Pressure-dependent OH yields in alkene + HO2 reactions: a theoretical study

The major bimolecular product of alkyl + O(2) reactions is alkene + hydroperoxyl radical (HO(2)), but in the reverse direction, the reactants are reformed to a very limited extent only. The most important products of the alkene + HO(2) reactions are alkylperoxy radical (ROO(?)), hydroxyl radical (OH) + cyclic ether, and the corresponding hydroperoxyalkyl ((?)QOOH) species. Moreover, abstraction of allylic hydrogens can compete with the addition, further complicating the possible outcome of this reaction type and its effect on low-temperature combustion chemistry. In this paper, six alkene + HO(2) reactions and the reaction between an unsaturated oxygenate and HO(2) are studied based on previously established potential energy surfaces. The studied unsaturated compounds are ethene, propene, 1-butene, trans-2-butene, isobutene, cyclohexene, and vinyl alcohol. Using multiwell master equations, temperature- (300-1200 K) and pressure-dependent rate coefficients and branching fractions are calculated for these reactions. The importance of this reaction type for the combustion of unsaturated compounds is also assessed, and we show that, to get reliable results, it is important to include the pressure-dependence of the rate coefficients in the calculations.     

Introducing transglycosylation activity into human salivary alpha-amylase (HSA)

Synthesis of 4-nitrophenyl 1-thio-beta-D-maltoside, maltotrioside, and maltotetraoside in yields up to 60% has been achieved by a Tyr151Met (Y151M) mutant of human salivary alpha-amylase. Y151M is capable of transferring maltose and maltotriose residues from a maltotetraose donor onto different p-nitrophenyl glycosides. (1)H and (13)C NMR studies revealed that the mutated enzyme preserved the stereo- and regioselectivity. The glycosylation took place at position 4 of the glycosyl acceptor, forming the alpha(1-4)glycosidic bond, exclusively. [reaction: see text]     

Dioxirane oxidation of benzopyrans,benzothiopyrans, and related compounds

Dioxiranes are shown to be powerful and convenient oxidation agents for benzopyrans, benzothiopyrans, and , -unsaturated ketones.Department of Organic Chemistry, KOssuth Lajos University, Egyetem ter 1, H-4010 Debrecen, Hungary. Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany. Technical Analytical Research Group of the Hungarian Academy of Sciences, Institute of General and Analytical Chemistry. Technical Univeristy, St. Gellert ter 4, H-1111 Budapest, Hungary. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1345–1349, October, 1995. Original article submitted July 16, 1995.     

Acovulparine,a New Norditerpene Alkaloid from Aconitum vulparia

Acovulparine (=1α,6β,7R,8β,14α,16β)‐4‐(hydroxymethyl)‐1,6,14,16‐tetramethoxyaconit‐19‐ene‐7,8‐diol; 1 ), a new norditerpene alkaloid, was isolated from the MeOH extract of the whole plants of Aconitum vulparia Reichenb ., together with the known compounds lycoctonine ( 2 ) and delcosine ( 3 ). The structures were established by HR‐EI‐MS and advanced 2D‐NMR, including ¹H‐NMR, JMOD, ¹H,¹H‐COSY, HSQC, and HMBC experiments. Acovulparine was found to contain the rare C(19)?N azomethine group.     

Quasiliving Carbocationic Polymerization. IV. Polymerization of p-tert-Butylstyrene

The mechanism of polymerization of p-tert-butylstyrene (ptBuSt) initiated by the cumyl chloride/BCl₃ initiating system in CH₂Cl₂ at -50°C has been investigated. At and below ～0.4 M ptBuSt, quasiliving polymerizations proceed, i.e., initiation is instantaneous, termination is absent or reversible, and chain transfer to monomer can be suppressed or eliminated. In the quasiliving range the M _n versus [ptBuSt]₀ plot is linear and passes through the origin, and a M _w/M _n decreases much below 2.0 with decreasing [ptBuSt]. GPC traces change from broad multimodal to narrow monomodal and the color of polymerization charges change from colorless to golden-yellow with decreasing [ptBuSt]. The effect of temperature jump subsequent to monomer addition has been examined; however, it does not explain the peculiar monomer concentration effect on the mechanism. Changes in the ionicity may be responsible for this phenomenon.     

Synthesis of arborescent (Dendritic) polystyrenes via controlled inimer‐type reversible addition‐fragmentation chain transfer polymerization

The reversible addition‐fragmentation chain transfer (RAFT) copolymerization of styrene and 4‐vinylbenzyl dithiobenzoate, a RAFT‐based inimer (initiator‐monomer), is described. Controlled polymerization was achieved in bulk conditions using thermal initiation at 110 °C to give arborescent polystyrene (arbPSt). The number‐average molecular weights of the polymers increased linearly with conversion and were much higher than theoretically calculated for a linear polymerization, reaching M_n = 364,000 g/mol with M_w/M_n = 2.65. Branching analysis by NMR showed an average of 3.5 branches per chain. SEC data, which were similar to those measured in arborescent polyisobutylene, supported the architectural analysis. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7621–7627, 2008     

Gold nanoparticle-quantum dot-polystyrene microspheres as fluorescence resonance energy transfer probes for bioassays

The paper describes the development of highly sensitive particle-based fluorescence resonance energy transfer (FRET) probes that do not use molecular fluorophores as donors and acceptors. In these probes, CdSe/ZnS luminescent quantum dots (QDs) were capped with multiple histidine-containing peptides to increase their aqueous solubility while maintaining their high emission quantum yield and spectral properties. The peptide-modified QDs (QD-His) were covalently attached to carboxyl-modified polystyrene (PS) microspheres to form highly emitting PS microspheres (QD-PS). Gold nanoparticles (AuNPs) were then covalently attached to the QD-PS surface to form AuNP-QD-PS composite microspheres that were used as FRET probes. Attachment of AuNPs to QD-PS completely quenched the QD emission through FRET interactions. The emission of QD-PS was restored when the AuNPs were removed from the surface by thiol ligand displacement. The new AuNP-QD-PS FRET platform is simple to prepare and highly stable, and it opens many new possibilities for carrying out FRET assays on microparticle-based platforms and in microarrays. The versatility of these assays could be greatly increased by replacing the linkers between the QDs and AuNPs with ones that selectively respond to specific cleaving agents or enzymes.     

Role of linear charge density and counterion quality in thermodynamic properties of strong acid type polyelectrolytes: divalent transition metal cations

Thermodynamic properties of aqueous solutions of poly[(vinyl alcohol)-co-(vinyl sulfate)] (PVAS) copolymer polyelectrolytes with divalent transition metal (Co(II), Ni(II), and Cu(II)) counterions have been determined by the gel deswelling method in the concentration range of 0.0005-0.12 mol of counterion/kg of water (0.09-9 w/w% of the polymer). The influence of the chemical nature of the counterion as well as the effect of the composition of the copolymer from small to medium linear charge density have been systematically studied. Solvent activity, reduced osmotic pressure, the Flory-Huggins pair interaction parameter, rational osmotic coefficients, and degrees of dissociation were calculated from the measured data. No difference could have been observed between the three counterions. Reduced osmotic pressure curves are found to be convex from above, as for Na+ counterions studied previously, which is contrary to the usual behavior of neutral polymers. Intercepts are increasing, and the calculated apparent molar masses and degrees of dissociation at infinite dilution are decreasing with increasing linear charge density of the polyelectrolytes. The pair interaction parameters show a considerable negative deviation from linearity, except for the high volume fraction region. From the differences, concentration dependence of degrees of dissociation could have been calculated. The values at infinite dilution are in good agreement with those obtained from the intercepts of the reduced osmotic pressure curves. Degrees of dissociation seem to decrease approximately linearly with increasing concentration and reach zero at finite concentrations. Rational osmotic coefficients have been calculated in three different ways, both regarding and neglecting the change in the degrees of dissociation.     

Synthesis of cellulose derivative based superabsorbent hydrogels by radiation induced crosslinking

Hydrogels with high water uptake were prepared by ionizing radiation induced crosslinking in aqueous solutions of four cellulose derivatives (carboxymethylcellulose sodium salt—CMC-Na, methylcellulose—MC, hydroxyethylcellulose—HEC and hydroxypropylcellulose—HPC). The gel fraction increased with absorbed dose, while water uptake decreased. At high polymer concentrations lower gel fractions were found due to the lower polymer chain mobility and inhomogeneity at low water content. The swelling rate gradually slowed down after 4–5 h. CMC and HEC gels reached equilibrium after 24 h, while HPC and MC gels required longer immersion times. Gels showed second-order swelling kinetics in water. The mechanism of the water diffusion proved to be anomalous. In pure water, CMC gels showed the highest, while HPC and MC gels the lowest water uptake. The derivatives had different sensitivities to ionic strength in the swelling solution. The salt type also proved to be a significant factor at uniform ionic strength. Thus different cellulose derivative based gels may be preferred at various applications depending on the environment.     

Green Polymer Chemistry VIII: Synthesis of Halo‐ester‐Functionalized Poly(ethylene glycol)s via Enzymatic Catalysis

Halo‐ester‐functionalized poly(ethylene glycol)s (PEGs) are successfully prepared by the transesterification of alkyl halo‐esters with PEGs using Candida antarctica lipase B (CALB) as a biocatalyst under the solventless conditions. Transesterifications of chlorine, bromine, and iodine esters with tetraethylene glycol monobenzyl ether (BzTEG) are quantitative in less than 2.5 h. The transesterification of halo‐esters with PEGs are complete in 4 h. ¹H and ¹³C NMR spectroscopy with MALDI‐ToF and ESI mass spectrometry confirm the structure and purity of the products. This method provides a convenient and “green” process to effectively produce halo‐ester PEGs.