Electrochemical behaviour of different redox probes on single wall carbon nanotube buckypaper-modified electrodes

In the present work, the electrochemical properties of single-walled carbon nanotube buckypapers (BPs) were examined in terms of carbon nanotubes nature and preparation conditions. The performance of the different free-standing single wall carbon nanotube sheets was evaluated via cyclic voltammetry of several redox probes in aqueous electrolyte. Significant differences are observed in the electron transfer kinetics of the buckypaper-modified electrodes for both the outer- and inner-sphere redox systems. These differences can be ascribed to the nature of the carbon nanotubes (nanotube diameter, chirality and aspect ratio), surface oxidation degree and type of functionalities. In the case of dopamine, ferrocene/ferrocenium, and quinone/hydroquinone redox systems the voltammetric response should be thought as a complex contribution of different tips and sidewall domains which act as mediators for the electron transfer between the adsorbate species and the molecules in solution. In the other redox systems only nanotube ends are active sites for the electron transfer. It is also interesting to point out that a higher electroactive surface area not always lead to an improvement in the electron transfer rate of various redox systems. In addition, the current densities produced by the redox reactions studied here are high enough to ensure a proper electrochemical signal, which enables the use of BPs in sensing devices.

Enzyme mediated synthesis of polypyrrole in the presence of chondroitin sulfate and redox mediators of natural origin

Polypyrrole (PPy) was synthesized by enzyme mediated oxidation of pyrrole using naturally occurring compounds as redox mediators. The catalytic mechanism is an enzymatic cascade reaction in which hydrogen peroxide is the oxidizer and soybean peroxidase, in the presence of acetosyringone, syringaldehyde or vanillin, acts as a natural catalysts. The effect of the initial reaction composition on the polymerization yield and electrical conductivity of PPy was analyzed. Morphology of the PPy particles was studied by scanning electron microscopy and transmission electron microscopy whereas the chemical structure was studied by X-ray photoelectron and Fourier transformed infrared spectroscopic techniques. The redox mediators increased the polymerization yield without a significant modification of the electronic structure of PPy. The highest conductivity of PPy was reached when chondroitin sulfate was used simultaneously as dopant and template during pyrrole polymerization. Electroactive properties of PPy obtained from natural precursors were successfully used in the amperometric quantification of uric acid concentrations. PPy increases the amperometric sensitivity of carbon nanotube screen-printed electrodes toward uric acid detection.

Total organic carbon content and redox-sensitiv trace metal concentrations of selected late Cenoman claystones from DSDP Hole 93-603B

The Cenomanian/Turonian (C/T) intervals at DSDP Sites 105 and 603B from the northern part of the proto-North Atlantic show high amplitude, short-term cyclic variations in total organic carbon (TOC) content. The more pronounced changes in TOC are also reflected by changes in lithology from green claystones (TOC<1%) to black claystones (TOC>1%). Although their depositional history was different, the individual TOC cycles at Sites 105 and 603B can be correlated using stable carbon isotope stratigraphy. Sedimentation rates obtained from the isotope stratigraphy and spectral analyses indicate that these cycles were predominately precession controlled. The coinciding variations in HI, OI, delta13Corg and the abundance of marine relative to terrestrial biomarkers, as well as the low abundance of lignin pyrolysis products generated from the kerogen of the black claystones, indicate that these cyclic variations reflect changes in the contribution of marine organic matter (OM). The cooccurrence of lamination, enrichment of redox-sensitive trace metals and presence of molecular fossils of pigments from green sulfur bacteria indicate that the northern proto-North Atlantic Ocean water column was periodically euxinic from the bottom to at least the base of the photic zone (<150 m) during the deposition of the black claystones. In contrast, the green claystones are bioturbated, are enriched in Mn, do not show enrichments in redox-sensitive trace metals and show biomarker distributions indicative of long oxygen exposure times, indicating more oxic water conditions. At the same time, there is evidence (e.g., abundance of biogenic silica and significant 13C-enrichment for OC of phytoplanktic origin) for enhanced primary productivity during the deposition of the black claystones. We propose that increased primary productivity periodically overwhelmed the oxic OM remineralisation potential of the bottom waters resulting in the deposition of OM-rich black claystones. Because the amount of oxygen used for OM remineralisation exceeded the amount supplied by diffusion and deep-water circulation, the northern proto-North Atlantic became euxinic during these periods. Both Sites 105 and 603B show trends of continually increasing TOC contents and HI values of the black claystones up section, which most likely resulted from both enhanced preservation due to increased anoxia and increased production of marine OM during oceanic anoxic event 2 (OAE2).

Measurement of redox potential and determination of ferrous iron in ground water /

Includes bibliographical references (p. 100-103).

The gamma analytical system at Redox /

"Work performed under Contract No. AT(45-1)-1350 between the Atomic Energy Commission and General Electric Company."

Cambial meristem dormancy in trees involves extensive remodelling of the transcriptome

The establishment of the dormant state in meristems involves considerable physiological and metabolic alterations necessary for surviving unfavourable growth conditions. However, a global molecular analysis of dormancy in meristems has been hampered by the difficulty in isolating meristem cells. We used cryosectioning to isolate purified cambial meristem cells from the woody plant Populus tremula during active growth and dormancy. These samples were used to generate meristem-specific cDNA libraries and for cDNA microarray experiments to define the global transcriptional changes underlying cambial dormancy. The results indicate a significant reduction in the complexity of the cambial transcriptome in the dormant state. Although cell division is terminated in the dormant cambium, the cell cycle machinery appears to be maintained in a skeletal state as suggested by the continued presence of transcripts for several cell cycle regulators. The downregulation of PttPIN1 and PttPIN2 transcripts explains the reduced basipetal polar auxin transport during dormancy. The induction of a member of the SINA family of ubiquitin ligases implicated in auxin signalling indicates a potential mechanism for modulation of auxin sensitivity during cambial dormancy. The metabolic alterations during dormancy are mirrored in the induction of genes involved in starch breakdown and the glyoxysomal cycle. Interestingly, the induction of RGA1 like gene suggests modification of gibberellin signalling in cambial dormancy. The induction of genes such as poplar orthologues of FIE and HAP2 indicates a potential role for these global regulators of transcription in orchestrating extensive changes in gene expression during dormancy.

Effects of revascularisation and contractile reserve on left ventricular remodelling in patients with impaired left ventricular function

Objective: We sought to define the influence of revascularisation and contractile reserve on left ventricular (LV) remodelling in patients with LV dysfunction after myocardial infarction. Revascularisation of viable myocardium is associated with improved regional function, but the effect on remodelling is undefined. Methods: We studied 70 patients with coronary artery disease and LV dysfunction, 31 of whom underwent revascularisation. A standard dobutamine stress echocardiogram (DbE) was carried out. All patients underwent standard medical treatment; the decision to revascularise was made clinically, independent of this study. LV volumes and ejection fraction were measured by 3D echocardiography at baseline and after an average of 40 weeks. Results: There was no significant difference in baseline ejection fraction or volumes between patients who underwent revascularisation and the remainder. Compared to medically treated patients, revascularised patients had significant improvements in ejection fraction and end-systolic volume in follow-up. The impact of baseline variables on remodelling was assessed by dividing patients into tertiles of LV ejection fraction and volumes. Revascularised patients in the lowest tertile of ejection fraction at baseline (<38%) had a significant improvement in end-systolic volume and ejection fraction, larger than obtained in medically treated patients with low ejection fraction. Revascularised patients with an ejection fraction >38% did not show significant improvement in volumes compared to baseline. Revascularised patients in the largest tertiles of end-systolic (>88 ml) or end-diastolic volume (>149 ml) at baseline had a significant improvement in end-systolic volume. Conclusion: Remodeling appears to occur independent of the presence of regional contractile reserve but does correlate with the volume response to low-dose dobutamine. (C) 2003 Elsevier Ireland Ltd. All rights reserved.

A numerical approach to modeling the catalytic voltammetry of surface-confined redox enzymes

A finite difference method for simulating voltammograms of electrochemically driven enzyme catalysis is presented. The method enables any enzyme mechanism to be simulated. The finite difference equations can be represented as a matrix equation containing a nonlinear sparse matrix. This equation has been solved using the software package Mathematica. Our focus is on the use of cyclic voltammetry since this is the most commonly employed electrochemical method used to elucidate mechanisms. The use of cyclic voltammetry to obtain data from systems obeying Michaelis-Menten kinetics is discussed, and we then verify our observations on the Michaelis-Menten system using the finite difference simulation. Finally, we demonstrate how the method can be used to obtain mechanistic information on a real redox enzyme system, the complex bacterial molybdoenzyme xanthine dehydrogenase.

The acidic nature of the CcmG redox-active center is important for cytochrome c maturation in Escherichia coli

Cytochrome c biogenesis in Escherichia coli is a complex process requiring at least eight genes (ccmABC DEFGH). One of these genes, ccmG, encodes a thioredoxin-like protein with unusually specific redox activity. Here, we investigate the basis for CcmG function and demonstrate the importance of acidic residues surrounding the redox-active center.

Cyclosporine A-induced changes to erythrocyte redox balance is time course-dependent

Cyclosporine A-treated transplant recipients develop pronounced cardiovascular disease and have increased oxidative stress and altered antioxidant capacity in erythrocytes and plasma. These experiments investigated the time-course of cyclosporine A-induced changes to redox balance in plasma and erythrocytes. Rats were randomly assigned to either a control or cyclosporine A-treated group. Treatment animals received 25 mg/kg of cyclosporine A via intraperitoneal injection for either 7 days or a single dose. Control rats were injected with the same volume of the vehicle. Three hours after the final injections, plasma was analysed for total antioxidant status, a-tocopherol, malondialdehyde, and creatinine. Erythrocytes were analysed for reduced glutathione (GSH), alpha-tocopherol, methaemoglobin, malondialdehyde, and the activities of superoxide dismutase, catalase, GSH peroxidase, and glucose-6-phosphate dehydrogenase (G6PD). Cyclosporine A administration for 7 days resulted in a significant increase (P < 0.05) in plasma malondialdehyde, methaemoglobin, and superoxide dismutase and catalase activities. There was a significant decrease (P < 0.05) in erythrocyte GSH concentration and G6PD activity in cyclosporine A animals. There were no significant differences (P > 0.05) between groups following a single dose of cyclosporine A in any of the measures. In summary, cyclosporine A alters erythrocyte redox balance after 7 days administration, but not after a single dose.

NmlR of Neisseria gonorrhoeae: a novel redox responsive transcription factor from the MerR family

A MerR-like regulator (NmlR -Neisseria merR-like Regulator) identified in the Neisseria gonorrhoeae genome lacks the conserved cysteines known to bind metal ions in characterized proteins of this family. Phylogenetic analysis indicates that NmlR defines a subfamily of MerR-like transcription factors with a distinctive pattern of conserved cysteines within their primary structure. NmlR regulates itself and three other genes in N. gonorrhoeae encoding a glutathione-dependent dehydrogenase (AdhC), a CPx-type ATPase (CopA) and a thioredoxin reductase (TrxB). An nmlR mutant lacked the ability to survive oxidative stress induced by diamide and cumene hydroperoxide. It also had > 50-fold lower NADH-S-nitrosoglutathione oxidoreductase activity consistent with a role for AdhC in protection against nitric oxide stress. The upstream sequences of the NmlR regulated genes contained typical MerR-like operator/promoter arrangements consisting of a dyad symmetry located between the -35 and -10 elements of the target genes. The NmlR target operator/promoters were cloned into a beta-galactosidase reporter system and promoter activity was repressed by the introduction of NmlR in trans. Promoter activity was activated by NmlR in the presence of diamide. Under metal depleted conditions NmlR did not repress P-AdhC (or P-CopA) promoter activity, but this was reversed in the presence of Zn(II), indicating repression was Zn(II)-dependent. Analysis of mutated promoters lacking the dyad symmetry revealed constitutive promoter activity which was independent of NmlR. Gel shift assays further confirmed that NmlR bound to the target promoters possessing the dyad symmetry. Site-directed mutagenesis of the four NmlR cysteine residues revealed that they were essential for activation of gene expression by NmlR.

Tuning the photophysical behavior of luminescent cyclam derivatives by cation binding and excited state redox potential

The emission from two photoactive 14-membered macrocyclic ligands, 6-((naphthalen-1-ylmethyl)-amino)trans-6,13-dimethyl- 13-amino- 1,4,8,11 -tetraaza-cyclotetradecane (L-1) and 6-((anthracen-9-ylmethyl)-amino)trans-6,13 -dimethyl - 13 -amino- 1,4,8, 1 1-tetraaza-cyclotetradecane (L-2) is strongly quenched by a photoinduced electron transfer (PET) mechanism involving amine lone pairs as electron donors. Time-correlated single photon counting (TCSPC), multiplex transient grating (TG), and fluorescence upconversion (FU) measurements were performed to characterize this quenching mechanism. Upon complexation with the redox inactive metal ion, Zn(II), the emission of the ligands is dramatically altered, with a significant increase in the fluorescence quantum yields due to coordination-induced deactivation of the macrocyclic amine lone pair electron donors. For [ZnL2](2+), the substituted exocyclic amine nitrogen, which is not coordinated to the metal ion, does not quench the fluorescence due to an inductive effect of the proximal divalent metal ion that raises the ionization potential. However, for [ZnL1](2+), the naphthalene chromophore is a sufficiently strong excited-state oxidant for PET quenching to occur.

Dipyridyl thiosemicarbazone chelators with potent and selective antitumor activity form iron complexes with redox activity

There has been much interest in the development of iron (Fe) chelators for the treatment of cancer. We developed a series of di-2-pyridyl ketone thiosemicarbazone (HDpT) ligands which show marked and selective antitumor activity in vitro and in vivo. In this study, we assessed chemical and biological properties of these ligands and their Fe complexes in order to understand their marked activity. This included examination of their solution chemistry, electrochemistry, ability to mediate redox reactions, and antiproliferative activity against tumor cells. The higher antiproliferative efficacy of the HDpT series of chelators relative to the related di-2-pyridyl ketone isonicotinoyl hydrazone (HPKIH) analogues can be ascribed, in part, to the redox potentials of their Fe complexes which lead to the generation of reactive oxygen species. The most effective HDpT ligands as antiproliferative agents possess considerable lipophilicity and were shown to be charge neutral at physiological pH, allowing access to intracellular Fe pools.