Mild Oxidative Conditions Research Articles

An Easy and Straightforward Oxidative Rearrangement of 1,2-Dialkoxy-Substituted Olefins

1,2-Dialkoxy-substituted olefins, easily available from dihydropyranylcarbene complexes, evolve under mild oxidative conditions to form new functionalized compounds through an oxidative rearrangement process. Depending on the substitution of the starting material, different rearranged final products including ­cyclopentanone, cyclopentenone, spiroketal and polycyclic derivatives are easily available.

Molecular basis for the redox control of nuclear transport of the structural chromatin protein Hmgb1

Oxidative stress can induce a covalent disulfide bond between protein and peptide thiols that is reversible through enzymatic catalysis. This process provides a post-translational mechanism for control of protein function and may also protect thiol groups from irreversible oxidation. High mobility group protein B1 (Hmgb1), a DNA-binding structural chromosomal protein and transcriptional co-activator was identified as a substrate of glutaredoxin. Hmgb1 contains 3 cysteines, Cys23, 45, and 106. In mild oxidative conditions, Cys23 and Cys45 readily form an intramolecular disulfide bridge, whereas Cys106 remains in the reduced form. The disulfide bond between Cys23 and Cys45 is a target of glutathione-dependent reduction by glutaredoxin. Endogenous Hmgb1 as well as GFP-tagged wild-type Hmgb1 co-localize in the nucleus of CHO cells. While replacement of Hmgb1 Cys23 and/or 45 with serines did not affect the nuclear distribution of the mutant proteins, Cys106-to-Ser and triple cysteine mutations impaired nuclear localization of Hmgb1. Our cysteine targeted mutational analysis suggests that Cys23 and 45 induce conformational changes in response to oxidative stress, whereas Cys106 appears to be critical for the nucleocytoplasmic shuttling of Hmgb1.

Ribosomal Synthesis of Dehydroalanine-Containing Peptides

Dehydroalanine is a nonproteinogenic amino acid, but it is a component of a wide variety of natural products with therapeutic activities. Indeed, this alpha,beta-unsaturated residue is a highly versatile building block due to its rigidifying effect on peptide backbones and its electrophilicity which allows site-specific thiol ligations of peptides with small molecules or proteins. To harness such versatility in genetically encoded, combinatorial peptide libraries, we report a simple and robust method for the ribosomal synthesis of dehydroalanine-containing peptides. Selenalysine, a selenium-containing lysine analogue, was recruited as a masked dehydroalanine equivalent. This residue is efficiently incorporated by a reconstituted Escherichia coli translation system at high fidelity and efficiency despite the presence of low levels of lysine. Mild oxidative conditions were used to convert selenalysine into dehydroalanine post-translationally. Using this method, we demonstrate the preparation of polyunsaturated and highly decorated peptides. This report is an important step toward the preparation and selection of large libraries of protein-reactive compounds with potential use as novel drugs or as analytical tools.

Novel Thiophenic Copolymer as a Multi‐Purpose Macromolecular Intermediate

AbstractThe preparation of a new thiophenic dimer partially derivatized with a bromohexylic chain is described. Its polymerization under mild oxidative conditions leads to a polythiophene with a degree of functionalization of 50% and highly soluble in organic solvents. The product has been fully characterized by FT‐IR, 1H and 13C NMR using one and two‐dimensional techniques as well as by SEC and thermal analysis. Solvatochromic effects of the new polymer have been investigated in different mixtures, underlining its self‐assembling capability even in solvated states.

The para‐Siletanylbenzyl (PSB) Ether: A Peroxide‐Cleavable Protecting Group for Alcohols and Phenols.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The para-siletanylbenzyl (PSB) ether: a peroxide-cleavable protecting group for alcohols and phenols

A novel arylmethyl protecting group that is electronically similar to benzyl (Bn) but that can be cleaved under mild oxidizing conditions in the presence of para-methoxybenzyl (PMB) is described herein. para-Siletanylbenzyl (PSB) ethers are formed in one or two steps from the corresponding alcohols and cleaved in one or two steps with basic peroxide. Alcohols and phenols have been protected in good yields and deprotected cleanly under mild oxidative conditions.

Carboamination: Additions of Imine C=N Bonds Across Alkynes Catalyzed by Imidozirconium Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Redox state alteration modulates astrocyte glucuronidation

We have investigated the effects of mild oxidative conditions on drug-metabolizing enzyme activity in rat cultured astrocytes. These experimental conditions promoting an oxidative environment were obtained by short exposure to a low concentration of menadione (5 μM) for a short duration (15 min). This resulted in the rapid and transient production of reactive oxygen species (+130%), associated with a decrease in GSH cellular content (−24%), and an increase in total protein oxidation (+26%), but promoted neither PGE 2 nor NO production. This treatment induced a rapid and persistent decrease in astrocyte glucuronidation activities, which was totally prevented by N-acetyl- l-cysteine. These oxidative conditions also affected the specific UGT1A6 activity measured in transfected V79-1A6 cells. Finally, the subsequent recovery of astrocyte glucuronidation activity may result from upregulation of UGT1A6 expression (+62%) as shown by RT-PCR and gene reporter assay. These results show that the catalytic properties and expression of cerebral UGT1A6 are highly sensitive to the redox environment. The protective effect of N-acetyl- l-cysteine suggests both a direct action of reactive oxygen species on the protein and a more delayed action on the transcriptional regulation of UGT1A6. These results suggest that cerebral metabolism can be altered by physiological or pathological redox modifications.

Inhibition of methyl linoleate autoxidation by phenolics and other related compounds under mild oxidative conditions

AbstractMethyl linoleate (MeLo) is a commercially available substrate widely used for studying the inhibitory properties of pure compounds and plant extracts against lipid oxidation. In this paper, 13 phenolic (benzoic and cinnamic acids, aldehydes and derivatives, and flavonoids) and other related compounds (butylated hydroxyanisole, butylated hydroxytoluene and tocopherols) as well as nine complex mixtures rich in phenolics (wines) or tocopherols (soybean oil extracts) were assayed for their inhibitory activity against MeLo autoxidation under mild conditions (40 °C, darkness and atmospheric pressure). Samples were also assayed for their free radical‐scavenging capacity against 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH•). Protocatechuic aldehyde, a compound that has not been previously evaluated by either of the two methods, showed the highest antioxidant activity. Some variations in the antioxidant ranking for some compounds were found between our results and those obtained by other authors using accelerated conditions for MeLo oxidation. Antioxidant activity of wines and soybean oil extracts was related to their richness in phenolics and tocopherols respectively. Correlation between antioxidant capacity measured by the MeLo and DPPH• methods was found for wines but not for the other samples studied. Therefore the measure of the free radical‐scavenging capacity of a compound is not always a reliable indicator of its lipid oxidation inhibitory ability. Copyright © 2004 Society of Chemical Industry

Fluorogenic peptide sequences--transformation of short peptides into fluorophores under ambient photooxidative conditions.

Long-lived proteins are susceptible to nonenzymatic chemical reactions and the evolution of fluorescence; however, little is known about the sequence-dependence of fluorogenesis. We synthesized a library of over half a million octapeptides and exposed it to light and air in pH 7.4 buffer to identify fluorogenic peptides that evolve under mild oxidative conditions. The bead-based peptide library was composed of the general sequence H(2)N-Ala-(Xxx)(6)-Ala-resin, where Xxx was one of nine representative amino acids: Asp, Gly, His, Leu, Lys, Pro, Ser, Trp, and Tyr. Next, we selected five highly fluorescent beads from the library and subjected them to microsequencing, revealing the sequence of the unreacted peptide. All five of the fluorogenic sequences were ionic; lacked Tyr, His, and Leu; and most of the sequences contained only one Trp. We then synthesized the five soluble peptides corresponding to the fluorogenic peptide sequences and exposed them to photooxidative conditions. In general, the soluble peptides reacted slowly, generating nonfluorescent monooxygenated and dioxygenated products. However, one peptide (H(2)N-AlaLysProTrpGlyGlyAspAla-CONH(2)) evolved into a highly fluorescent photoproduct as well as a nonfluorescent monooxygenated photoproduct. The fluorescent photoproduct consisted of a 2-carboxy-quinolin-4-yl moiety fused to the N-terminus of GlyGlyAspAla. The formation of this photoproduct requires cleavage of the peptide backbone and a dramatic reorganization of tryptophan. This work demonstrates that sequencing unreacted peptide on beads can reveal sequences with unique nonenzymatic reactivity. The study also confirms that peptide fluorogenesis is dependent on sequence and not merely on the presence of tryptophan. The potential importance of fluorogenic peptide sequences is two-fold. First, fluorogenic sequences that arise through mutation could prove to be hot spots for human aging. Second, fluorogenic sequences, particularly those compatible with intracellular conditions, may serve as fluorescent tags for proteins or as fluorescent biomaterials.

First demonstration of differential inhibition of lectin binding by synthetic tri- and tetravalent glycoclusters from cross-coupling of rigidified 2-propynyl lactoside.

The interplay of mammalian lectins such as galectins with cellular glycoconjugates is intimately involved in crucial reaction pathways including tumor cell adhesion, migration or growth regulation. These clinically relevant functions explain the interest in designing glycoclusters with potent activity to interfere with lectin binding. In view of the perspective for medical applications the following objective arises: to correlate topological factors of ligand display most favorably to reactivity against endogenous lectins. To date, plant agglutinins have commonly been used as models. Properly addressing this issue we first prepared di- to tetravalent clusters from 2-propynyl lactoside under mild oxidative homocoupling conditions and using the Sonogashira palladium-catalyzed cross-coupling reaction with triiodobenzene or pentaerythritol cores. These products were tested for bioactivity in a competitive solid-phase assay using different labeled sugar receptors as probes, i,e. the beta-trefoil mistletoe lectin, the natural lactoside-binding immunoglobulin G fraction from human serum and three mammalian galectins from two subgroups. The lactose headgroups in the derivatives retained ligand properties. Differences in inhibitory capacity were marked between the galectins. In contrast to homodimeric proto-type galectins-1 and -7 significant inhibition of galectin-3 binding with a 7-fold increase in relative potency was observed for the trivalent compound. In comparison, the binding of the beta-trefoil mistletoe agglutinin was reduced best by tetravalent substances The result for galectin-3 was independently confirmed by haemagglutination and cytofluorometric cell binding assays. These data underline the feasibility of galectin-type target selectivity by compound design despite using an identical headgroup (lactose) in synthesis.

A new protecting group for 5'-hydroxyl function of nucleotides in oligonucleotide synthesis without acid treatment utilizing unique properties of tritylthio group.

New protecting groups having a 2-aminomethylbezoyl skeleton, in which the reactive amino functions were blocked further by a tritylthio-type protecting group, were developed for the protection of the 5'-hydroxyl function of nucleosides during the oligonucleotide synthesis. These benzoate-type protecting groups were designed to be removed via an intramolecular cyclization following the removal of the tritylthio-type protecting group under mild oxidative conditions using diluted aqueous iodine solution. The new protecting groups would enable us to synthesize oligonucleotides without using any acid treatment. Furthermore, they can reduce the reaction steps required for a chain elongation cycle from four to three because the deprotection of the 5'-hydroxyl protecting group and the oxidation of the phosphite intermediate can be performed simultaneously by a single aqueous iodine treatment.

Thioredoxin-mediated reductive activation of a protein kinase for the regulatory phosphorylation of C4-form phosphoenolpyruvate carboxylase from maize.

The activity of phosphoenolpyruvate carboxylase (PEPC, EC4.1.1.31) for the C4 photosynthesis is known to be regulated mainly in response to light/dark transitions through reversible phosphorylation by a specific protein kinase (PK). PEPC-PK with an M(r) of 30 kDa was purified about 1.4 million-fold to homogeneity from maize leaves and characterized. The purified PEPC-PK was readily inactivated under mild oxidative conditions, but the activity could be recovered by dithiothreitol (DTT). The recovery by DTT was strongly accelerated by thioredoxin (Trx) from E. coli. Trxs of plant origin such as Trx-m from spinach chloroplast and Trx-h from rice cytoplasm were also effective. These results suggest the possibility of PEPC-PK being redox-regulated via Trx in vivo.

Solid-phase synthesis of dehydropeptide, AM-toxin II, using a novel selenyl linker by side-chain tethered strategy

The cyclic dehydrodepsipeptide, AM-toxin II, was efficiently synthesized by a solid-phase method using a novel selenyl linker. The cleavage from the resin with formation of a double bond is successfully achieved under mild oxidative conditions with TBHP.

A New Synthesis of Pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-diones by Oxidative N-N Bond Formation of 6-Amino-5-(N-aryliminomethyl)uracils Using Iodobenzene Diacetate

The intramolecular cyclizations of 6-amino-5-(N-aryliminomethyl)-1,3-dimethyluracils (2) involving the N-N bond formation were effected via a hypervalent iodine oxidation using iodobenzene diacetate. This method enabled a facile synthesis of 2-aryl-5,7-dimethylpyrazolo[3,4-d]pyrirnidine-4,6(5H,7H)-diones (3) in moderate to excellent yields. The primary advantage of this N-N bond formation method is that various 2-aryl-substituted pyrazolo[3,4d]pyrimidines can be provided under mild oxidative conditions.

Regeneration of Carbonyl Compounds by Oxidative Cleavage of Hydrazones with Zr(O-t-Bu)4/tert-Butyl Hydroperoxide

A catalytic method with Zr(O-t-Bu)4 as the catalyst and tert-butyl hydroperoxide as the oxidant is described to cleave phenylhydrazones 6—9, 14—16 to the parent carbonyl compounds under mild and neutral oxidative conditions.

Sulfur Removal from Gediz Lignite Using Aqueous Sodium Hydroxide Solutions Under Mild Oxidative Conditions

Sulfur removal from a high-sulfur Turkish lignite (Gediz) using aqueous sodium hydroxide solutions having dissolved oxygen was investigated under mild oxidative conditions. Effects of the parameters such as sodium hydroxide lignite weight ratio, temperature, and partial pressure of oxygen were investigated within the ranges of 0.05 - 0.8, 423 - 498 K, and 1 - 2 MPa, respectively. Optimum values of these parameters were determined regarding sulfur removal and coal recovery. Influences of dry oxidation of the lignite sample as a pretreatment at 573 K and subsequent washing of some treated lignite samples with 1 N HCl were investigated.

Hydrogen peroxide produced during gamma-glutamyl transpeptidase activity is involved in prevention of apoptosis and maintainance of proliferation in U937 cells.

It has been reported in several cell lines that exposure to low levels of reactive oxygen species can exert a stimulatory effect on their proliferation. We have previously shown that mild oxidative conditions can also counteract apoptotic stimuli. A constitutive cellular production of low levels of superoxide and hydrogen peroxide originates from various sources; among these, gamma-glutamyl transpeptidase (GGT), the plasma membrane-bound activity in charge of metabolizing extracellular reduced glutathione, has recently been included. Since the inhibition of GGT is a sufficient stimulus for the induction of apoptosis in selected cell lines, we investigated whether this effect might result from the suppression of the mentioned GGT-dependent prooxidant reactions, on the theory that the latter may represent a basal antiapoptotic and proliferative signal for the cell. Experiments showed that: 1) GGT activity in U937 monoblastoid cells is associated with the production of low levels of hydrogen peroxide, and two independent GGT inhibitors cause a dose-dependent decrease of such GGT-dependent production of H2O2; 2) GGT inhibition with acivicin results in cell growth arrest, and induces cell death and DNA fragmentation with the ladder appearance of apoptosis; 3) treatment of cells with catalase--and even more with Trolox C--is able to decrease their proliferative rate; 4) GGT inhibition (with suppression of H2O2 production) results in a down-regulation of poly(ADP-ribose) polimerase (PARP) activity, which precedes the proteolytic cleavage of PARP molecule, such as that typically induced by caspases. The reported data suggest that the low H2O2 levels originating as a by-product during GGT activity are able to act as sort of a 'life signal' in U937 cells, insofar as they can maintain cell proliferation and protect against apoptosis, possibly through an up-regulation of PARP activity.

Preparation of Novel (Fluoroaryl)silanes and Their Applications in the Organoyttrium-Catalyzed Hydrosilylation of Alkenes and Oxidation to Alcohols

The synthesis of novel (fluoroaryl)silanes from the respective aryl Grignard reagents and silyl triflates is reported. The new silanes were used in the hydrosilylation of alkenes catalyzed by the organoyttrium complex Cp*2YMe·THF (THF = tetrahydrofuran). (Fluoroaryl)silanes were found to react faster than phenylsilane and in good to near quantitative yields. The resulting [(fluoroaryl)silyl]alkane products were found to be labile under very mild oxidative conditions, with both disubstituted alkenes and silyl-protected alcohol functionalities tolerating the oxidation.

Aryl hydrazides as linkers for solid phase synthesis which are cleavable under mild oxidative conditions

We have developed and acid/base stable aryl hydrazide linker which is readily coupled to solid phase resins. Cleavage is specific and facile, requiring a copper (II) catalyst, base and a nucleophile to proceed. The conditions are compatible with all 20 proteinogenic amino acids and quantitative cleavage is achieved within 2 hours at 20°C to give peptides with C-terminal acid, amide or ester functionalities. Aryl hydrazides also offer scope as simple “traceless” linkers.