Kinetics of the Gas-Phase Reaction of Hydroxyl Radicals with Dimethyl Methylphosphonate (DMMP) over an Extended Temperature Range (273-837 K).

The kinetics of the reaction of hydroxyl radical (OH) with dimethyl methylphosphonate (DMMP, (CHO)CHPO) (reaction 1) OH + DMMP → products (1) was studied at the bath gas (He) pressure of 1 bar over the 295-837 K temperature range. Hydroxyl radicals were produced in the fast reaction of electronically excited oxygen atoms O(D) with HO. The time-resolved kinetic profiles of hydroxyl radicals were recorded via UV absorption at around 308 nm using a DC discharge HO/Ar lamp.

Disproportionation Channel of the Self-reaction of Hydroxyl Radical, OH + OH → HO + O, Revisited.

The rate constant of the disproportionation channel 1a of the self-reaction of hydroxyl radicals OH + OH → HO + O (1a) was measured at ambient temperature as well as over an extended temperature range to resolve the discrepancy between the IUPAC recommended value ( = 1.48 × 10 cm molecule s, discharge flow system, Bedjanian et al. 1999, 103, 7017) and a factor of ca.

Pressure-Dependent Kinetics of the Reaction between CHO and OH: TRIOX Formation.

Reaction of methyl peroxy radicals with hydroxyl radicals (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy at 296 K over the 1-100 bar pressure range (bath gas He): CHO + OH → CHO + HO (1a), CHO + OH → CHOO + HO (1b), and CHO + OH → CHOOOH (TRIOX) (1d). Channel 1a is the dominant channel under ambient conditions, while the contribution of channel 1b is less than 5% at 1 bar and 296 K. Channel 1c is strongly pressure-dependent and becomes dominant at high pressures.

Kinetics and Thermodynamics of DNA Processing by Wild Type DNA-Glycosylase Endo III and Its Catalytically Inactive Mutant Forms.

Endonuclease III (Endo III or Nth) is one of the key enzymes responsible for initiating the base excision repair of oxidized or reduced pyrimidine bases in DNA. In this study, a thermodynamic analysis of structural rearrangements of the specific and nonspecific DNA-duplexes during their interaction with Endo III is performed based on stopped-flow kinetic data. 1,3-diaza-2-oxophenoxazine (tC), a fluorescent analog of the natural nucleobase cytosine, is used to record multistep DNA binding and lesion recognition within a temperature range (5-37 °C).

Kinetics of the Reaction of CH3O2 Radicals with OH Studied over the 292-526 K Temperature Range.

Reaction of methyl peroxy radicals with hydroxyl radicals, CH3O2 + OH → CH3O + HO2 (1a) and CH3O2 + OH → CH2OO + H2O (1b) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 292-526 K temperature range and pressure 1 bar (bath gas He). Hydroxyl radicals were generated in the reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N2O at 193.3 nm, with H2O.

Reaction CH3 + CH3 → C2H6 Studied over the 292-714 K Temperature and 1-100 bar Pressure Ranges.

Reaction of recombination of methyl radicals, CH3 + CH3 → C2H6 (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 292-714 K temperature and 1-100 bar pressure ranges (bath gas He), very close to the high-pressure limit. Methyl radicals were produced by photolysis of acetone at 193.3 nm or in the reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N2O at 193.

Thermodynamics of the DNA damage repair steps of human 8-oxoguanine DNA glycosylase.

Human 8-oxoguanine DNA glycosylase (hOGG1) is a key enzyme responsible for initiating the base excision repair of 7,8-dihydro-8-oxoguanosine (oxoG). In this study a thermodynamic analysis of the interaction of hOGG1 with specific and non-specific DNA-substrates is performed based on stopped-flow kinetic data. The standard Gibbs energies, enthalpies and entropies of specific stages of the repair process were determined via kinetic measurements over a temperature range using the van't Hoff approach.

Kinetics of the gas phase reaction CH3 + HO2.

Reaction of methyl radicals with hydroperoxy radicals, CH3 + HO2 → products (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy at 295 K and 1 bar (He). Photolysis of N2O/H2O2/CH4/H2O/He mixtures at 193.3 nm (ArF excimer laser) was used to simultaneously produce methyl radicals and hydroperoxy radicals in reactions of electronically excited oxygen atoms O((1)D) with CH4 and OH radicals with H2O2, respectively.

Disproportionation channel of self-reaction of hydroxyl radical, OH + OH → H2O + O, studied by time-resolved oxygen atom trapping.

The disproportionation channel of the self-reaction of hydroxyl radicals, OH + OH → H(2)O + O (1a) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 298-414 K temperature and 3-10 bar pressure ranges (bath gas He). To distinguish channel 1a from the recombination channel 1b, OH + OH → H(2)O(2) (1b), time-resolved trapping of oxygen atoms, produced in channel 1a, was used. The ozone produced in the reaction of oxygen atoms with molecular oxygen was measured using strong UV absorption at 253.

Reaction CH3 + OH studied over the 294-714 K temperature and 1-100 bar pressure ranges.

Reaction of methyl radicals with hydroxyl radicals, CH(3) + OH → products (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 294-714 K temperature and 1-100 bar pressure ranges (bath gas He). Methyl radicals were produced by photolysis of acetone at 193.3 nm.

Thermodynamics of the multi-stage DNA lesion recognition and repair by formamidopyrimidine-DNA glycosylase using pyrrolocytosine fluorescence--stopped-flow pre-steady-state kinetics.

Formamidopyrimidine-DNA glycosylase, Fpg protein from Escherichia coli, initiates base excision repair in DNA by removing a wide variety of oxidized lesions. In this study, we perform thermodynamic analysis of the multi-stage interaction of Fpg with specific DNA-substrates containing 7,8-dihydro-8-oxoguanosine (oxoG), or tetrahydrofuran (THF, an uncleavable abasic site analog) and non-specific (G) DNA-ligand based on stopped-flow kinetic data. Pyrrolocytosine, highly fluorescent analog of the natural nucleobase cytosine, is used to record multi-stage DNA lesion recognition and repair kinetics over a temperature range (10-30°C).

New cross-linking quinoline and quinolone derivatives for sensitive fluorescent labeling.

A variety of contemporary analytical platforms, utilized in technical and biological applications, take advantage of labeling the objects of interest with fluorescent tracers-compounds that can be easily and sensitively detected. Here we describe the synthesis of new fluorescent quinoline and quinolone compounds, whose light emission can be conveniently tuned by simple structural modifications. Some of these compounds can be used as sensitizers for lanthanide emission in design of highly sensitive luminescent probes.

Reaction OH + OH studied over the 298-834 K temperature and 1-100 bar pressure ranges.

Self-reaction of hydroxyl radicals, OH + OH → H(2)O + O (1a) and OH + OH → H(2)O(2) (1b), was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 298-834 K temperature and 1-100 bar pressure ranges (bath gas He). A heatable high-pressure flow reactor was employed. Hydroxyl radicals were prepared using reaction of electronically excited oxygen atoms, O((1)D), produced in photolysis of N(2)O at 193 nm, with H(2)O.

Branching ratios in the hydroxyl reaction with propene.

The branching ratios for the reactions of attachment of hydroxyl radical to propene and hydrogen-atom abstraction were measured at 298 K over the buffer gas pressure range 60-400 Torr (N(2)) using a subatmospheric pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass spectrometer. Isotopically enriched water H(2)(18)O was used to produce (18)O-labeled hydroxyl radicals in reaction with fluorine atoms. The β-hydroxypropyl radicals formed in the attachment reactions 1a and 1b , OH + C(3)H(6) → CH(2)(OH)C(•)HCH(3) (eq 1a ) and OH + C(3)H(6) → C(•)H(2)CH(OH)CH(3) (eq 1b ), were converted to formaldehyde and acetaldehyde in a sequence of secondary reactions in O(2)- and NO-containing environment.

Luminescent probes for ultrasensitive detection of nucleic acids.

Novel amino-reactive derivatives of lanthanide-based luminescent labels of enhanced brightness and metal retention were synthesized and used for the detection of cDNA oligonucleotides by molecular beacons. Time-resolved acquisition of the luminescent signal that occurs upon hybridization of the probe to the target enabled the avoidance of short-lived background fluorescence, markedly enhancing the sensitivity of detection, which was less than 1 pM. This value is about 50 to 100 times more sensitive than the level achieved with conventional fluorescence-based molecular beacons, and is 10 to 60 times more sensitive than previously reported for other lanthanide-based hybridization probes.

Rate constants and hydrogen isotope substitution effects in the CH3 + HCl and CH3 + Cl2 reactions.

The kinetics of the CH3 + Cl2 (k2a) and CD3 + Cl2 (k2b) reactions were studied over the temperature range 188-500 K using laser photolysis-photoionization mass spectrometry. The rate constants of these reactions are independent of the bath gas pressure within the experimental range, 0.6-5.

In situ optical monitoring of RDX nanoparticles formation during rapid expansion of supercritical CO2 solutions.

Nanoparticles of RDX (cyclotrimethylenetrinitramine) generated by RESS (rapid expansion of supercritical solutions) using supercritical CO2 were characterized in situ by a pulsed laser light scattering imaging technique using a gated ICCD (intensified CCD) camera. The absolute sensitivity calibration was performed using Rayleigh light scattering from air as well as light scattering from standard polystyrene spheres. The size distribution functions of the particles formed in the RESS jet were determined using the calibrated sensitivity.

Complete thermodynamically consistent kinetic model of particle nucleation and growth: numerical study of the applicability of the classical theory of homogeneous nucleation.

A complete thermodynamically consistent elementary reaction kinetic model of particle nucleation and growth from supersaturated vapor was developed and numerically evaluated to determine the conditions for the steady-state regime. The model treats all processes recognized in the aerosol science (such as nucleation, condensation, evaporation, agglomerationcoagulation, etc.) as reversible elementary reactions.

Thermochemistry is not a lower bound to the activation energy of endothermic reactions: a kinetic study of the gas-phase reaction of atomic chlorine with ammonia.

The rate constant for Cl + NH3 --> HCl + NH2 has been measured over 290-570 K by the time-resolved resonance fluorescence technique. Ground-state Cl atoms were generated by 193 nm excimer laser photolysis of CCl4 and reacted under pseudo-first-order conditions with excess NH3. The forward rate constant was fit by the expression k1 = (1.

Modified transition state theory and negative apparent activation energies of simple metathesis reactions: application to the reaction CH3 + HBr --> CH4 + Br.

A modified transition state theory (MTST) has been developed for gas-phase reactions with "negative barriers". The theory was applied to the reactions CH3 + HBr(DBr) --> CH4(CH3D) + Br (1a, 1b), which exhibit negative temperature dependences. Accurate ab initio calculations performed with coupled cluster theory extrapolated to the complete basis set limit revealed a transition state located at -2.