Rate Constants For Trapping Research Articles

Chemistry and DNA alkylation reactions of aziridinyl quinones: development of an efficient alkylating agent of the phosphate backbone.

Described herein are detailed hydrolytic studies of a series of aziridinyl quinones, which trap nucleophiles when protonated. This study provided a compilation of the rate constants for nucleophile trapping and of the pKa values for the protonated aziridinyl quinones. A linear free energy relationship, including the antitumor agent DZQ, as well as other synthetic quinone derivatives, was obtained as a result of this study. Protonated DZQ has the relatively high pKa value of 3.8, which explains the enhanced cross-linking of DNA by DZQ and other related aziridinyl quinones at pH 4. The literature often shows aziridinyl quinone protonation occurring at the aziridinyl nitrogen, but the dependence of pKa values on quinone substituents indicates the presence of delocalization, which must arise from O-protonation. Also investigated were the DNA alkylation reactions of protonated aziridinyl quinones. At the outset of this study, we postulated that these "hard" electrophiles would alkylate the phosphate backbone of DNA. Bulk DNA is up to 35% alkylated by protonated aziridinyl quinones as judged by the incorporation of the quinone chromophore into the DNA. The presence of phosphate alkylation was verified by a 1H-31P NMR correlation experiment with DZQ-alkylated hexamer. Our modeling studies present a new picture of DZQ alkylation of DNA, where there is competition between N(7) and phosphate alkylation. The conclusions of this part of our study are that the phosphate backbone should be considered as a possible target of any DNA-alkylating agent and that an assessment of phosphate alkylation is best made with a 1H-31P NMR correlation experiment. Finally, the benzimidazole-based aziridinyl quinone 2 was observed to undergo aziridine ring opening followed by hydrolytic removal of the aminoethyl group from the quinone ring. This reaction was used to tag the phosphate backbone of DNA with aminoethyl groups. Such tags render anionic phosphates cationic and could also be employed as points of attachment for chromophores, spin labels, or other moieties to DNA.

Diffusivity, solubility and trapping behavior of hydrogen in alloys 600, 690tt and 800

The permeation technique and a potentiostatic pulse technique were used to investigate the diffusivity, solubility, and trapping behavior of hydrogen (H) in three austenitic alloys: 600, 690tt and 800. The apparent diffusivity increases with nickel content, whereas the solubility decreases. The diffusivity decreases with cold work. In the alloys with 50% cold work, H is reversibly trapped in the stress field of dislocations. The rate constants for irreversible trapping paralleled the observed resistance to environmentally assisted cracking of the non-cold-worked alloys. The principal irreversible traps appeared to be carbide/carbonitride precipitates. Microvoids provided additional irreversible traps in precharged, cold-worked alloy 600.

Absolute Rate Constants for alpha-Amide Radical Reactions.

The 1-(diethylcarbamoyl)-6,6-diphenyl-5-hexenyl radical (4a), the 1-(diethylcarbamoyl)-7,7-diphenyl-6-heptenyl radical (4b), and the 1-(diethylcarbamoyl)-1-methyl-6,6-diphenyl-5-hexenyl radical (4c) were produced from the corresponding PTOC esters (anhydrides of the carboxylic acid and N-hydroxypyridine-2-thione) by laser flash photolysis methods. The kinetics of cyclizations of radicals 4a and 4b were measured at various temperatures, and that of cyclization of 4c was measured at ambient temperature. Radicals 4a and 4b were employed as radical clocks in indirect kinetic studies to determine rate constants for reaction of secondary alpha-amide radicals with Bu(3)SnH. The calibrated tin hydride trapping reaction was then employed to determine rate constants for cyclization of the 1-(diethylcarbamoyl)-5-hexenyl radical (12). The rate constants for 5-exo cyclizations of secondary alpha-amide radicals are similar to those of their isostructural alkyl radical analogues. The rate constants for the 5-exo cyclization of tertiary alpha-amide radical 4c and the 6-exo cyclization of the secondary alpha-amide radical 4b are smaller than those of the analogous alkyl radicals and alpha-ester substituted radicals, apparently due to steric effects. The rate constants for tin hydride trapping of secondary alpha-amide radicals are similar to those for reactions with secondary alpha-ester radicals.

Adiabatic transfer of net electron polarization to nuclear polarization

A new type of spin chemical experiment is suggested which utilizes adiabatic reversion of a low magnetic B 0 field for an efficient transfer of net electron spin polarization, as arising from some CIDEP mechanism, to the nuclear polarization of radicals. After trapping the radicals in diamagnetic products and transferring the sample to a NMR probe, the adiabatic polarization transfer effect can be measured as induced CIDNP, as in the recently established SNP method. The optimum conditions concerning delay time and duration of the B 0 field switching, as well as the trapping rate constant are analysed quantitatively for a radical with one nuclear spin I = 1/2. Owing to their efficiency and some parameters that can be controlled in the experiment, adiabatic polarization transfer effects should have clear advantages over experiments relying on DNP effects due to the Overhauser mechanism.

Rate Constants for Termination and TEMPO Trapping of Some Resonance Stabilized Hydroaromatic Radicals in the Liquid Phase

Rate Constants for Termination and TEMPO Trapping of Some Resonance Stabilized Hydroaromatic Radicals in the Liquid Phase

Absolute rates of heterocyclic singlet biradical reactions determined by nanosecond time-resolved absorption spectroscopy. Dimerizations and cycloadditions to alkenes and to dioxygen

Singlet biradicals of the 3,4-dimethyleneheterocycle series, where the heterocycle is furan, thiophene, or N-substituted pyrrole, with chromophores in the UV-vis region (560-640 nm), are generated by nanosecond laser flash photolysis of diazene precursors in fluid solution. The rate constants for disappearance of these transients are determined by time-resolved absorption spectroscopy using the long-wavelength band. The rate constants for dimerization are near the encounter-controlled rate. Rates of trapping by alkenes are well described by a frontier orbital model. Oxygen trapping rate constants range between 10 7 and 10 9 M -1 s -1

Effect of mud penetration on borehole skin properties

Clay particles in drilling muds can form a filter cake (skin) on the walls of the well and migrate into the adjacent formation. In general, filter cakes are formed when a liquid phase containing solid particles is forced through a pervious surface which allows liquid transport, while retaining solid particles. Following a literature survey, the governing equations for the concentration of suspended solid particles in the pore fluid, cake permeability, and solid velocity are obtained by considering the mass balance equations for the fluid phase and suspended and captured solid particles. The capture mechanism is represented by a kinetic equation with rate constants for particle trapping and cake erosion. Numerical solutions for the cake permeability, cake resistance, solid particle velocity (cake compression rate), and concentration of suspended particles are obtained by utilizing experimentally available porosity and pressure variations along the dimensionless cake thickness. The results compared for a simplified experimental case, show excellent match. A sensitivity analysis shows that cake permeability and cake resistance are more sensitive to the rate constant of cake erosion than they are to the rate constant of particle capture. It was also determined that the permeability has larger values along the entire cake thickness when the rate constant of particle capture is zero in comparison to values obtained with a nonzero rate constant. Cake permeability is small when the rate constant of cake erosion is large.

Numerical solution and sensitivity analysis of filter cake permeability and resistance to model parameters

The migration and capture of solid particles in porous media occur in fields as diverse as water and wastewater treatment, well drilling, and in various liquid-solid separation processes. Filter cakes are formed when a liquid containing solid particles is forced through a pervious surface which allows the liquid transport while retaining solid particles. Following a literature survey, a governing equation for the cake thickness is obtained by considering the instantaneous mass balance. Later, numerical solutions for the cake thickness, cake permeability, cake resistance, solid particle velocity (cake compression rate) and concentration of suspended particles are obtained and a sensitivity analysis is conducted. The sensitivity analysis shows that the cake permeability and cake resistance are more sensitive to the rate constant of cake erosion than they are to the rate constant of particle capture. However, the concentration of suspended solid particles, and the solid velocity are mostly sensitive to the slurry parameter and the rate constant of particle trapping. Moreover, cake permeability, compressibility, concentration of suspended particles, and the solid velocity are very sensitive to the concentration at the filter septum. Finally, as expected, with a thicker slurry, more particles are captured inside the cake, thus forming a thicker and more resistant cake. Also, as more particles are being filtered at the filter septum, a thinner cake is formed and a smaller effluent concentration is achieved.

Picosecond radical kinetics. Rate constants for reaction of benzeneselenol with primary alkyl radicals and calibration of the 6-cyano-5-hexenyl radical cyclization

The cyclopropylcarbinyl radical ring opening was used as a radical clock to determine rate constants for benzeneselenol trapping in THF and in toluene. Hydrogen atom transfer trapping from PhSeH appeared to be partially diffusion controlled. An operational Arrhenius function for trapping in THF is log (k T .M s)=11.03-2.27/2.3RT. The recommended function for PhScH trapping in other low-viscosity organic solvents is log (k T .M s)=10.87-2.10/2.3RT. The rate constant for trapping at 25 o C is 2.1×10 9 M -1 s -1

A method for estimation of drug affinity constants to the open conformational state of calcium channels.

The affinity of D600 to calcium channels in the open state has been examined in isolated smooth muscle cells of the rabbit ear artery. Calcium channel currents were measured in high external barium solution by means of the patch-clamp technique. The current inhibition in various D600 concentrations (3-100 microM) on application of trains of short test pulses (20-80 ms) has been studied in nonmodified calcium channels and in cells where the calcium channels were modified by the agonist dihydropyridine (+) 202,791 (100 nM). The kinetics of the peak current decay has been analyzed with a mathematical model which is based on the experimental finding that D600 interacts primarily with calcium channels in the open conformational state. The model approach allows the estimation of drug affinity constants of D600 to the calcium channel in the open conformation. An association rate constant to the open conformational state of D600 of 6.16 x 10(4) M-1 s-1 was estimated. The association rate of the drug was not significantly changed after the calcium channels have been modified with 100 nM (+) 202,791. A method for correction of rate constants for possible drug trapping is discussed.

Thermolysis of α-hydroperoxyalkyl diazenes. Spin trapping of radical intermediates and spin trapping kinetics

α-Hydroperoxyalkyl diazenes (Me2C(OOH)N=NR, 1, R = CH2CF3, CH2CH2OMe, CH(Me)2, CMe3, CH2Ph, Ph, CH2CH2OPh, and c-C3H5CD2) decompose in benzene, at 50 °C or less, by a mechanism involving free radical (R•) intermediates. The radicals were trapped with 1-methyl-4-nitroso-3,5-diphenylpyrazole, 2, to afford spin adducts (nitroxyls) that were observed by ESR spectroscopy. When the solvent was ethyl vinyl ether, radicals from 1 (R = CH2CH2OPh) were trapped by the solvent and the adduct radicals so formed were spin trapped by 2. These observations support free radical mechanisms for thermolysis of 1 and for the hydroxyalkylations that occur when 1 are decomposed in solutions containing enol ethers or other unsaturated substrates. The ring-opening of cyclopropylmethyl radicals (cpm) to 3-butenyl radicals was used to estimate the rate constant for radical trapping by 2. For cpm the rate constant is given by log kcpm = (10.7 ± 0.4) − (3.9 ± 0.5)/θ where θ = 2.3 RT kcal mol−1. At 25 °C, the spin trapping rate constant has the value 6.9 × 107 M−1 s−1. Key words: hydroperoxyalkyl diazenes; radicals, spin trapping; spin trapping, rate constant.

Picosecond radical kinetics. Benzeneselenol as a fast radical trapping agent and rate constants for ring opening of the trans-(2-phenylcyclopropyl)carbinyl radical

Picosecond radical kinetics. Benzeneselenol as a fast radical trapping agent and rate constants for ring opening of the trans-(2-phenylcyclopropyl)carbinyl radical

Relative phototrapping rates of the two bacteriopheophytins in the photoreaction center of Ectothiorhodospira sp.

The photoreaction center of Ectothiorhodospira sp. includes a bound c-type cytochrome containing four hemes. When reduced, this cytochrome competes with the reduced primary electron acceptor for charge recombination with the oxidized primary donor. This allows the phototrapping of reduced intermediary species. While only the A arm bacteriopheophytin anion was phototrapped at temperatures above 250 K, both bacteriopheophytin anions were phototrapped at 219 K. The trapping and decay rate constants of the two bacteriopheophytin anions were obtained from the kinetics were obtained from the kinetics of their light-induced absorbance change and EPR signal. The bacteriopheophytin situated in the A arm was phototrapped 274-times faster than the one in the B arm. Both phototrapped anions decay by charge recombination with the oxidized cytochrome.

Hydrogen Ingress in High-Strength Alloys Exposed to Different Electrolytes

Abstract A diffusion/trapping model for hydrogen ingress was applied to three high-strength alloys, 4340 (UNS G43400) steel, Monel K-500 (UNS N05500), and MP35N (UNS R30035), in electrolytes ranging in pH from 2.9 to 9.0. Hydrogen ingress was found to occur under interface control, although in some cases, the data were affected by corrosion or by a change in the surface film on the alloy. The apparent rate constants for hydrogen trapping (ka were self-consistent for the steel in an acetate buffer, but anomalies were observed with other electrolytes due to significant metal dissolution or film formation. In contrast, the interface control model was applicable to alloy K-500 in various electrolytes. The value of ka for alloy K-500 was independent of the electrolyte, indicating that the model adequately represents hydrogen ingress in this alloy over a range of test conditions. The interface control model can be fitted also to the data for alloy MP35N in different electrolytes, and ka was determined in each c...

Triplet photophysical properties of poly(N-vinylcarbazole) in fluid solutions and in solid films in the submicrosecond time regime

Triplet photophysical properties of poly(N-vinylcarbazole) (PVCA) have been examined in fluid solutions and in solid films using time resolution of a few hundred nanoseconds to several microseconds. In fluid solutions the intensity of delayed excimer fluorescence (def) increases with time following excitation and goes through a maximum at a delay time near 300 ns. A kinetic model describing the formation and decay of the various triplet species is proposed, which reproduces the experimental observations using the rate constant for trapping of mobile triplet excitons as a fitting parameter. The best fit yields a minimum value for the trapping rate constant of 2.0 {times} 10{sup 9} M{sup {minus}1}s{sup {minus}1}, assuming every chromophore is a potential trap site. When transient triplet-triplet absorption methods are used, the second-order rate constant for the triplet-triplet annihilation (TTA) in fluid solution is found to be 2.5{times}10{sup 8} M{sup {minus}1}s{sup {minus}1}. Solid films at 10 K also show a maximum in the intensity of def recorded as a function of time. For solid films a best fit to the experimental results is achieved with a trapping rate constant of 8{times}10{sup 5}M{sup {minus}1}s{sup {minus}1}. This rate constant is similar to that recently determined for triplet quenching in copolymersmore » containing the carbazolyl group and is probably controlled by the rate of energy migration.« less

Reaction dynamics of photosubstitution intermediates of the triruthenium carbonyl cluster Ru3(CO)12 as studied by flash photolysis with infrared detection

The reaction dynamics of intermediates formed by 308-nm XeCl excimer-laser excitation of the triruthenium cluster Ru{sub 3}(CO){sub 12} in room-temperature isooctane solution were studied by using a flash-photolysis apparatus with a tunable infrared laser probe source and Hg/Cd/Te fast-risetime ir detector. The primary photoreaction is the dissociation of CO to give Ru{sub 3}(CO){sub 11}, which is rapidly trapped in second-order reactions by CO or by an added nucleophile such as tetrahydrofuran. The rate constants for such trapping are shown t be 2.4 {times} 10{sup 9} and 6.1 {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1} for CO and THF, respectively. The intermediate Ru{sub 3}(CO){sub 11}(THF) reacts with CO to re-form Ru{sub 3}(CO){sub 12} via an apparent first-order dissociation of the THF with a rate constant about 2 {times} 10{sup 6} s{sup {minus}1}. The flash-photolysis apparatus with the ir detection system is described in detail. 17 refs., 6 figs.

Delayed fluorescence and delayed excimer fluorescence from fluid solutions of poly(2-vinylnaphthalene) in the nanosecond time regime

The time dependence of delayed fluorescence (DF) and delayed excimer fluorescence (DEF) of poly (2-vinylnaphthalene) (P2VN) has been measured in fluid solution at ambient temperature. Within the time resolution of the apparatus, the DF decreases monotonically with time but DEF exhibits a maximum intensity near 300 ns. The time dependence of the DEF signal suggests that one of the triplet partners must be trapped prior to the annihilative event. Model calculations have been carried out yielding a trapping rate constant of (2.0±0.5)×10 9 M −1 s −1. The rate-controlling step for triplet trapping may involve internal rotation of the chain backbone.

Hydrogen retention and release dynamics of amorphous carbon films exposed to a hydrogen plasma

H2 and DH release was investigated in a pulsed deuterium discharge over a carbon thin-film deposited wall. A large quantity of hydrogen was desorbed by 300 eV D+2 irradiation from hydrogenated amorphous carbon (a-C:H) deposited by a hydrogen-admixed methane discharge. On the other hand, a strong pumping effect was observed in the case of films deposited by a helium-admixed discharge. The wall pumping effect was attributed to the presence of interstitial vacancies rather than dangling bonds in the films. A linear dependence of the hydrogen release rate on the ion bombarding current suggests that molecular formation is by direct hydrogen abstraction instead of recombination of two free hydrogen atoms. A dynamic model of hydrogen release and retention for a-C:H layers during hydrogen implantation is proposed. Corresponding calculations yield satisfactory agreement with observations when using appropriate rate constants for trapping and detrapping.

Directed picosecond excitation transport in purple photosynthetic bacteria

Picosecond spectrally resolved fluorescent measurements together with the coupled kinetic rate equation model simulations of the data have been employed to determine the rates and pathways of heterogeneous excitation transport in the membranes of photosynthetic bacteria Rhodobacter sphaeroides and Chromatium minutissimum. Excitation transport from bacteriochlorophyll molecules B800 to B850, belonging to the same pigment-pigment-protein complex B800–850, proceeds in 1–2 ps at room temperature and at 77 K. The intercomplex excitation transport from B850 to B875 takes about 10 ps for most excitations. In the minor part of B800–850 complexes, the excitation transport to B875 complex takes much longer, about 50 ps. The macroscopic rate constant of excitation trapping by open reaction centres is shown to be the same for all the bacteria studied, although the number of antenna molecules per reaction centre differs significantly. This seems to be an indication of the intrinsic homogeneity of the long-wavelength bacteriochlorophyll band, which facilitates an additional localization of excitations in the vicinity of the reaction centre and, due to the shortening of the trapping time, increases the overall quantum yield of photosynthesis.

Nitroxide Formation in the Solid State of the Liquid Crystal 4-Pentyl-4'-Cyanobiphenyl

Abstract The formation of nitroxides during photolysis of 2-methyl-2-nitrosopropane in nematic liquid crystal 4-pentyl-4'-(5CB) has been studied by ESR at temperatures 183–343K. The kinetics of nitroxide formation is sensitive to the phase state of liquid crystalline systems. In the solid state of 5CB the kinetic manifestation of hysteresis phenomena has been established. In nematic phase 5CB the equalization of rate constants for the tert-butyl radical trapping by nitrosocompounds of different chemical structures has been observed.