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The thermally assisted reaction of Cr(NO 3 ) 3 ·9H 2 O, 4-Cl-pzH, and Et 3 N yielded electrochemically active trinuclear Cr III -oxo-pyrazolate complexes with formula (Ph 4 P) 2 [Cr 3 (μ 3 -O)(μ-4-Cl-pz) 6 X 3 ], X = Cl (1) and Br (2). Magnetic susceptibility measurements and EPR spectroscopy show that 1 has an antiferromagnetically coupled Cr 3 O-core with an S = 1/2 ground state and isotropic exchange of J = -12.7 cm -1 . ( H ex = -2 JS 1 S 2 ). Cyclic voltammetry reveals a facile reversible oxidation to the formally CrIII2Cr IV analogue of 1 . The UV-Vis-NIR spectra of CrIII3 and CrIII2Cr IV species have been assigned with the help of DFT calculations.

The tetramanganese-calcium cluster of the oxygen-evolving complex of photosystem II adopts electronically and magnetically distinct but interconvertible valence isomeric forms in its first light-driven oxidized catalytic state, S 2 . This bistability is implicated in gating the final catalytic states preceding O-O bond formation, but it is unknown how the biological system enables its emergence and controls its effect. Here we show that the Mn 4 CaO 5 cluster in the resting (dark-stable) S 1 state adopts orientational Jahn-Teller isomeric forms arising from a directional change in electronic configuration of the "dangler" Mn III ion. The isomers are consistent with available structural data and explain previously unresolved electron paramagnetic resonance spectroscopic observations on the S 1 state. This unique isomerism in the resting state is shown to be the electronic origin of valence isomerism in the S 2 state, establishing a functional role of orientational Jahn-Teller isomerism unprecedented in biological or artificial catalysis., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Among the intermediate catalytic steps of the water-oxidizing Mn 4 CaO 5 cluster of photosystem II (PSII), the final metastable S 3 state is critically important because it binds one substrate and precedes O 2 evolution. Herein, we combine X- and Q-band EPR experiments on native and methanol-treated PSII of Spinacia oleracea and show that methanol-treated PSII preparations of the S 3 state correspond to a previously uncharacterized high-spin (S=6) species. This is confirmed as a major component also in intact photosynthetic membranes, coexisting with the previously known intermediate-spin conformation (S=3). The high-spin intermediate is assigned to a water-unbound form, with a Mn IV 3 subunit interacting ferromagnetically via anisotropic exchange with a coordinatively unsaturated Mn IV ion. These results resolve and define the structural heterogeneity of the S 3 state, providing constraints on the S 3 to S 4 transition, on substrate identity and delivery pathways, and on the mechanism of O-O bond formation., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Curcumin and quercetin are two of the most prominent natural polyphenols with a diverse spectrum of beneficial properties, including antioxidant, anti-inflammatory, chemopreventive and chemotherapeutic activity. The complexation of these natural products with bioactive transition metal ions can lead to the generation of novel metallodrugs with enhanced biochemical and pharmacological activities. Within this framework, the synthesis and detailed structural and physicochemical characterization of two novel complex assemblies of Cu(II) with curcumin and quercetin and the ancillary aromatic chelator 2,2'-bipyridine is presented. The two complexes represent the only crystallographically characterized structures with Cu(II) as the central metal ion and curcumin or quercetin as the ligands. The new complexes were biologically evaluated in vitro for their antioxidant potential, both exhibiting strong scavenging activity in the 2,2-diphenyl-1-picrylhydrazyl assay, and their plasmid DNA binding/cleavage properties. Both complexes appear to be non-toxic in the eukaryotic experimental model Saccharomyces cerevisiae and merit further investigation of their pharmacological profile., Competing Interests: Declaration of competing interest No conflict of interest was declared by the authors., (Copyright © 2020 Elsevier Inc. All rights reserved.)

This study aims at the further expansion of knowledge on the antimicrobial activities of the tetramic acid moiety and the effect of metal complexation. Complexes of the N-acetyl-3-acetyl-5-benzylidenetetramic acid with Mn, Co, Ni, Cu, Zn and Cd were synthesized and screened against 5 key ESKAPE pathogens (Escherichia coli, methicillin-resistant Staphylococcus aureus (MRSA), Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa) and 2 fungi (Cryptococcus neoformans and Candida albicans). The cadmium complex was found to effectively inhibit the fungus Cryptococcus neoformans with minimum inhibitory concentration (MIC) of 8 μg/mL, with no human cell toxicity and hemolytic activity within the tested concentration range. The biologically active tetramic acid‑cadmium complex was structurally characterized by single-crystal X-ray analysis. Furthermore, the thermal stability of the ligand and the complexes was investigated along with NMR and EPR studies of the Cd(II) and Co(II) complexes respectively., (Copyright © 2019 Elsevier Inc. All rights reserved.)

In biological water oxidation, a redox-active tyrosine residue (D1-Tyr161 or Y Z ) mediates electron transfer between the Mn 4 CaO 5 cluster of the oxygen-evolving complex and the charge-separation site of photosystem II (PSII), driving the cluster through progressively higher oxidation states S i ( i = 0-4). In contrast to lower S-states (S 0 , S 1 ), in higher S-states (S 2 , S 3 ) of the Mn 4 CaO 5 cluster, Y Z cannot be oxidized at cryogenic temperatures due to the accumulation of positive charge in the S 1 → S 2 transition. However, oxidation of Y Z by illumination of S 2 at 77-190 K followed by rapid freezing and charge recombination between Y Z • and the plastoquinone radical Q A •- allows trapping of an S 2 variant, the so-called S 2 trapped state (S 2 t ), that is capable of forming Y Z • at cryogenic temperature. To identify the differences between the S 2 and S 2 t states, we used the S 2 t Y Z • intermediate as a probe for the S 2 t state and followed the S 2 t Y Z • /Q A •- recombination kinetics at 10 K using time-resolved electron paramagnetic resonance spectroscopy in H 2 O and D 2 O. The results show that while S 2 t Y Z • /Q A •- recombination can be described as pure electron transfer occurring in the Marcus inverted region, the S 2 t → S 2 reversion depends on proton rearrangement and exhibits a strong kinetic isotope effect. This suggests that Y Z oxidation in the S 2 t state is facilitated by favorable proton redistribution in the vicinity of Y Z , most likely within the hydrogen-bonded Y Z -His190-Asn298 triad. Computational models show that tautomerization of Asn298 to its imidic acid form enables proton translocation to an adjacent asparagine-rich cavity of water molecules that functions as a proton reservoir and can further participate in proton egress to the lumen.

An artificial oxygen-evolving complex (OEC) contains a tetranuclear Co III 4 O 4 cubic cluster ligated with acetate and pyridine molecules, a light-activated Ru(bpy) 3 2+ moiety (bpy = 2,2'-bipyridine), and the sacrificial electron acceptor S 2 configuration of the Co(II) species. The measurement of the EPR spectrum at a wider magnetic field range in comparison to that reported recently shows the presence of an additional signal that contributes to the spectrum of the Co(II) center. The theoretical simulation of this spectrum reveals that an isotropic g value and considerably small zero-field splitting parameters describe the high-spin Co(II) ion in a unique way, which supports a tetrahedral crystal field symmetry. On the basis of the spin-Hamiltonian parameters, the looping transitions that lead to the experimental EPR signals are determined. Additionally, a possible role of the symmetry of the Co(II) species and a proposed model that explains its formation during the O 8 2- side. A recent EPR investigation of this system showed the formation of the high-spin Co(II) ion under visible-illumination conditions. It has been supported that this center originates from the cluster and is involved in the oxygen-evolving process. The present report is focused on the further characterization of the high-spin d 7 configuration of the Co(II) species. The measurement of the EPR spectrum at a wider magnetic field range in comparison to that reported recently shows the presence of an additional signal that contributes to the spectrum of the Co(II) center. The theoretical simulation of this spectrum reveals that an isotropic g value and considerably small zero-field splitting parameters describe the high-spin Co(II) ion in a unique way, which supports a tetrahedral crystal field symmetry. On the basis of the spin-Hamiltonian parameters, the looping transitions that lead to the experimental EPR signals are determined. Additionally, a possible role of the symmetry of the Co(II) species and a proposed model that explains its formation during the O 2 -evolving process of the Co 4 O 4 cubic molecules are discussed.

The intermediates trapped during the transitions between the consecutive S-states of the oxygen-evolving complex (OEC) of photosystem II (PSII) contain the free radical TyrZ • interacting magnetically with the Mn-cluster (Mn 4 Ca). In this paper, we present a theoretical study of the EPR spectrum of the S 3 TyrZ • metalloradical intermediate state, which has been recently detected in MeOH-containing PSII preparations. For this analysis, we use two different approximations: the first, simpler one, is the point-dipole approach, where the two interacting spins are the S = 1/2 of TyrZ • and the ground spin state of S = 3 of the OEC being in the S 3 state. The second approximation is based on previous proposals indicating that the ground spin state (S G  = 3) of the S 3 state arises from an antiferromagnetic exchange coupling between the S = 9/2 of the Mn(IV) 3 CaO 4 and the S = 3/2 of the external Mn(IV) of the OEC. Under the above assumption, the second approximation involves three interacting spins, denoted S A (Mn(IV) 3 Ca) = 9/2, S B (Mn(IV)) = 3/2 and S C (TyrZ • ) = 1/2. Accordingly, the tyrosine radical is exposed to dipolar interactions with both fragments of the OEC, while an antiferromagnetic exchange coupling within the "3 + 1" structural motif of the OEC is also considered. By application of the first-point-dipole approach, the inter-spin distance that simulates the experimental spectrum is not consistent with the theoretical models that were recently reported for the OEC in the S 3 state. Instead, the recent models are consistent with the results of the analysis that is performed by using the second, more detailed, approach.

We report the trapping of two metalloradical intermediates corresponding to the transitions S2 to S3 and S3 to S0 of the oxygen evolving complex (OEC) of Photosystem II (PSII), in preparations containing methanol, at temperatures near that of half inhibition of the respective S-state transitions. The first intermediate, with an EPR width of 160 G, is assigned to S2YZ, based on its similarity to the one previously characterized after trapping at 10 K. The second with a splitting of ∼80 G is tentatively assigned to S3YZ. The S3YZ EPR signal is weaker than the S2YZ one, and both are stable at cryogenic temperatures., (Copyright © 2014 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)

Over the last decade the importance of nitric oxide (NO) in plant signaling has emerged. Despite its recognized biological role, the sensitivity and effectiveness of the methods used for measuring NO concentration in plants are still under discussion. Among these, electron paramagnetic resonance (EPR) is a well-accepted technique to detect NO. In the present work we report the constraints of using 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) in biological samples as spin trap for quantitative measurement of NO. EPR analyses on Arabidopsis cell cultures and seedlings show that cPTIO(NNO) is degraded in a matter of few minutes while the (INO) compound, produced by cPTIO and NO reaction, has not been detected. Limitations of using this spin trap in plant systems for quantitative measurements of NO are discussed. As NO scavenger, cPTIO is widely used in combination with 4-amino-5-methylamino-2('),7(')-difluorofluorescein (DAF-FM) fluorescent dye in plant research. However, the dependence of DAF-FM fluorescence on cPTIO and NO concentrations is not clearly defined so that the range of concentrations should be tightly selected. In this context, a systematic study on cPTIO NO scavenging properties has been performed, as it was still lacking for plant system applications. The results of this systematic analysis are discussed in terms of reliability of the use of cPTIO in the quantitative determination and scavenging of NO in plants and plant cultured cells.

The paper extends earlier studies on the S(2)Y(Z)* intermediate that is trapped by illumination in the temperature range 77 K to 190 K of untreated samples poised in the S(2)...Q(A) state. X-band EPR experiments on untreated and glycerol (50% v/v) treated samples at 10 K indicate that the intermediate consists of two components. A wide one with a splitting of ca 170 G, and a narrow one characterized by a splitting of ca 120 G (untreated), or 124 G (glycerol-treated samples). Lower temperatures of illumination in the above temperature range favor the wide component, which at 10 K decays faster than the narrow one. Re-illumination at 10 K after decay of the signal trapped at 77-190 K induces only the narrow component. Rapid scan experiments in the temperature range 77-190 K reveal high resolution spectra of the isolated tyz Z* radical and no evidence of alternative radicals. The two split signals are accordingly assigned to different conformations of the S(2)Y(Z)* intermediate A point-dipole simulation of the spectra yields "effective distances" between the spin densities of Y(Z)* and the Mn(4)Ca center of 5.7 Å for the wide and 6.4 Å for the narrow component. The results are discussed on the basis of a molecular model assuming two sequential proton transfers during oxidation of tyr Z. The wide component is assigned to a transient S(2)Y(Z)* conformation, that forms during the primary proton transfer., (Copyright © 2011 Elsevier B.V. All rights reserved.)

The oxygen evolving complex of Photosystem II undergoes four light-induced oxidation transitions, S(0)-S(1),...,S(3)-(S(4))S(0) during its catalytic cycle. The oxidizing equivalents are stored at a (Mn)(4)Ca cluster, the site of water oxidation. EPR spectroscopy has yielded valuable information on the S states. S(2) shows a notable heterogeneity with two spectral forms; a g=2 (S=1/2) multiline, and a g=4.1 (S=5/2) signal. These oscillate in parallel during the period-four cycle. Cyanobacteria show only the multiline signal, but upon advancement to S(3) they exhibit the same characteristic g=10 (S=3) absorption with plant preparations, implying that this latter signal results from the multiline configuration. The fate of the g=4.1 conformation during advancement to S(3) is accordingly unknown. We searched for light-induced transient changes in the EPR spectra at temperatures below and above the half-inhibition temperature for the S(2) to S(3) transition (ca 230K). We observed that, above about 220K the g=4.1 signal converts to a multiline form prior to advancement to S(3). We cannot exclude that the conversion results from visible-light excitation of the Mn cluster itself. The fact however, that the conversion coincides with the onset of the S(2) to S(3) transition, suggests that it is triggered by the charge-separation process, possibly the oxidation of tyr Z and the accompanying proton relocations. It therefore appears that a configuration of (Mn)(4)Ca with a low-spin ground state advances to S(3).

IspG is a [4Fe-4S] cluster-containing protein, and the [4Fe-4S](+) species is proposed to be the catalytically relevant species. However, attempts reported in the literature failed to detect the [4Fe-4S](+) species. In this study, using a potent reduction system, we have successfully detected the [4Fe-4S](+) species with X-band EPR spectroscopy. In addition, we have improved the Escherichia coli IspG activity to 550 nmol min(-1) mg(-1), which is approximately 20-fold greater than that of the NADPH-Fpr-FldA system in the literature.

The O2-evolving complex of photosystem II, Mn 4Ca, cycles through five oxidation states, S0,..., S4, during its catalytic function, which involves the gradual abstraction of four electrons and four protons from two bound water molecules. The direct oxidant of the complex is the tyrosine neutral radical, YZ(*), which is transiently produced by the highly oxidizing power of the photoexcited chlorophyll species P680. EPR characterization of YZ(*) has been limited, until recently, to inhibited (non-oxygen-evolving) preparations. A number of relatively recent papers have demonstrated the trapping of YZ(*) in O2-evolving preparations at liquid helium temperatures as an intermediate of the S0 to S1, S1 to S2, and S2 to S3 transitions. The respective EPR spectra are broadened and split at g approximately 2 by the magnetic interaction with the Mn cluster, but this interaction collapses at temperatures higher than about 100K [Zahariou et al. (2007) Biochemistry 46, 14335 -14341]. We have conducted a study of the Tyr Z(*) transient in the temperature range 77-240 K by employing rapid or slow EPR scans. The results reveal for the first time high-resolution X-band spectra of Tyr Z(*) in the functional system and at temperatures close to the onset of the S-state transitions. We have simulated the S 2Y Z(*) spectrum using the simulation algorithm of Svistunenko and Cooper [(2004) Biophys. J. 87, 582 -595]. The small g(x) = 2.00689 value inferred from the analysis suggests either a H-bonding of Tyr Z (*) (presumably with His190) that is stronger than what has been assumed from studies of Tyr D(*) or Tyr Z(*) in Mn-depleted preparations or a more electropositive environment around Tyr Z(*). The study has also yielded for the first time direct information on the temperature variation of the YZ(*)/QA(-) recombination reaction in the various S states. The reaction follows biphasic kinetics with the slow phase dominating at low temperatures and the fast phase dominating at high temperatures. It is tentatively proposed that the slow phase represents the action of the YZ(*)/YZ(-) redox couple while the fast phase represents that of the YZ(*)/YZH couple; it is inferred that Tyr Z at elevated temperatures is protonated at rest. It is also proposed that YZ(*)/YZH is the couple that oxidizes the Mn cluster during the S1-S2 and S2-S3 transitions. A simple mechanism ensuring a rapid (concerted) protonation of Tyr Z upon oxidation of the Mn cluster is discussed, and also, a structure-based molecular model suggesting the participation of His190 into two hydrogen bonds is proposed.

Tetronic acids, 4-hydroxy-5H-furan-2-ones, constitute a class of heterocyclic compounds with potent biological and pharmacological activity. The beta, beta'-tricarbonyl moiety plays an integral role in biological systems and forms a variety of metal complexes. In this report, we present the complexation reactions of 3-ethoxycarbonyl tetronic acids with acetates and chlorides of Cu(II) and Co(II). These complexes have been studied by means of EPR spectroscopy and magnetic susceptibility measurements. From the obtained results, a preliminary complexation mode of the ligand is proposed.

Tyr Z of photosystem II mediates electron transfer from the water splitting site, a Mn4Ca cluster, to the specialized chlorophyll assembly P680. Due to its proton-limited redox properties and the proximity to the Mn cluster, it is thought to play a critical role in the proton-coupled electron transfer reactions that constitute the four-step oxidation mechanism (so-called S-state transitions) of water to molecular oxygen. Spectroscopic evidence for the Tyr Z radical has been scarce in intact preparations (it is difficult to probe it optically, and too short-lived for EPR characterization) until recently. Advances in recent years have allowed the trapping at liquid helium temperatures and EPR characterization of metalloradical intermediates, attributed to tyrosyl Z* magnetically interacting with the Mn cluster. We have extended these studies and examined the evolution of the spectra of five intermediates: S0YZ*, S0YZ* (with 5% MeOH), S1YZ*, S2YZ*, and S2YZ* (with 5% MeOH) in the temperature range of 11-230 K. A rapid-scan EPR method has been applied at elevated temperatures. The tyrosyl radical decouples progressively from Mn, as the Mn relaxation rate increases with an increase in temperature. Above approximately 100 K, the spectra collapse to the unperturbed spectrum of Tyr Z*, which is found to be somewhat broader than that of the stable Tyr D* radical. This study provides a simple means for recording the spectrum of Tyr Z* and extends earlier observations that link the photochemistry at liquid helium temperatures to the photochemistry at temperatures that support S-state transitions.

Photosystem II preparations poised in the S(2)...Q(A) state produce no detectable intermediate during straightforward illumination at liquid helium temperatures. However, upon flash illumination in the range of 77-190 K, they produce a transient state which at -10 degrees C advances to S(3) or after rapid cooling to 10 K gives rise to a 116 G wide metalloradical EPR signal. The latter decays with half-times on the order of a few minutes, presumably by charge recombination, and can be regenerated repeatedly by illumination at 10 K. The constraints for Tyr Z oxidation are attributed to the presence of excess positive charge in S(2). Elevated temperatures are required presumably to overcome a thermal barrier in the deprotonation of Tyr Z(+) or most likely to allow secondary proton transfer away from the base partner of Tyr Z. Treatment with 5% (v/v) MeOH appears to remove the constraints for Tyr Z oxidation, and a 160 G wide metalloradical EPR signal is produced by illumination at 10 K, which decays with a half-time of ca. 80 s. Formation of the metalloradical signals is accompanied by reversible changes in the Mn multiline signal. The intermediates are assigned to Tyr Z(*) magnetically interacting with the Mn cluster in S(2), S(2)Y(Z)(*). A molecular model which extends an earlier suggestion and provides a plausible explanation of a number of observations, including the binding of small molecules to the Mn cluster, is presented.