研究离子浓度及阳离子共存对BC700吸附Cd²⁺的影响,结果显示: NaCl浓度越高,对Cd²⁺的吸附抑制越明显;共存阳离子中,Ca²⁺和Mg²⁺对Cd²⁺的吸附抑制更明显,而K⁺几乎无影响.因此,以蚕丝被废弃物制备的生物炭作为去除水体中Cd²⁺的吸附剂具有较强的应用潜力.     

Inactivation of chloroplast photosynthetic electron-transport activity by Ni

Ni²⁺ inhibits electron-transport activity of isolated barley chloroplasts and this inhibition of electron transport by Ni²⁺ is distinctly different from other heavy metal ion (e.g., Pb²⁺, Cd²⁺, Zn²⁺)-induced inhibition of chloroplast function. Ni²⁺ inactivates Photosystem II (PS II) activity at a lower concentration than that required for the same extent of inhibition of Photosystem I (PS I)-mediated electron flow. Ni²⁺ induces changes in chlorophyll a (Chl a) emission characteristics and brings about a lowering of the Chl a fluorescence yield, and this lowering of Chl a fluorescence intensity is not relieved by the exogenously supplied electron donor NH₂OH which donates electrons very close to the PS II reaction centres. Immobilization of the chloroplast membrane structure with glutaraldehyde fails to arrest the Ni²⁺-induced loss of PS II activity. Also, Ni²⁺-treated chloroplasts do not regain the ability to photoreduce 2,6-dichlorophenolindophenol even after washing of chloroplasts with buffer. These results indicate that unlike Zn²⁺ or Pb²⁺, Ni²⁺ induces alterations in the chloroplast photosynthetic apparatus resulting in an irreversible loss of electron-transport activity.     

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