Synthesis, characterization and redox reactivity of l-tartrato bridged dinuclear manganese complex with 2,2′-bipyridine

A new l-tartrato manganese(III) complex was synthesized and characterized as the dinuclear dimanganese(III) structure with a stereospefically formed [Λ-Λ] absolute configuration around Mn(III) ions. The thermal and photo decomposition gave the first example of dihydrogen gas evolution besides CO and CO₂ gas associated with cis-[Mn^II(ox)(bpy)(H₂O)₂]. A proposed redox reaction proceeds from Mn(III) to Mn(II) via intermediate Mn(IV) and Mn(II) with CO anion radical species followed by oxidation of tartrate ligands.     

Cobalt(II) and μ-oxodiiron(III,IV) complexes of salen analog with aromatic thioether pendant group,N, N′-disalicylidene-2-methyl-4-(2-methylthiophenyl)- 1,2-butanediamine

A pentadentate salen analog containing a thioether group in the pendant tail, N,N′-disalicylidene-2- methyl-4-(2-methylthiophenyl)- 1,2-butanediamine, has been synthesized. The cobalt(II) complex of this ligand retains a planar configuration free from the coordination of the pendant group at room temperature but adopts a square-pyramidal configuration with the thioether at the apex near liquid nitrogen temperature. The iron complex obtained with this ligand is shown to be a μ-oxodiiron(III, IV) complex comprised of high-spin iron(III) and low-spin iron(IV), based on cryomagnetic data (80–300 K), ESR, and M:ossbauer spectra. An antiferromagnetic spin-exchange interaction (J = − 13.0 cm ⁻¹) operates between the metal ions.     

N-(3,5-Dichloro-4-(2,4,6-trichlorophenoxy)phenyl)benzenesulfonamide: A new dual-target inhibitor of mitochondrial complex II and complex III via structural simplification

Mitochondrial complex II and complex III are two promising targets for the development of numerous pharmaceuticals and pesticides. Although tremendous inhibitors of either complex II or complex III were identified, compounds which are capable of prohibiting the activities of both complexes have been rarely reported. Since multi-target drugs can interact with several drug targets simultaneously, we were keen on discovering new and potent dual-target inhibitors of both complex II and complex III. Therefore, a new series of structurally simplified sulfonamides bearing a diaryl ether scaffold were designed and synthesized in this paper. Afterwards, the biological activities of the newly synthesized compounds were evaluated. The results implied that several compounds demonstrated outstanding potency against succinate-cytochrome c reductase (SCR, a mixture of complex II and complex III). Further studies confirmed that N-(3,5-Dichloro-4-(2,4,6-trichlorophenoxy)phenyl)benzenesulfonamide (3f), a representative compound herein, was identified as a dual-target inhibitor of both complexes. Furthermore, computational simulations were also performed to have a better understanding about binding of 3f to the enzyme complexes, which concluded that 3f should bind to complex II and the Q_o site of complex III. Consequently, we harbor the idea that this work can be beneficial for the synthesis and discovery of more dual- or multi-target inhibitors.     

Synthesis and properties of different metal complexes of the siderophore desferriferricrocin

Desferriferricrocin is a cyclic hexa-peptide siderophore with three hydroxamates as primary coordination groups. It forms metal complexes with Fe(III), Cr(III), Al(III), Ga(III), Cu(II), and Zn(II). These complexes were prepared and characterized using UV–vis, circular dichroism spectroscopy (CD), nuclear magnetic resonance spectroscopy (NMR), and electrospray ionization mass spectroscopy (ESI-MS). The mononuclear trivalent metal complexes of desferriferricrocin were stable in aqueous solutions, and their coordination centers primarily adopted the Λ configuration. The formation of multinuclear complexes of desferriferricrocin was determined by ESI-MS. Desferriferricrocin was able to bind up to three Cu(II) and two Zn(II) respectively. Heteronuclear complexes containing one trivalent and one divalent were also determined. In these complexes, amide nitrogens were utilized as alternative binding groups of desferriferricrocin in addition to the primary binding groups, the hydroxamates. Published online December 2004     

Complex II phosphorylation is triggered by unbalanced redox homeostasis in cells lacking complex III

A marked stimulation of complex II enzymatic activity was detected in cybrids bearing a homoplasmic MTCYB microdeletion causing disruption of both the activity and the assembly of complex III, but not in cybrids harbouring another MTCYB mutation affecting only the complex III activity. Moreover, complex II stimulation was associated with SDHA subunit tyrosine phosphorylation. Despite the lack of detectable hydrogen peroxide production, up-regulation of the levels of mitochondrial antioxidant defenses revealed a significant redox unbalance. This effect was also supported by the finding that treatment with N-acetylcysteine dampened the complex II stimulation, SDHA subunit tyrosine phosphorylation, and levels of antioxidant enzymes. In the absence of complex III, the cellular amount of succinate, but not fumarate, was markedly increased, indicating that enhanced activity of complex II is hampered due to the blockage of respiratory electron flow. Thus, we propose that complex II phosphorylation and stimulation of its activity represent a molecular mechanism triggered by perturbation of mitochondrial redox homeostasis due to severe dysfunction of respiratory complexes. Depending on the site and nature of the damage, complex II stimulation can either bypass the energetic deficit as an efficient compensatory mechanism, or be ineffectual, leaving cells to rely on glycolysis for survival.     

Preparation and characterization of oxovanadium(IV), acetatomanganese(III), chloroiron(III), nickel(II), copper(II), zinc(II) and palladium(II) complexes of 3,3′-(1,2-phenylenediimino)diacrolein

The oxovanadium(IV), acetatomanganese(III), chloroiron(III), nickel(II), copper(II), zinc(II) and palladium(II) of 3,3′-(1,2-phenylenediimino)diacrolein were prepared and investigated by means of mass, electronic, vibrational, NMR and ESR spectroscopy as well as magnetic susceptibility measurements. The acetatomanganese(III) and chloroiron(III) complexes were confirmed to be of high spin type. The absorption bands appearing in the energy range greater than 23 000 cm⁻¹ were attributed to π→π^* transitions within a ligand molecule and charge- transfer transitions from metal to ligand. The metal complexes assume the square-planar configuration type since the ligand-field bands were detected in the 12 700–18 500 cm⁻¹ region. Strong bands appearing at 1601 and 1627 cm⁻¹ were assigned to the CC and CO stretching vibrational modes, respectively, and were shifted to lower frequency upon metal-coordination. A VO stretching band was observed at 982 cm⁻¹ for the oxovanadium(IV) complex and a CO stretching band was observed at 1547 cm⁻¹ for the acetatomanganese(III) complex. Upon complex formation the amine proton signal is found to vanish and the aldehydic methine proton signal in the lowest field is shifted upfield for the nickel(II), zinc(II) and palladium(II) complexes. ¹³C NMR spectra support the coordination structure of the complexes which is revealed by ¹H NMR spectra. As judged by the spin Hamiltonian parameters, the oxovanadium(IV) complex is of a square- planar type with an unpaired electron in the d_xy orbital and the copper(II) complex assumes a distorted square-planar coordination due to the presence of five- and six-membered chelate rings with an unpaired electron in the d_x2−y2 orbital.     

Synthesis of new 4-aryloxy-N-arylanilines and their inhibitory activities against succinate-cytochrome c reductase

Succinate-cytochrome c reductase (SCR) is composed of a mixture of mitochondrial complex II (succinate-ubiquinone oxidoreductase) and complex III (cytochrome bc₁ complex). Meanwhile, complexes II and III are two promising targets of numerous antibiotics and fungicides. With an aim to identify new lead structures for SCR, complex II or III, a new series of 4-aryloxy-N-arylanilines were synthesized by introducing a 4-aryloxy phenyl group as one of the aryl groups in diaryl amines. With the economic Cu(OAc)₂·H₂O as the optimal copper promoter, a simple and facile protocol was utilized to afford 24 target products in 56–93% yields. Furthermore, extensive screening results suggested variable inhibitory activities of these compounds against SCR. Exceptionally, compounds 7k–7n showed excellent inhibition potency with their IC₅₀ values in the nanomolar range, demonstrating higher potency than the commercial controls (penthiopyrad and azoxystrobin) by over one order of magnitude.     

Central imidazolidine ring hydrolysis of a binucleating amine phenol ligand during complex formation with manganese(III): synthesis, structure and electron transfer properties of mononuclear MnN4O2 complex

The reaction of μ-bis(tetradentate) ligand H₃L^′, formed from imidazolidine ring insertion within the parent hexadentate Schiff base ligand of salicylaldehyde and triethylenetetramine, with manganese(II) salt in air spontaneously transforms the binucleating ligand, with respect to the removal of the phenol substituted imidazolidine ring, affording the mononuclear manganese(III) complex of the parent hexadentate ligand (H₂L). The mononuclear complex has MnN₄O₂ coordination sphere as established from a crystal structure determination. The imine nitrogen pairs are coordinated trans to each other in [Mn^IIIL]⁺. The Mn-N(imine), Mn-N(amine), and Mn-O(phenol) distances are, respectively, 2.001 (6), 2.246 (8) and 1.949 (6) Å. The mononuclear complex has a magnetic moment corresponding to the high-spin 3d⁴ configuration. In dimethylformamide solution, two quasireversible couples for manganese(IV)-manganese(III) and manganese(III)-manganese(II) are observed in cyclic voltammetry with E_1/2 values of 0.66 and 0.02 V versus SCE, respectively. One-electron nature of these two couples can be verified from current height and room-temperature solution coulometry data. The X-band EPR spectrum of the coulometrically oxidized and frozen methanol-toluene solution of [Mn^IIIL]⁺ consists of weak and strong resonances at g∼4 and g∼2, respectively, the latter resonance shows ⁵⁵Mn hyperfine lines and forbidden lines are also resolved. The broad shoulder around 615 nm ( ε ∼ 200 l mol⁻¹cm⁻¹) may be assigned to a d-d transition for Mn(III) in pseudotetragonal environment.     

The influence of carotenoids on the conformation of chlorophyll-protein complexes isolated from the cyanobacterium Plectonema boryanum. Absorption and circular dichroism study

Thylakoid membranes of the cyanobacterium Plectonema boryanum solubilized with Triton X-100 can be resolved into three fractions of pigment-protein complexes (Hladík, J. and Sofrová, D. (1981) Photosynthetica 15, 490–503). Fraction I contained relatively the highest amount of carotenoids as well as monomeric forms of chlorophyll a, Fractions II and III contained chlorophyll-protein complexes with a characteristic exciton-split circular dichroism in the red region. It has been shown that fraction III is an oligomeric form of the chlorophyll-protein complex of fraction II. Circular dichroism spectra indicate that, different from fraction II, fraction III contains specifically oriented and space-fixed molecules of carotenoids. Thermal dissociation of fracion III to fraction II is accompanied by disappearance of the positive circular dichroism effect of carotenoids in the 500–550 nm region, thus causing deorganization of the carotenoids, proceeding in parallel to the geometrical rearrangement of chlorophyll molecules. Extraction of the carotenoids of fraction III with heptane is acompanied by dissociation of fraction III. We assume that the observed effects are due to binding of the two pigments to the protein component of the complex and that carotenoids can mediate a part of the interactions which stabilize the structure of pigment-protein complexes. Thus, besides the light-harvesting and protective functions, carotenoids can also play a structural role.     

Synthesis and characterization of different complexes derived from Schiff base and evaluation as a potential anticancer,antimicrobial, and insecticide agent

The condensation of (1H-benzimidazole-2-yl) methanamine, with 2-hydroxy naphthaldehyde lead to Schiff base ligand (H2L) (1). This was later reacted with metal salts (ZnCl₂, CrCl₃·6H₂O, and MnCl₂·4H₂O) to afford the corresponding metal complexes. Biological activity findings indicate that the metal complexes have promising activity against Escherichia coli and Bacillus subtilis and modest activity against Aspergillus niger. The in vitro anticancer activities of Zn (II), Cr (III), and Mn (II) complexes were investigated and the best results were observed with Mn (II) complex as the most potent cytotoxic agent toward human cell lines colorectal adenocarcinoma HCT 116, hepatocellular carcinoma HepG₂ and breast adenocarcinoma MCF-7 with 0.7, 1.1 and 6.7 μg of inhibitory concentration IC₅₀ values respectively. Consequently, the Mn (II) complex and ligand were docked inside the energetic site of ERK2 and exhibited favorable energy for binding. The investigation of biological tests towards mosquito larvae indicates that Cr (III) and Mn (II) complexes manifest strong toxicity against Aedes aegypti larvae with 3.458 and 4.764 ppm values of lethal concentration LC₅₀, respectively.     

Heterometallic complex formation on p-sulfonatothiacalix[4]arene platform resulting in pH- and redox-modification of [Ru(bpy)3] luminescence

The pH-dependent heterometallic complex formation with p-sulfonatothiacalix[4]arene (TCAS) as bridging ligand in aqueous solutions was revealed by the use of spectrophotometry, nuclear magnetic relaxation and fluorimetry methods. The novelty of the structural motif presented is that the appendance of emission metal center ([Ru(bpy)₃]²⁺) is achieved through the cooperative non-covalent interactions with the upper rim of TCAS. The second metal block (Fe(III), Fe(II) and Mn(II)), bound with the lower rim of TCAS in the inner sphere coordination mode is serving as quencher of [Ru(bpy)₃]²⁺ emission. The difference between the complex ability of Fe(III) and Fe(II) ions provides pH conditions for redox-dependent emission of [Ru(bpy)₃]²⁺.     

N-(4-(2-chloro-4-(trifluoromethyl)phenoxy)phenyl)picolinamide as a new inhibitor of mitochondrial complex III: Synthesis,biological evaluation and computational simulations

Mitochondrial complex III is one of the most promising targets for a number of pharmaceuticals and fungicides. Due to the wide-spread use of complex III-inhibiting fungicides, a considerable increase of resistance has occurred worldwide. Therefore, inhibitors with novel scaffolds and potent activity against complex III are still in great demand. In this article, a new series of amide compounds bearing the diaryl ether scaffold were designed and prepared, followed by the biological evaluation. Gratifyingly, several compounds demonstrated potent activity against succinate-cytochrome c reductase (SCR, a mixture of mitochondrial complex II and complex III), with compound 3w possessing the best inhibitory activity (IC₅₀ = 0.91 ± 0.09 μmol/L). Additional studies verified that 3w was a new inhibitor of complex III. Moreover, computational simulations elucidated that 3w should bind to the Q_o site of complex III. We believe this work will be valuable for the preparation and discovery of more complex III inhibitors.     

Suberoylanilide hydroxamic acid, a potent histone deacetylase inhibitor; its X-ray crystal structure and solid state and solution studies of its Zn(II), Ni(II), Cu(II) and Fe(III) complexes

Reaction of the potent hydroxamate-based histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid (SAHA), with hydrated metal salts of Fe(III), Cu(II), Ni(II) and Zn(II) yielded a tris-hydroxamato complex in the case of Fe(III) and bis-hydroxamato complexes in the case of Cu(II), Ni(II) and Zn(II) both in the solid state and in solution. Reaction of the secondary hydroxamic acid, N-Me-SAHA, also yielded a tris-hydroxamato complex in the case of Fe(III) and bis-hydroxamato complexes in the case of Cu(II), Ni(II) and Zn(II) in solution. These metal complexes have the hydroxamato moiety coordinated in an O,O’-bidentate fashion. Stability constants of the metal complexes formed with SAHA and N-Me-SAHA in a DMSO/H₂O 70/30%(v/v) mixture are described. A novel crystal structure of SAHA together with a novel synthesis for N-Me-SAHA are also reported.     

Specific disintegration of complex II succinate:ubiquinone oxidoreductase links pH changes to oxidative stress for apoptosis induction

The formation of reactive oxygen species (ROS) and the change of the intracellular pH (pH_i) are common phenomena during apoptosis. How they are interconnected, however, is poorly understood. Here we show that numerous anticancer drugs and cytokines such as Fas ligand and tumour necrosis factor α provoke intracellular acidification and cause the formation of mitochondrial ROS. In parallel, we found that the succinate:ubiquinone oxidoreductase (SQR) activity of the mitochondrial respiratory complex II is specifically impaired without affecting the second enzymatic activity of this complex as a succinate dehydrogenase (SDH). Only in this configuration is complex II an apoptosis mediator and generates superoxides for cell death. This is achieved by the pH_i decline that leads to the specific dissociation of the SDHA/SDHB subunits, which encompass the SDH activity, from the membrane-bound components of complex II that are required for the SQR activity.     

Immunization with cytomegalovirus envelope glycoprotein M and glycoprotein N DNA vaccines can provide mice with complete protection against a lethal murine cytomegalovirus challenge

Human cytomegalovirus virions contain three major glycoprotein complexes (gC I, II, III), all of which are required for CMV infectivity. These complexes also represent major antigenic targets for anti-viral immune responses. The gC II complex consists of two glycoproteins, gM and gN. In the current study, DNA vaccines expressing the murine cytomegalovirus (MCMV) homologs of the gM and gN proteins were evaluated for protection against lethal MCMV infection in a mouse model. Humoral and cellular immune responses, spleen viral titers, and mice survival and body-weight changes were examined. The results showed that immunization with gM or gN DNA vaccine alone was not able to offer good protection, whereas co-immunization with both gM and gN induced an effective neutralizing antibody response and cellular immune response, and provided mice with complete protection against a lethal MCMV challenge. This study provides the first in vivo evidence that the gC II (gM-gN) complex may be able to serve as a protective subunit antigen for future HCMV vaccine development.     

Polymer‐Supported Optically Active fac(S)‐Tris(thiotato)rhodium(III) Complex for Sulfur‐Bridging Reaction With Precious Metal Ions

The optically active mixed‐ligand fac(S)‐tris(thiolato)rhodium(III) complexes, Δ_L‐fac(S)‐[Rh(aet)₂(L‐cys‐N,S)]^? (aet = 2‐aminoethanethiolate, L‐cys = L‐cysteinate) ( 1 ) and Δ_LL‐fac(S)‐[Rh(aet)(L‐cys‐N,S)₂]^2? were newly prepared by the equatorial preference of the carboxyl group in the coordinated L‐cys ligand. The amide formation reaction of 1 with 1,10‐diaminodecane and polyallylamine gave the diamine‐bridged dinuclear Rh(III) complex and the single‐chain polymer‐supported Rh(III) complex with retention of the Δ_L configuration of 1 , respectively. These Rh(III) complexes reacted with Co(III) or Co(II) to give the linear‐type trinuclear structure with the S‐bridged Co(III) center and the two Δ‐Rh(III) terminal moieties. The polymer‐supported Rh(III) complex was applied not only to the CD spectropolarimetric detection and determination of a trace of precious metal ions such as Au(III), Pt(II), and Pd(II) but also to concentration and extraction of these metal ions into the solid polymer phase. Chirality 28:85–91, 2016. © 2015 Wiley Periodicals, Inc.     

Structure and characterization of a μ-phenoxo-bis μ-acetato dinuclear Mn(II,II) complex [Mn2(LO)(μ-OAc)2](ClO4) (LOH = 2,6-bis{bis(2-(2-pyridyl)ethyl)aminomethyl}-4-methylphenol): Substituent effects

The synthesis, structure and characterization of the dinuclear Mn(II) complex [Mn₂(LO)(μ-OAc)₂](ClO₄) (1) where LOH = 2,6-bis{bis(2-(2-pyridyl)ethyl)aminomethyl)}-4-methylphenol are reported. The reaction of Mn(ClO₄)₂ · 6H₂O with the dinucleating ligand LOH and H₃CCOONa in the presence of NEt₃ in dry, degassed methanol and under an argon atmosphere, yields 1 as a colorless powder. The crystal structure of 1, determined by X-ray diffraction methods, shows a dinuclear Mn(II) complex in which two Mn(II) ions, each in six-coordinate approximate octahedral coordination, are bridged by the phenolate oxygen of LO⁻ and by two acetate ions in a syn,syn-1,3-bridging mode. The Mn-Mn distance is 3.557(1) Å and Mn-O_phenolate-Mn angle is 112.50(9)°. Cyclic voltammetry of 1 in acetonitrile solution shows a quasi-reversible wave at E_1/2 = 0.65 V, for the Mn₂(II,II)/Mn₂(II,III) redox process, and an irreversible oxidation peak at E_p,c = 1.22 V versus Ag/AgCl for the Mn₂(II,III) to Mn₂(III,III) oxidation process. Controlled potential electrolysis of 1 in acetonitrile solution at 0.85 V (versus Ag/AgCl) takes up 1 F of charge per mole of 1 to yield a brown solution of the Mn₂(II,III) state of the complex, which, however, is unstable and reverts back to the Mn₂(II,II) state in solution at room temperature. Least square fitting of the variable temperature magnetic susceptibility measurements on powdered sample of 1 is obtained with g = 1.888, J = −2.75 cm⁻¹, Par = 0.008, TIP = 0. The low −J value and the room temperature calculated magnetic moment of the complex (5.30 BM per Mn(II)), which is less than the spin-only moment of Mn(II), show that the two Mn(II) ions are weakly antiferromagnetically coupled.     

Morphology of the procarboxypeptidase A-S6 complex

Bovine pancreatic procarboxypeptidase A is secreted as a non-covalent association of three different proteins (pro CPA-S6). The free native subunits can be obtained by dissociation of the complex by dimethylmaleylation. Moreover, two specific binary complexes resulting from the high affinity of procarboxypeptidase A (subunit I) for its other two partners (subunits II and III) can also be obtained.In order to better understand the function of the association, an investigation of the morphology of the ternary complex by solution X-ray scattering has been carried out. The radii of gyration of all the molecular species have been obtained and the experimental results have been interpreted in terms of compact objects of simple shape. The various components correspond to globular particles as shown by the value of the ratio Rg/M^1/3. This is confirmed by the moderate anisotropy of the simple geometric shapes determined using an assumed value of 0.3 g H₂O/g protein for the hydration. The distances between the centres of gravity of pairs of species strongly suggest that the components are in the closest distance configuration or close to it. However, the binary complex I–III appears to be more open than the complex I–II. Finally, a model of the interaction between carboxpeptidase A and its activation peptide has been constructed by comparing the hypothetical geometric model of subunit I to the crystallographically determined structure of carboxypeptidase A.Abbreviations pro CPA procarboxypeptidase A - pro CPA-S6 (or T.C.) ternary complex with a sedimentation coefficient of 6S - CPA carboxypeptidase A     

The molecular mechanism of induction of unfolded protein response by Chlamydia

The unfolded protein response (UPR) contributes to chlamydial pathogenesis, as a source of lipids and ATP during replication, and for establishing the initial anti-apoptotic state of host cell that ensures successful inclusion development. The molecular mechanism(s) of UPR induction by Chlamydia is unknown. Chlamydia use type III secretion system (T3SS) effector proteins (e.g, the Translocated Actin-Recruiting Phosphoprotein (Tarp) to stimulate host cell's cytoskeletal reorganization that facilitates invasion and inclusion development. We investigated the hypothesis that T3SS effector-mediated assembly of myosin-II complex produces activated non-muscle myosin heavy chain II (NMMHC-II), which then binds the UPR master regulator (BiP) and/or transducers to induce UPR. Our results revealed the interaction of the chlamydial effector proteins (CT228 and Tarp) with components of the myosin II complex and UPR regulator and transducer during infection. These interactions caused the activation and binding of NMMHC-II to BiP and IRE1α leading to UPR induction. In addition, specific inhibitors of myosin light chain kinase, Tarp oligomerization and myosin ATPase significantly reduced UPR activation and Chlamydia replication. Thus, Chlamydia induce UPR through T3SS effector-mediated activation of NMMHC-II components of the myosin complex to facilitate infectivity. The finding provides greater insights into chlamydial pathogenesis with the potential to identify therapeutic targets and formulations.     

pH Dependence of the Efficiency of Binding of Iron Cations to the Donor Side of Photosystem II

Light-induced interaction of Fe(II) cations with the donor side of Mn-depleted photosystem II (PS II(–Mn)) results in the binding of iron cations and blocking of the high-affinity (HA_Z) Mn-binding site. The pH dependence of the blocking was measured using the diphenylcarbazide/2,6-dichlorophenolindophenol test. The curve of the pH dependence is bell-shaped with pK ₁ = 5.8 and pK ₂ = 8.0. The pH dependence of the O₂-evolution mediated by PS II membranes is also bellshaped (pK ₂ = 7.6). The pH dependence of the process of electron donation from exogenous donors in PS II(–Mn) was studied to determine the location of the alkaline pH sensitive site of the electron transport chain. The data of the study showed that the decrease in the iron cation binding efficiency at pH > 7.0 during blocking was determined by the donor side of the PS II(–Mn). Mössbauer spectroscopy revealed that incubation of PS II(–Mn) membranes in a buffer solution containing ⁵⁷Fe(II) + ⁵⁷Fe(III) was accompanied by binding only Fe(III) cations. The pH dependence of the nonspecific Fe(III) cation binding is also described by the same bell-shaped curve with pK ₂ = 8.1. The treatment of the PS II(–Mn) membranes with the histidine modifier diethylpyrocarbonate resulted in an increase in the iron binding strength at alkaline pH. It is suggested that blocking efficiency at alkaline pH is determined by competition between OH^– and histidine ligand for Fe(III). Because the high-affinity Mn-binding site contains no histidine residue, this fact can be regarded as evidence that histidine is located at another (other than high-affinity) Fe(III) binding site. In other words, this means that the blockage of the high-affinity Mn-binding site is determined by at least two iron cations. We assume that inactivation of oxygen-evolving complex and inhibition of photoactivation in the alkaline pH region are also determined by competition between OH^– and a histidine residue involved in coordination of manganese cation outside the high-affinity site.