Stable Redox States Research Articles

Multichannel Molecular Switch with a Surface-Confined Electroactive Radical Exhibiting Tunable Wetting Properties

A multichannel surface molecular switch based on self-assembled monolayers (SAMs) on gold of a novel polychlorotriphenylmethyl radical has been fabricated. These SAMs are electrically commutable between two stable redox states that reveal distinct magnetic, optical, and wetting properties that can be employed as read-out mechanisms. The high stability and reversibility of systems like the one reported here further supports the vision of using molecules in the electronic devices of the future.

Physicochemical properties of solutions and ultrathin films of triple-decker gadolinium tetra-15-crown-5-phthalocyaninate

The ability of the triple-decker gadolinium complex with tetra-15-crown-5-phthalocyanine Gd2(R4Pc)3, (R = 15-crown-5) (1) to form monolayers and Langmuir-Blodgett films (LBFs) was studied for the first time. The charge characteristics of molecules of a triple-decker phthalocyaninate in monolayer, as well as their orientation and adhesion to the water subphase, were controlled by changing the surface pressure, pH, and subphase composition (aqueous solutions containing triethylenetetramine (TETA) and metal cations). It was shown that the presence of Na+ and protonated TETA aminogroups in resulted in an increasing limiting monolayer area and significant decreasing of the monolayer liquid state region. It is proven that the observed effects are caused by the conformational and charge transitions of peripheral crown ethers induced by their interaction with cations. A comparison of the differential reflectance spectra of the complex monolayer on the deionized water surface with UV-Vis absorption spectra of three-layer Gd2(R4Pc)3 LBF and complex solution in chloroform shows that partial complex oxidation and intensive stacking formation occurs already at the stage of monolayer formation on the subphase surface. Electrochemical studies of three-layer LBFs performed at the indium-tin-oxide electrodes (ITO-electrodes) using cyclic voltammetry (CV) showed three reversible redox waves in the potential range −200 to +1100 mV (vs. Ag+/AgCl). All registered peaks remain the same position and intensity upon multiple cycling. plasmon resonance (SPR) measurements allow to register three stable redox states of studied LBF upon applying the external potential. Such behavior shows the possibility to use multistep redox transformations of studied complex LBF for developing of stable and reproducible switchable optoelectronic systems.

Tetrathiafulvalene-Based Architectures: From Guests Recognition to Self-Assembly

The tetrathiafulvalene (TTF) unit has been successfully used for an incredibly broad range of applications. Beyond the well-established conducting properties of the corresponding cation-radical salts, this unit has appeared as a key redox-active component for various applications supported by its remarkable redox properties: a high π-donating ability and occurrence of three stable redox states. This article reviews the main contribution of the group of Angers to this field, highlighting results obtained in terms of redox-sensing as well as efforts carried out to reach new self-assembled TTF-based architectures.

Near infrared spectroscopy: Blood and tissue oxygenation in renal ischemia-reperfusion injury in rats

Near infrared spectroscopy was used to monitor blood (HbO2, Hb, and Hbtot) and tissue oxygenation (oxidized cyt aa3) of both left and right kidneys of ratsin vivo simultaneously, during either 45 or 80 minutes of left renal ischemia followed by 4.5 hours of reperfusion. Ischemia significantly reduced HbO2 (p< 0.0001) and increased Hb (p<0.05) concentration change in the left kidney compared with the right kidney (control). Reperfusion with oxygenated blood reached control levels after 1 hour. The rate of change in HbO2 (p<0.05) and Hbtot (p<0.05) concentration during reperfusion was dependent upon the duration of ischemia, being slower after the longer ischemic period of 80 minutes. The concentration change of oxidized cyt aa3 of the 80-minute ischemic group slowly increased compared with the stable redox state of the 45-minute ischemic group, during the latter stages of reperfusion (p<0.05). Near infrared spectroscopy is a promising new development that will enable the effects of interventional treatment upon ischemia and reperfusion injury to be studied.

The Microbial Sulfur Cycle

Sulfur is one example of an element whose transformation and fate in the environment are critically dependent upon microbial activities. Sulfur is the 10th most abundant element in the universe and the sixth most abundant element in microbial biomass. By virtue of its chemical properties, particularly the wide range of stable redox states, sulfur plays important roles in central biochemistry as a structural element, redox center, and carbon carrier. In addition, redox reactions involving reduced and oxidized inorganic sulfur compounds can be utilized by microbes for the generation and conservation of biochemical energy. Microbial transformation of both inorganic and organic sulfur compounds has had a profound effect on the properties of the biosphere and continues to affect geochemistry today. While sulfur microbiology has a rich history, the last decade has changed our views on many aspects of microbially driven biogeochemical cycles, including the global sulfur cycle, in that tremendous advances in methods, techniques, and approaches have enabled the discovery of novel processes and characterization of the organisms and molecular mechanisms that facilitate them. This progress is documented in a collection of 20 articles at the leading edge of sulfur microbiology, focusing on microbes, reactions, and compounds of geochemical significance. Whether students or peer researchers in the field, it is the intent of this Research Topic issue to reach a wide audience whose interests and expertise spans from the molecular biochemistry of single enzymes and their reactions through pure culture physiology and genetic analysis to community level metagenomics and metatranscriptomics. This collection is especially timely given the recent publication describing one of the oldest microfossils to date that likely lived a sulfur-dependent chemolithoautotrophic lifestyle (Wacey et al., 2011). Microbial sulfur cycling may have had a hand in starting it all. This collection of papers can be viewed as falling roughly into four clusters that deal with enzymes/activities, organisms/pathways/comparative approaches, symbiosis, and environments: At the level of enzymes and activities, new data are reported on a variety of systems. Structural analyses are reported for a disproportionating sulfur oxygenase/reductase and a dissimilatory sulfite reductase. Two independent papers focus on tetrathionate hydrolase, one of bacterial and one of archeal origin. Electron flow for the Sor pathway of thiosulfate oxidation has been further defined in Sinorhizobium meliloti. Finally, another article provides the first functional evidence in a phototrophic bacterium for the function of a quinone-interacting membrane-bound oxidoreductase that participates in the oxidation of sulfite. At the level of pathways and organisms, detailed reviews have been produced. For the sulfate reducing bacteria, the foci are metabolic flexibility, energy metabolism, and electron flow. In the phototrophic bacteria, putative sulfur oxidation pathways in sequenced Chlorobi are examined. Finally, the latest developments in microbial pathways for the degradation of DMSP, an extremely important component of the marine sulfur cycle are summarized. Three research articles focus on the regulation of sulfur oxidation genes in the purple sulfur bacterium Allochromatium vinosum, growth on tetrathionate vs. S(0) in Acidithiobacillus caldus and new Mycobacteria isolates from sandstone monuments that appear to grow as sulfur oxidizers. Multi-organism interactions and symbiosis encompass the third category. Papers on exclusively prokaryotic symbioses and those that cross domain boundaries involving microbial eukaryotes and the symbionts of marine fauna provide a broad perspective in this arena grounded in the application of traditional (label tracing experiments to examine substrate exchange) and cutting edge (transcriptomic analysis of uncultured microbial symbionts) techniques. Finally, this issue presents three papers on sulfur cycling at the level of ecosystems and communities. Two extreme environments are examined. Sorokin et al. (2011) summarize their long track record of work in soda lakes that combine high salinity with high pH. Both the reductive and oxidative arms of the sulfur cycle are discussed, and new insights into sulfidogenesis are presented for these fascinating systems. Yang et al. (2011) compare microbial communities and chemolithoautotrophic activity in relatively understudied freshwater hydrothermal vents in Yellowstone Lake. Finally, Luther et al. (2011) present a detailed analysis of the chemical parameters that influence abiotic oxidation of sulfide in natural environments. Sulfide oxidizing microbes must compete with this abiotic process and this report analyzes both literature and fresh experimental data to show that biotic sulfide oxidation will almost always occur at significantly faster rates due to kinetic restrictions on abiotic oxidation. Biology rules after all, as should be evident not just from this paper, but from the entire collection presented here. Together, these papers represent the forefront of research of this field. We hope you enjoy them as much as we enjoyed working with this talented group of researchers to put this volume together.

Ferrocene-Terminated Monolayers Covalently Bound to Hydrogen-Terminated Silicon Surfaces. Toward the Development of Charge Storage and Communication Devices

The combination of monocrystalline silicon's well-defined structure and the ability to prepare hydrogen-terminated surfaces (Si-H) easily and reproducibly has made this material a very attractive substrate for immobilizing functional molecules. The functionalization of Si-H using the covalent attachment of organic monolayers has received intense attention due to the numerous potential applications of controlled and robust organic/Si interfaces. Researchers have investigated these materials in diverse fields such as molecular electronics, chemistry, and bioanalytical chemistry. Applications include the preparation of surface insulators, the incorporation of chemical or biochemical functionality at interfaces for use in photovoltaic conversion, and the development of new chemical and biological sensing devices. Unlike those of gold, silicon's electronic properties are tunable, and researchers can directly integrate silicon-based devices within electronic circuitry. Moreover, the technological processes used for the micro- and nanopatterning of silicon are numerous and mature enough for producing highly miniaturized functional electronic components. In this Account, we describe a powerful approach that integrates redox-active molecules, such as ferrocene, onto silicon toward electrically addressable systems devoted to information storage or transfer. Ferrocene exhibits attractive electrochemical characteristics: fast electron-transfer rate, low oxidation potential, and two stable redox states (neutral ferrocene and oxidized ferrocenium). Accordingly, ferrocene-modified silicon surfaces could be used as charge storage components with the bound ferrocene center as the memory element. Upon application of a positive potential to silicon, ferrocene is oxidized to its corresponding ferrocenium form. This redox change is equivalent to the change of a bit of information from the "0" to "1" state. To erase the stored charge and return the device to its initial state, a low potential must be applied to reduce the whole generated ferrocenium. In this type of application, the electron is transferred from the ferrocene headgroups to the underlying conducting silicon surface by a tunneling process across the monolayer. To produce a stable and reproducible electrical response, this process must be efficient, fast, and reversible. The stability, charge density, and capacitance performances of high-quality ferrocene-terminated monolayers could compete with those of the existing semiconductor-based memory devices, such as dynamic random access memories, DRAMs. Moreover, we provide experimental evidence that a series of immobilized ferrocene centers can efficiently communicate via a lateral electron hopping process. Using these modified interfaces, we demonstrate that the thin redox-active monolayer can behave as a purely conducting material, highlighting an unprecedented very fast electron communication between immobilized redox groups. Perhaps more importantly, the surface coverage of ferrocene allows us to precisely control the rate of this process. Such characteristics are relevant not only for electrocatalytic reactions but also for widening the potential applications of these assemblies to novel molecular electronic devices (e.g. chemiresistors, chemically sensitive field-effect transistors (CHEMFETs)) and redox chemistry on insulating surfaces.

Redox-Switchable Second-Order Nonlinear Optical Responses of Push−Pull Monotetrathiafulvalene-Metalloporphyrins

The redox-active tetrathiafulvalene (TTF) is a good electron donor, and porphyrin is highly delocalized in cyclic pi-conjugated systems. The direct combination of the two interesting building units into the same molecule provides an intriguing molecular system for designing nonlinear optical (NLO) molecular materials. In the present paper, the second-order NLO properties of a series of monoTTF-porphyrins and metalloporphyrins have been calculated by density functional theory (DFT) combined with the finite field (FF) method. Our calculations show that these compounds possess considerably large static first hyperpolarizabilities, approximately 400 x 10(-30) esu. Since the TTF unit is able to exist in three different stable redox states (TTF, TTF(*+), and TTF(2+)), the redox switching of the NLO response of the zinc(II) derivative of monoTTF-metalloporphyrin has been studied, and a substantial enhancement in static first hyperpolarizability has been obtained in its oxidized species according to our DFT-FF calculations. The beta values of one- and two-electron-oxidized species are 3.6 and 8.7 times as large as that of the neutral compound, especially for two-electron-oxidized species, with a value of 3384 x 10(-30) esu. This value is about 3 times that for a push-pull metalloporphyrin, which has an exceptionally large hyperpolarizability among reported organic NLO chromophores. Meanwhile, to give a more intuitive description of band assignments of the electron spectrum and trends in NLO behavior of these compounds, the time-dependent (TD)DFT method has been adopted to calculate the electron spectrum. The TDDFT calculations well-reproduce the soret band and Q-type bands of the monoTTF-porphyrin, and these absorption bands can be assigned to the pi --> pi* transition of the porphyrin core. On the other hand, the oxidized process significantly affects the geometrical structures of the TTF unit and porphyrin ring, and the two-electron-oxidized species has a planar TTF unit and a high conjugative porphyrin ring. This effect reduces the excited energy, changes the CT feature, and thus enhances its static first hyperpolarizability.

Tetrathiafulvalene (TTF) derivatives: key building-blocks for switchable processes

Besides a traditional use for the development of organic conducting materials, the tetrathiafulvalene (TTF) unit and its derivatives have recently appeared as key constituents for new applications, exploiting remarkable redox properties: a high pi-donating ability and occurrence of three stable redox states. Indeed, in very recent years, an impressive variety of switchable TTF-based molecular and supramolecular (multifunctional) architectures have been designed and synthesized. In this feature article, we discuss recent developments of TTF-based molecular and supramolecular systems in this respect, including molecular sensors, redox-fluorescent switches, multi-input systems for logic gates, electrochemically-driven conformational controls, molecular clips and tweezers, and redox-controlled gelation processes.

Dithienylcyclopentenes-Containing Transition Metal Bisterpyridine Complexes Directed toward Molecular Electronic Applications

There is continuing interest in the design and synthesis of functional materials for applications in molecular electronics and information storage. Of particular interest are systems that can provide multiple means for controlling transport through well-defined and stable electronic and/or redox states. We report herein the synthesis and characterization of a system containing transition-metal complexes along with dithienylethene (DTE) units so as to achieve photo and redox control of transport. A facile synthetic methodology was developed to assemble and couple metal terpyridine (M-tpy) complexes with the photochromic DTE unit in a linear structure with M-DTE-M or DTE-M-DTE arrangements, with emphasis on the latter series. The photochromic properties of these assemblies were examined by monitoring the changes in their UV/vis spectra upon irradiation at specific wavelengths capable of triggering the open/closed isomerization in the DTE units. Their electrochromic properties were studied via cyclic voltammetry and controlled potential electrolysis experiments. Complexes 10 (PhDTE-Fe-DTEPh) and 11 (PhDTE-Co-DTEPh) with phenyl-ending groups were found to be both photochromic and electrochromic, so they represent excellent candidates for further elaborations. The Fe(II)-containing complex 8 (ClDTE-Fe-DTECl) with chloride-ending groups was photochromically inactive but could undergo the electrochemically induced open-to-closed isomerization. On the contrary, the electrochromically inactive complex 9 (ClDTE-Co-DTECl) underwent cyclization under ultraviolet irradiation.

Redox-controlled multistability of double-decker cerium tetra-(15-crown-5)-phthalocyaninate ultrathin films

The optical and electrochemical properties of novel double-decker cerium bis-tetra-15-crown-5-phthalocyaninate [ Ce ( R 4 Pc 2−)2]0 (R4Pc2− = [4,5,4',5',4",5",4'",5'"-tetrakis-(1,4,7,10,13-pentaoxapentadecamethylene)-phthalocyaninate-anion]) Langmuir-Blodgett and cast films were investigated. The particular feature of cerium ion in complex with tetra-15-crown-5-phthalocyanine is the stability of oxidation state +4 unlike other lanthanide metal centers. Cyclic voltammetry curves exhibited three stable redox states in the Langmuir-Blodgett and cast films. Redox processes in Langmuir-Blodgett films are reversible and reproducible at multiple scan procedures. The mechanisms of redox transformations in Langmuir-Blodgett films are suggested. We demonstrated that the well-defined structure of Langmuir-Blodgett film is essential for fast electron transfer within the planar system, in which the charge is delocalized along the conjugated assembly of uniformly ordered stacks of discotic crown-phthalocyaninate. Fast charge relaxation was observed in highly ordered Langmuir-Blodgett film whereas the electrochemically written redox states remained unchanged in unordered cast film. The combination of electrochemistry with surface plasmon resonance spectroscopy allowed us to demonstrate that stepwise change of potential in the range 200-850 mV induced the respective optical response, which can be observed as the change in resonance angle value. High-speed response and reversibility of the switching process between stable states may be utilized as the basis for switchable optoelectronic devices. Electrochemical multistability of cerium crown-phthalocyaninate provided a basis for developing a simple strategy to fabricate nanoelectromechanical systems with high efficiency and fast response. Our approach relies on the modulation of the distance between decks in a complex stack via redox-controlled change of metal center size that results in change of linear dimensions of the stacks. The reported results are valuable, not only because of their potential applications, for instance, in OFET and MEMS fabrication, but also from a fundamental point of view since they illustrate the interplay between the orientation of stacks bearing discotic aromatic molecules and charge transfer within such a planar assembly.

Rapid and Quantitative Activation of Chlamydia trachomatis Ribonucleotide Reductase by Hydrogen Peroxide

We recently showed that the class Ic ribonucleotide reductase (RNR) from the human pathogen Chlamydia trachomatis ( Ct) uses a Mn (IV)/Fe (III) cofactor in its R2 subunit to initiate catalysis [Jiang, W., Yun, D., Saleh, L., Barr, E. W., Xing, G., Hoffart, L. M., Maslak, M.-A., Krebs, C., and Bollinger, J. M., Jr. (2007) Science 316, 1188-1191]. The Mn (IV) site of the novel cofactor functionally replaces the tyrosyl radical used by conventional class I RNRs to initiate substrate radical production. As a first step in evaluating the hypothesis that the use of the alternative cofactor could make the RNR more robust to reactive oxygen and nitrogen species [RO(N)S] produced by the host's immune system [Högbom, M., Stenmark, P., Voevodskaya, N., McClarty, G., Gräslund, A., and Nordlund, P. (2004) Science 305, 245-248], we have examined the reactivities of three stable redox states of the Mn/Fe cluster (Mn (II)/Fe (II), Mn (III)/Fe (III), and Mn (IV)/Fe (III)) toward hydrogen peroxide. Not only is the activity of the Mn (IV)/Fe (III)-R2 intermediate stable to prolonged (>1 h) incubations with as much as 5 mM H 2O 2, but both the fully reduced (Mn (II)/Fe (II)) and one-electron-reduced (Mn (III)/Fe (III)) forms of the protein are also efficiently activated by H 2O 2. The Mn (III)/Fe (III)-R2 species reacts with a second-order rate constant of 8 +/- 1 M (-1) s (-1) to yield the Mn (IV)/Fe (IV)-R2 intermediate previously observed in the reaction of Mn (II)/Fe (II)-R2 with O 2 [Jiang, W., Hoffart, L. M., Krebs, C., and Bollinger, J. M., Jr. (2007) Biochemistry 46, 8709-8716]. As previously observed, the intermediate decays by reduction of the Fe site to the active Mn (IV)/Fe (III)-R2 complex. The reaction of the Mn (II)/Fe (II)-R2 species with H 2O 2 proceeds in three resolved steps: sequential oxidation to Mn (III)/Fe (III)-R2 ( k = 1.7 +/- 0.3 mM (-1) s (-1)) and Mn (IV)/Fe (IV)-R2, followed by decay of the intermediate to the active Mn (IV)/Fe (III)-R2 product. The efficient reaction of both reduced forms with H 2O 2 contrasts with previous observations on the conventional class I RNR from Escherichia coli, which is efficiently converted from the fully reduced (Fe 2 (II/II)) to the "met" (Fe 2 (III/III)) form [Gerez, C., and Fontecave, M. (1992) Biochemistry 31, 780-786] but is then only very inefficiently converted from the met to the active (Fe 2 (III/III)-Y (*)) form [Sahlin, M., Sjöberg, B.-M., Backes, G., Loehr, T., and Sanders-Loehr, J. (1990) Biochem. Biophys. Res. Commun. 167, 813-818].

Arsenate and Arsenite Sorption on Carbonate Hosted Precious Metals Ore

The societal impacts of As in water resources in the arid western USA are potentially acute as a consequence of the combined effects of limited water supplies and the pervasive occurrence of naturally occurring As in subsurface geologic formations, including the carbonate‐hosted, disseminated gold‐bearing formations of the Carlin Trend. The prevalence of As in secondary minerals in gold‐bearing carbonate‐hosted ores is of interest because of the potential for As release as a result of ore development. A key component to gold mining is the engineering and construction of large‐scale heap‐leach and waste‐rock containment structures that are characterized by variably saturated hydrology. Estimating As release behavior from these structures with a variably saturated reactive flow and transport numerical model requires the quantification of the significant differences in the sorption behavior for the stable redox states for As. Therefore, the objective of this study was to quantify this sorption behavior and to represent the observed behavior with an isotherm formulation. The pH‐dependent sorption behavior of arsenite, As(III), and arsenate, As(V), onto two carbonate‐hosted gold ores is presented. The experimentally determined pH‐dependent sorption behavior for both As(III) and As(V) is consistent with sorption on metal oxides as reported in studies on rock and soils with similar bulk mineralogical properties. The experimental sorption data are represented with two modified isotherm formulations. Modified formulations of the Langmuir isotherm and of the Sips isotherm are presented that include the pH of the sorbate solution as an additional model parameter. These formulations are applied to both As(III) and As(V) sorption data to generate an isotherm surface. The pH‐dependent isotherm methodology can be incorporated readily into numerical models for the purposes of estimating As transport behavior in field‐scale, variably saturated environments.

Retrograde Plastid Redox Signals in the Expression of Nuclear Genes for Chloroplast Proteins of Arabidopsis thaliana

Excitation imbalances between photosystem I and II generate redox signals in the thylakoid membrane of higher plants which induce acclimatory changes in the structure of the photosynthetic apparatus. They affect the accumulation of reaction center and light-harvesting proteins as well as chlorophylls a and b. In Arabidopsis thaliana the re-adjustment of photosystem stoichiometry is mainly mediated by changes in the number of photosystem I complexes, which are accompanied by corresponding changes in transcripts for plastid reaction center genes. Because chloroplast protein complexes contain also many nuclear encoded components we analyzed the impact of such photosynthetic redox signals on nuclear genes. Light shift experiments combined with application of the electron transport inhibitor 3-(3',4'-dichlorophenyl)-1,1'-dimethyl urea have been performed to induce defined redox signals in the thylakoid membrane. Using DNA macroarrays we assessed the impact of such redox signals on the expression of nuclear genes for chloroplast proteins. In addition, studies on mutants with lesions in cytosolic photoreceptors or in chloroplast-to-nucleus communication indicate that the defective components in the mutants are not essential for the perception and/or transduction of light-induced redox signals. A stable redox state of glutathione suggest that neither glutathione itself nor reactive oxygen species are involved in the observed regulation events pointing to the thylakoid membrane as the main origin of the regulatory pathways. Our data indicate a distinct role of photosynthetic redox signals in the cellular network regulating plant gene expression. These redox signals appear to act independently and/or above of cytosolic photoreceptor or known chloroplast-to-nucleus communication avenues.

Thermodynamic and EPR studies of slowly relaxing ubisemiquinone species in the isolated bovine heart complex I

Previously, we investigated ubisemiquinone (SQ) EPR spectra associated with NADH-ubiquinone oxidoreductase (complex I) in the tightly coupled bovine heart submitochondrial particles (SMP). Based upon their widely differing spin relaxation rate, we distinguished SQ spectra arising from three distinct SQ species, namely SQNf (fast), SQNs (slow), and SQNx (very slow). The SQNf signal was observed only in the presence of the proton electrochemical gradient (ΔμH+), while SQNs and SQNx species did not require the presence of ΔμH+. We have now succeeded in characterizing the redox and EPR properties of SQ species in the isolated bovine heart complex I. The potentiometric redox titration of the gz,y,x=2.00 semiquinone signal gave the redox midpoint potential (Em) at pH 7.8 for the first electron transfer step [Em1(Q/SQ)] of −45 mV and the second step [Em2(SQ/QH2)] of −63 mV. It can also be expressed as [Em(Q/QH2)] of −54 mV for the overall two electron transfer with a stability constant (Kstab) of the SQ form as 2.0. These characteristics revealed the existence of a thermodynamically stable intermediate redox state, which allows this protein-associated quinone to function as a converter between n=1 and n=2 electron transfer steps. The EPR spectrum of the SQ species in complex I exhibits a Gaussian-type spectrum with the peak-to-peak line width of ∼6.1 G at the sample temperature of 173 K. This indicates that the SQ species is in an anionic Q− state in the physiological pH range. The spin relaxation rate of the SQ species in isolated complex I is much slower than the SQ counterparts in the complex I in situ in SMP. We tentatively assigned slow relaxing anionic SQ species as SQNs, based on the monophasic power saturation profile and several fold increase of its spin relaxation rate in the presence of reduced cluster N2. The current study also suggests that the very slowly relaxing SQNx species may not be an intrinsic complex I component. The functional role of SQNs is further discussed in connection with the SQNf species defined in SMP in situ.

Cold hardiness in Ostrinia nubilalis (Lepidoptera: Pyralidae): Glycerol content, hexose monophosphate shunt activity, and antioxidative defense system

Many insects in temperate regions overwinter in diapause, during which they are cold hardy. In these insects, one of the metabolic adaptations to the unfavorable environmental conditions is the synthesis of cryoprotectants/anhydroprotectants. The aim of this study was to investigate the connection between the antioxidative system and synthesis of cryoprotectants (mainly glycerol) in diapausing larvae of the European corn borer, Ostrinia nubilalis (Lepidoptera: Pyralidae). At two periods during diapause (November and February), in three groups of insects (kept under field conditions; -12°C for two weeks; 8°C for two weeks), the activity of key enzymes of the antioxidative system and oxidative part of the hexose monophosphate shunt were measured: super- oxide dismutase, catalase, non selenium glutathione peroxidase, glutathione S-transferase, glutathione reductase, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, as well that of the antioxidative components: total glutathione and ascorbate, and dehydroascorbate reductase. There was a higher activity of antioxidative enzymes at the beginning of the dia- pause period (November) compared to late diapause (February), while glutathione and ascorbate were higher in February. Similarly, there was a lower activity of the hexose monophosphate shunt enzymes in February. Exposure of larvae to -12°C resulted in an ele- vation of hexose monophosphate shunt enzyme activity, especially in November. This was accompanied by a significant increase in glycerol content in February. Changes in ascorbate levels and dehydroascorbate reductase activity in both experimental groups (-12°C and 8°C) suggest a connection between the antioxidative system, metabolism during diapause and cold hardiness. Our results support the notion that antioxidative defense in larvae of Ostrinia nubilalis is closely connected with metabolic changes character- istic of diapause, mechanisms of cold hardiness involved in diapause and the maintenance of a stable redox state.

A Thin-Film Electrochromic Device Based on a Polyoxometalate Cluster

The realization of molecular-based switching and display devices faces two entirely different challenges that require a scientific remedy. First, components possessing addressable states with distinct physical properties have to be identified and synthesized. The components must operate reversibly with long-term stability and suitable response times. The stimulus threshold and the power consumption for switching between states ought to be low. Second, one or several active components have to be positioned in a predefined way into the actual device’s structure. An efficient and rational approach towards this goal is to use strategies from supramolecular chemistry. The future fabrication of such materials may, therefore, rely on principles of molecular self-organization. [1] Most likely, thin-film technologies will play a major role in future applications. [2] In terms of possible components for electrochromic devices, polyoxometalates (POMs) are promising candidates due to their ability to act as an electron reservoir, thereby giving rise to colored mixed-valence state species while retaining their structural integrity. [3‐7] In contrast to many semiconductor nanoparticles and quantum dots, POMs are discrete, molecularly defined metal‐oxide clusters with an extensive range of structures and properties. For the realization of such devices it will, therefore, be necessary to identify existing and to synthesize novel POMs with sizeable electrochromic response and stable redox states. [8] Despite the potential of POMs, their implementation in advanced materials has remained elusive, mainly due to the fact that they are obtained as crystalline solids, which are difficult to process. [9] Traditionally, thin films of POMs are made by spin coating, the Langmuir‐Blodgett technique, electrodeposition, or simply by compressing POM solids against an indium tin oxide (ITO)-coated glass slide. [10] More recent approaches involve the use of surfactant-encapsulated POM clusters. [11] A simple

Stoichiometric redox titrations of complex metalloenzymes

This chapter describes stoichiometric redox titration method and points out difficulties and avoidable pitfalls. This method may provide reliable evidence for a spectroscopically silent redox center in an enzyme, as long as the reduction potential of the center is within the potential range probed. Such centers may consume redox equivalents and thus alter the shape of the titration curves of spectroscopically active centers. Spectroscopically observable stable redox states are commonly viewed as being more important than those which cannot be easily observed, but this need not be the case. Thus, the results of stoichiometric titration studies may be critical for formulating the correct mechanism of catalysis. Finally, despite best efforts, some uncertainty in the results and analysis will inevitably remain. This ultimately translates into some persistent uncertainty regarding the redox/spectroscopic system present in the enzyme.

Towards understanding the chemistry of photosynthetic oxygen evolution: dynamic structural changes, redox states and substrate water binding of the Mn cluster in photosystem II

This mini-review summarizes my postdoctoral research in the labs of T. Wydrzynski/C.B. Osmond, J.H.A. Nugent/M.C.W. Evans and V.K. Yachandra/K. Sauer/M.P. Klein. The results are reported in the context of selected data from the literature. Special emphasis is given to the mode of substrate water binding, Mn oxidation states and the structures of the Mn cluster in the four (meta)stable redox states of the oxygen evolving complex. The paper concludes with a working model for the mechanism of photosynthetic water oxidation that combines μ-oxo bridge oxidation in the S 3 state (V.K. Yachandra, K. Sauer, M.P. Klein, Chem. Rev. 96 (1996) 2927–2950) with O-O bond formation between two terminal Mn-hydroxo ligands during the S 3→(S 4)→S 0 transition.

The activity of oxidized bovine spleen purple acid phosphatase is due to an Fe(III)Zn(II) 'impurity'.

Bovine spleen purple acid phosphatase (BSPAP) is a dinuclear iron protein with two stable redox states. The Fe3+Fe2+ state is the active state, while the fully oxidized protein (BSPAPox) has been reported to retain 5-10% activity, corresponding to a kcat of ca. 150 s-1 [Dietrich, M., Münstermann, D., Suerbaum, H., and Witzel, H. (1991) Eur. J. Biochem. 199, 105-113]. Here we show that this activity does not originate from Fe3+Fe3+-BSPAP, but rather from an 'impurity' of FeZn-BSPAP. The FeZn form of BSPAP was prepared from apo-BSPAP following a new procedure, and its kinetic properties were carefully determined for comparison to those of BSPAPox. For the hydrolysis of p-NPP at pH 6.00, both kcat and KM were affected by the Fe2+-to-Zn2+-substitution [Fe3+Fe2+-BSPAP, kcat = (1.8 +/- 0.1) x 10(3) s-1 and KM = 1.2 +/- 0.2 mM; Fe3+Zn2+-BSPAP; kcat = (2.8 +/- 0.2) x 10(3) s-1 and KM = 3.3 +/- 0.4 mM]. The KM of BSPAPox was the same as that of FeZn-BSPAP. pH profiles of BSPAPox and FeZn-BSPAP were both shifted to lower pH compared to that of BSPAPred. FeZn-BSPAP, FeZn-BSPAP.PO4, and FeZn-BSPAP.MoO4 all showed characteristic EPR spectra similar to the corresponding complexes of FeZn-Uf. The same species could also be observed in concentrated samples of native BSPAP. Spin integration of these spectra showed a quantitative relation between the spin concentration of the FeZn-BSPAP 'impurity' and the residual phosphatase activity after oxidation. Since all activity found after oxidation of BSPAP could be attributed to FeZn-BSPAP, there is no direct evidence that Fe3+Fe3+-BSPAP is catalytically active. These results set an upper limit to the possible catalytic activity of the Fe3+Fe3+ form of </=1% of that of the Fe3+Fe2+ form, a finding that is important for understanding the fundamental chemistry by which these dinuclear enzymes catalyze the hydrolysis of phosphate esters.

16] Noninvasive measurement of thiol levels in cellsand isolated organs

Publisher Summary Changes in thiol levels of tissues are sensitive indicators of the metabolic challenges of intoxication with heavy metals and arsenic compounds and of an imbalance between pro- and antioxidants known as “oxidative stress.” A noninvasive analysis of low- and high-molecular-weight thiols in a particular organ or in distinct biological systems cannot be performed with the existing methods that use optical or chromatographic methods, which require the preparation of tissue samples after the destruction of intact functional systems by homogenization, centrifugation, freeze clamping, or other procedures. The biradical method can be used to determine the changes in steady state levels of thiol pools without the need to disrupt the biological system. Thiol–disulfide exchange reactions between low-molecular-weight thiols (GSH) and high-molecular-weight disulfides are important biological processes for maintaining stable tissue thiol redox states. The biradical method is based on a reaction mechanism in which the reactant used is a symmetrical paramagnetic disulfide that rapidly undergoes thiol–disulfide exchange reactions with tissue thiols. The use of this biradical for a noninvasive assessment of thiol levels in any biological system requires its penetration through the phospholipid membranes.