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PubMed1975

Synthesis and conformational studies in solution of sequential copolypeptides: poly(L-prolyl-L-alpha-phenylglycyl-L-proline).

High molecular weight poly(L-porlyl-L-alpha-phenylglycyl-L-proline) was synthesized by condensation of L-prolyl-L-alpha-phenylglycyl-L-proline pentachlorophenyl ester in N,N-dimethylformamide. The solution properties of this material have been investigated by circular dichroism and 220-MHz nuclear magnetic resonance spectroscopy in solvents such as trifluoroethanol (F3EtOH), chloroform, trifluoroacetic acid (F3CCOOH), chloroform-F3CCOOH and F3EtOH-F3CCOOH mixtures. Ordered structures can be formed in F3EtOH and chloroform, which we believe to be similar to the poly-L-proline II form. The ordered form easily undergoes a conformational transition to an unordered form upon addition of small amounts of F3CCOOH to the chloroform solution. The ordered structure is not disrupted upon addition of 12 per cent F3CCOOH to a F3EtOH solution of the polypeptide.

Chloroform↗
PubMed1975

The kinetics of flavine oxidation-reduction. II. Metal ion interactions.

The oxidation-reduction reactions of tetraacetylriboflavine in the presence of various metal ions in dimethylformamide have been investigated using the stopped-flow technique under anaerobic conditions. Dismutation kinetics in the presence of redox-inactive dissociated divalent metal ions such as Cd2+, Zn2+, and Fe2+ are typically triphasic. Metal ions act primarily upon an intermediate flavine dimer formed by fast association of flavoquinone and flavohydroquinone, resulting in a parallel formation and neutral and chelated radicals. A competition between metal ions and proton donors, e.g. the neutral flavohydroquinone (FredH3), is observed at the level of this intermediate complex. Small spectral changes occur secondarily as an ill-defined intermediate phase which could correspond to the reorganization of the solvation of radical chelate. The neutral radical is finally chelated at a much slower rate, the yield of total radical formation remaining almost unchanged during this kinetic phase. The oxidation of flavohydroquinone by ferric ions, either dissociated or strongly coordinated within a porphyrin, is complete and proceeds through biphasic kinetics. The first phase (Fred leads to F) is much faster than the second one (F leads to Fox). Dismutation resulting from the transient accumulation of neutral flavosemiquinone competes with the direct oxidation with ferric ions for the completion of the second oxidation step. The relative rate of dismutation is essentially limited by acidic-basic reactions in the absence of an excess of ferrous ion. The kinetic analysis of the direct oxidation reactions favors an outer-sphere mechanism for the electron transfer to the ferric ion, either free or strongly coordinated. The formation of a ferrous radical chelate can result from the dismutation reactions only when the amount of ferric ion initially present is not sufficient for complete oxidation.

Cadmium↗
PubMed2005

Synthesis and properties of the chromophore of the asFP595 chromoprotein from Anemonia sulcata.

A model compound for the chromophore within the purple nonfluorescent GFP-like chromoprotein asFP595 was synthesized. The postulated structure of the chromophore, 2-acetyl-4-(p-hydroxybenzylidene)-1-methyl-5-imidazolone, was taken from the high-resolution crystal structure analysis of intact asFP595 [Quillin, M. L., Anstrom, D., Shu, X., O'Leary, S., Kallio, K., Lukyanov, K. A., and Remington, S. J. (2005) Kindling Fluorescent Protein from Anemonia sulcata: Dark-State Structure at 1.38 A Resolution, Biochemistry 44, 5774-5787]. Erlenmeyer lactonization and oxidation of the methylene group attached to the heteroaromatic moiety with selenium dioxide were used at the key stages of the synthesis. The spectral properties of the model chromophore in solution and their dependence on the pH and polarity of the solvent were investigated. In water, the chromophore was found to exist in two forms, neutral and anionic, with a pK(a) of 7.1. In a dimethylformamide solution, the spectral properties of the anionic form closely match those of the native protein, demonstrating that under these conditions, the compound is an excellent model for the chromophore within native asFP595.

Animals↗
PubMed2005

Redox potentials of chlorophylls in the photosystem II reaction center.

Water oxidation generating atmospheric oxygen occurs in photosystem II (PSII), a large protein-pigment complex located in the thylakoid membrane. The recent crystal structures at 3.2 and 3.5 A resolutions provide novel details on amino acid side chains, especially in the D1/D2 subunits. We calculated the redox potentials for one-electron oxidation of the chlorophyll a (Chla) molecules in PSII, considering the protein environment in atomic detail. The calculated redox potentials for the dimer Chla (P(D1/D2)) and accessory Chla (Chl(D1/D2)) were 1.11-1.30 V relative to the normal hydrogen electrode at pH 7, which is high enough for water oxidation. The D1/D2 proteins and their cofactors contribute approximately 390 mV to the enormous upshift of 470 mV compared to the redox potential of monomeric Chla in dimethylformamide. The other subunits are responsible for the remaining 80 mV. The high redox potentials of the two accessory Chla Chl(D1/D2) suggests that they also participate in the charge separation process.

Chlorophyll↗
PubMed2000

Horseradish peroxidase mediated free radical polymerization of methyl methacrylate.

This paper reports the free radical polymerization of methyl methacrylate (MMA) catalyzed by horseradish peroxidase (HRP). A novel method was developed whereby MMA polymerization can be carried out at ambient temperatures in the presence of low concentrations of hydrogen peroxide and 2,4-pentanedione in a mixture of water and a water-miscible solvent. Polymers of MMA formed were highly stereoregular with predominantly syndiotactic sequences (syn-dyad fractions from 0.82 to 0.87). Analyses of the chloroform-soluble fraction of syndio-PMMA products by GPC showed that they have number-average molecular weights, Mn, that range from 7500 to 75,000. By using 25% v/v of the cosolvents dioxane, tetrahydrofuran, acetone, and dimethylformamide, 85, 45, 7 and 2% product yields, respectively, resulted after 24 h. Increasing the proportion of dioxane to water from 1:3 to 1:1 and 3:1 resulted in a decrease in polymer yield from 45 to 38 and 7%, respectively. Increase in the enzyme concentration from 70 to 80 and 90 mg/mL resulted in increased reaction kinetics. By adjustment of the molar ratio of 2,4-pentanedione to hydrogen peroxide between 1.30:1.0 and 1.45:1.0, the product yields and Mn values were increased. On the basis of the catalytic properties of HRP and studies herein, we believe that the keto-enoxy radicals from 2,4-pentanedione are the first radical species generated. Then, initiation may take place through this radical or by the radical transfer to another molecule.

Free Radicals↗
PubMedDate not supplied

Chemoselective protection of the amino groups of chitosan by controlled phthaloylation: facile preparation of a precursor useful for chemical modifications.

A simple and convenient procedure for chemoselectively protecting the amino groups of chitosan has been developed to provide N-phthaloyl-chitosan that is indispensable as a soluble N-protected precursor for further controlled modification reactions of chitosan. Although the conventional N-phthaloylation of chitosan in N,N-dimethylformamide was accompanied by partial phthaloylation of the hydroxy groups, the addition of a small amount of hydroxy-containing compounds effectively suppressed the O-phthaloylation. Of some compounds examined, water proved particularly suitable, resulting in the formation of chemoselectively N-phthaloylated chitosan without any appreciable O-phthaloyl groups. The resulting N-phthaloyl-chitosan was found to be crystalline despite the presence of a bulky substituent. A solubility test indicated that N-phthaloyl-chitosan exhibited considerable affinity for organic solvents.

Acetylation↗
PubMedDate not supplied

Enzymatic polymerization of tyrosine derivatives. Peroxidase- and protease-catalyzed synthesis of poly(tyrosine)s with different structures.

Polymerization of tyrosine derivatives has been carried out by using two enzymes, peroxidase and protease, as catalyst to give poly(tyrosine)s with different structures. Tyrosine ester hydrochlorides were oxidatively polymerized by a peroxidase in a buffer. Using a high buffer concentration produced the polymer in good yields. The resulting polymer was soluble in N,N-dimethylformamide, dimethyl sulfoxide, and methanol but was insoluble in acetone, tetrahydrofuran, and water. The ester moiety of the polymer was subjected to the alkaline hydrolysis, yielding a water-soluble polymer having the amino acid group in the side chain. The peroxidase also catalyzed the oxidative polymerization of N-acetyltyrosine to give the polymer soluble in water. The polymerization of tyrosine ester hydrochlorides proceeded in the presence of papain catalyst to give a polymer of alpha-peptide structure. The polymerization in the buffer of high phosphate concentration efficiently produced the polymer. On the other hand, the polymer formation was not observed in the low buffer concentration. The molecular weight was several thousands and almost constant during the reaction. The morphology of the precipitated polymer was examined. The product of the initial reaction stage was amorphous. After 24 h, the precipitates exhibiting clear birefringence were formed. Scanning electron microscopy observation of the polymer after 72 h showed the formation of a globular crystal in a diameter larger than 50 microm, which was not found by recrystallization of poly(tyrosine).

Biocompatible Materials↗
PubMedDate not supplied

Synthesis, characterization, and mesophase formation of phenylacetoxy cellulose and its halogenated derivatives.

A series of phenylacetoxy cellulosics with degrees of substitution (DS) between 1.4 and 3.0 and different halogenation (2-chloro, 3-chloro, 4-chloro, 2,4-dichloro, 3,4-dichloro, and 4-bromo) were synthesized. All the prepared phenylacetoxy cellulosics were soluble in dimethylformamide (DMF) and DMAc. The solubility increased with increasing DS. Mesophases were observed for all of the phenylacetoxy cellulosics with low to medium DS (DS < 2.5) in DMF and DMAc. Non- or mono-halogeneated phenylacetoxy cellulosics with high DS (DS > 1.9) were soluble in methylene chloride (CH2Cl2), whereas those with very low DS or di-halogenation on the phenyl ring were only slightly swollen or partially soluble in CH2Cl2. Non- and mono-halogenated phenylacetoxy cellulosics were soluble in DMSO and formed liquid crystals regardless of the DS, in contrast to CH2Cl2 solutions which display liquid crystalline behavior at medium to high DS (DS > 1.9) only. The solubility of the di-halogenated phenylacetoxy cellulosics in DMSO was limited to approximately 40 wt %.

Cellulose↗
PubMedDate not supplied

Synthesis, characterization, and antibacterial activities of novel methacrylate polymers containing norfloxacin.

A novel methacrylate monomer containing a quinolone moiety was synthesized and homopolymerized in N,N-dimethylformamide (DMF) by using azobisisobutyronitrile (AIBN) as an initiator. The new monomer was copolymerized with poly(ethylene glycol) methyl ether methacrylate (MPEGMA) in DMF using the same initiator. The monomer, homopolymer, and copolymer were characterized by elemental analysis, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), size exclusion chromatography (SEC), FTIR, (13)C NMR, and (1)H NMR. The antibacterial activities of the monomer as well as polymers were investigated against Staphylococcus aureus and Escherichia coli, which are representative of Gram-positive and Gram-negative bacteria, respectively. All compounds showed excellent antibacterial activities against these two types of bacteria. The antibacterial activities were determined using the shaking flask method, where 25 mg/mL concentrations of each compound were tested against 10(5) CFU/mL bacteria solutions. The number of viable bacteria was calculated by using the spread plate method, where 100 microL of the incubated antibacterial agent in bacteria solutions were spread on agar plates and the number of viable bacteria was counted after 24 h of incubation period at 37 degrees C.

Anti-Bacterial Agents↗
PubMedDate not supplied

Hydrophilic polymer supports for solid-phase synthesis: preparation of poly(ethylene glycol) methacrylate polymer beads using "classical" suspension polymerization in aqueous medium and their application in the solid-phase synthesis of hydantoins.

Lightly cross-linked hydrophilic polymer beads representing new types of supports for solid-phase synthesis have been prepared from commercial oligoethylene glycol monomethacrylates using an aqueous suspension polymerization process and specifically designed polymerization mixtures. These beads swell extensively in solvents with a wide range of polarities from dichloromethane, tetrahydrofuran, and water to dimethylformamide, and they enable high functional loadings of 1.2-1.8 mmol g(-1). Their ability to serve as supports was demonstrated in the model solid-phase synthesis of a small library of hydantoins. This four-step synthesis using primary amines readily affords yields of over 70%.

Amines↗
PubMedDate not supplied

Comparative resin kinetics using in situ fluorescence measurements.

The aminolysis of a novel activated ester resin was utilized for kinetic study via continuous in situ fluorescence measurements. A variety of resin compositions (polystyrene, JandaJel, ArgoPore, TentaGel, NovaGel, and PEGA) and solvents (dimethylformamide, acetonitrile, tetrahydrofuran, 1,2-dichlorethane, and toluene) were tested to compare their effects on the reaction rate. A linear relationship between the reaction rate and (solvent polarity x swelling of resin) was elucidated for the aminolysis reaction.

Amides↗
PubMedDate not supplied

The use of solid-phase supported 1-N-piperazine-4-N-carboxaldehyde in Vilsmeier reactions.

A Vilsmeier salt supported on solid phase was prepared using piperazine bound to Merrifield resin. Piperazine was selected because it contains two secondary amines: one of the amines is protected upon binding to the resin, and the second was formylated to give resin-1-N-piperazine-4-N-carboxaldehyde (9). Activation of the formamide with either bis(trichloromethyl)carbonate (BTC) or POCl(3) afforded the Vilsmeier salt 10. Several olefins were used to test the supported Vilsmeier reagent. The in-solution activation with BTC and POCl(3) of various secondary amides was also evaluated: dimethylformamide (1), N-methyformanilide (4), 4-formylmorpholine (5), and 1,4-dicarboxylpiperazine (6), which showed that amides with one additional heteroatom increase the yields in the Vilsmeier salt formation.

Journal Article↗
PubMedDate not supplied

Functionalized cross-linked poly(vinyl alcohol) resins as reaction scavengers and as supports for solid-phase organic synthesis.

New hydrophilic poly(vinyl alcohol) (PVA-OH) resins were prepared by an inverse suspension polymerization using epichlorohydrin as a cross-linker. These novel resins swell in a variety of solvents commonly used in solid-phase organic synthesis, such as dicholomethane, dioxane, methanol, tetrahydrofuran, and dimethylformamide. In addition, PVA-OH shows excellent swelling in water. The cross-linked PVA-OH beads were functionalized with an aldehyde group and were tested as scavengers for primary amines in three different reactions: amide bond formation, reductive amination reaction, and urea formation. With 1-2 equiv of the PVA aldehyde resin, all the excess primary amines were successfully scavenged. The utility of PVA-OH resins as solid supports in mono- and dipeptide synthesis was also investigated using symmetrical anhydride and MSNT/MeIm (2,4,6-mesitylenesulfonyl-3-nitro-1,2,4-triazolide in the presence of 1-methylimidazol) methods.

Journal Article↗
PubMed2001

Photoreductive dehalogenation of halogenated benzene derivatives using ZnS or CdS nanocrystallites as photocatalysts.

ZnS nanocrystallites (nc-ZnS) prepared in N,N-dimethylformamide (DMF) photocatalyze dehalogenation of halogenated benzenes to benzene as the final product from chlorinated benzenes and to difluorobenzenes from fluorinated benzenes in the presence of triethylamine (TEA) as an electron donor under UV light irradiation (lambda > 300 nm). When CdS nanocrystallites (nc-CdS) are used as a photocatalyst (lambda > 400 nm), halogenated benzenes are photoreductively dehalogenated, yielding trichlorobenzene from hexachlorobenzene and tetrafluorobenzene isomers from hexafluorobenzene as the final products. Photoformed electrons on nc-ZnS and nc-CdS have such negative reduction potentials that these electrons reduce polyhalogenated benzenes, leading to the successive dehalogenation. nc-ZnS exhibits higher photocatalytic activitythan nc-CdS due to the more negative potential of the electrons on nc-ZnS than that on nc-CdS. The higher activities of nc-ZnS and nc-CdS compared to their bulk forms are explained as being due to their quantum size effects and the adsorptive interaction between the substrates and the nanosized photocatalysts.

Benzene Derivatives↗
PubMed2001

Benzo(a)pyrene, benzo(a)anthracene, and dibenzo(a,h)anthracene emissions from coal and waste tire energy generation at atmospheric fluidized bed combustion (AFBC).

The main aim of this work was the analyzing of the release to the atmosphere of benzo[a]pyrene (BaP), dibenzo(a,h)anthracene (D(a,h)A), and benzo[a]anthracene (BaA), three of the most carcinogenic PAHs listed by US-EPA as priority pollutants, emitted from combustion at the last generation reactors used nowadays in power generation, fluidized bed reactors, trying to establish their incidence when waste materials are used as "new fuels". BaP, D(a,h)A, and BaA emissions, once collected in the sampling system, have been analyzed by fluorescence spectroscopy in the synchronous mode (FS) after extraction by ultrasonic bath with dimethylformamide (DMF) as solvent. Concerning to the combustion variables influence, the conclusion reached was that, in coal combustion, the lowest emissions of BaP are generated at percentages of excess oxygen of 20%, at flows corresponding to good fluidization conditions, 860 L/h (double of the minimum fluidization velocity) and temperatures out of 850 degrees C; the lowest emission of D(a,h)A are emitted at 20% excess oxygen, 900 L/h and out of 750 degrees C, and the lowest emission of BaA are generated at 20% excess oxygen, 860 L/h and out of the range 750-850 degrees C. Regarding tire as nonfossil fuel, higher emissions of BaP, BaA, and D(a,h)A are detected in comparison to coal combustion. In coal-tire blend, the BaP, BaA, and D(a,h)A emissions are more similar to the values obtained in coal combustion and not intermediate values as it could be expected by the blend composition.

Air Pollutants↗
PubMed2002

Formation of N-nitrosodimethylamine (NDMA) from dimethylamine during chlorination.

Chlorine disinfection of secondary wastewater effluent and drinking water can result in the production of the potent carcinogen N-nitrosodimethylamine (NDMA) at concentrations of approximately 100 and 10 parts per trillion (ng/L), respectively. Laboratory experiments with potential NDMA precursors indicate that NDMA formation can form during the chlorination of dimethylamine and other secondary amines. The formation of NDMA during chlorination may involve the slow formation of 1,1-dimethylhydrazine by the reaction of monochloramine and dimethylamine followed by its rapid oxidation to NDMA and other products including dimethylcyanamide and dimethylformamide. Other pathways also lead to NDMA formation during chlorination such as the reaction of sodium hypochlorite with dimethylamine. However, the rate of NDMA formation is approximately an order of magnitude slower than that observed when monochloramine reacts with dimethylamine. The reaction exhibits a strong pH dependence due to competing reactions. It may be possible to reduce NDMA formation during chlorination by removing ammonia prior to chlorination, by breakpoint chlorination, or by avoidance of the use of monochloramine for drinking water disinfection.

Chloramines↗
PubMed2005

Hydrolysis of environmental contaminants as an experimental tool for indication of their persistency.

To predict the persistency of a chemical in the environment, the chemical's physical-chemical properties and its reactivity in the environment need to be known or at least estimated. The partitioning of a chemical can be described on the basis of its water solubility, its octanol/water partitioning coefficient, and its vapor pressure. The mechanisms by which a chemical can be transformed may be categorized as being hydrolysis, oxidation, reduction, and photolysis. This study establishes a method for estimating the relative susceptibility of some potential environmental pollutants to undergo hydrolysis reactions. The method used the second-order rate constant for the reaction with sodium methoxide in methanol/N,N-dimethylformamide (DMF) as an indicator of relative susceptibility toward hydrolysis. The decabromodiphenyl ether is rapidly hydrolyzed, that is, undergoes nucleophilic aromatic substitution, while the rate of reaction of less brominated diphenyl ethers decreased by roughly a factor of 10 for each decrease in the level of bromination. Hexachlorobenzene was found to have a similar rate to a nonabromodiphenyl ether. 2,2-Bis(4-chlorophenyl)-1,1,1-trichloroethane (DDT) was transformed to 2,2-bis(4-chlorophenyl)-1,1-dichloroethene (DDE) immediately under these conditions, while DDE showed no apparent reaction. The results show that chemicals that can undergo elimination reactions are rapidly transformed, as are perhalogenated chemicals that can undergo substitution reactions. These chemicals are not likely to persist in the environment, while those that did not show any observable reactivity under similar hydrolytic conditions may persist for a very long time.

Hydrolysis↗
PubMed2000

Copper(II) and copper(I) complexes with an open-chain N4 Schiff base ligand modeling CuZn superoxide dismutase: structural and spectroscopic characterization and kinetics of electron transfer.

The structure of the complex [CuII(PuPy)](ClO4)2 (PuPy = L = 1,8-bis(2-pyridyl)-2,7-diazaoctadiene-1,7) and the structure of the corresponding copper(I) complex were determined. In CuIIL(ClO4)2, a model compound with CuZnSOD activity, the unit CuIIL2+ has a tetrahedrally distorted square-planar N4 coordination geometry. The copper(I) complex with L was found to be dimeric, (CuIL)2(ClO4)2.DMF (DMF = N,N-dimethylformamide). The binuclear unit (CuIL)2(2+) has a helical structure with two ligands L bridging the two copper atoms to provide tetrahedral N4 coordination of each copper(I). In solutions of (CuIL)2(ClO4)2.DMF, solvent-dependent dissociation occurs according to D reversible 2M (D = (CuIL)2(2+); M = CuILSx+; S = solvent). Stopped-flow spectrophotometry was used to determine the rate constants for the dissociation of the dimer D (kM) and dimerization of the monomer M (kD) for S = acetonitrile and DMF. Equilibrium constants Kdim = kM/kD were determined spectrophotometrically. In aqueous solution, the oxidation of the dimer (CuIL)2(2+) by CoIII(NH3)5Cl2+ and cis- and trans-CoIII(en)2Cl2+ follows a second-order rate law, rate = kox[(CuIL)2(2+)][Co(III)]. Data for rate constant kox and for the activation parameters delta H++ and delta S++ are presented. In DMF, the oxidation of (CuIL)2(2+) by CoIII(NH3)5Cl2+ occurs via the monomer CuIL(DMF)x+ and the dissociation of (CuIL)2(2+) becomes rate-controlling. The reduction of CuIIL2+ by RuII(edta)H2O2- was found to be too fast to be resolved by stopped-flow spectrophotometry. The kinetic results are discussed mechanistically in terms of the redox switch aspects of the system.

Copper↗