Deactivation Step Research Articles

The chemical and enzymatic interaction of glutathione with the fungal metabolite, fusarin C

Glutathione (GSH) interacts both chemically and enzymatically with fusarin C, a mutagenic metabolite produced by Fusarium moniliforme. The chemical reaction, which is pH-dependent, results in the formation of both fusarin A and a compound that lacks the 2-pyrrolidone moiety thereby suggesting an interaction at the C-13–C-14 epoxide. Enzymatic interaction of fusarin C with GSH also appears to occur at this site as fusarins A and D, which lack the epoxide, do not serve as substrates for GSH- S-transferases. The interaction of GSH with fusarin C appears to be an important deactivation step which could explain the lack of carcinogenicity observed for fusarin C in rats.

Performance of Pt−Re/Al2O3 catalysts with different radial distribution profiles

Pt−Re/Al2O3 prepared under different impregnating conditions were characterized by TPR and tested in naphtha and n-heptane reforming. Catalysts showed different Pt and Re radial distributions and reduction profiles. When both metals interact, higher dehydrocyclization and conversion are retained after a deactivation step, meanwhile, when Re is segregated from Pt the catalytic performance is negatively affected.

24-Hydroxylation of 1,25-dihydroxyergocalciferol. An unambiguous deactivation process.

1,24,25-Trihydroxyergocalciferol was isolated from bovine kidney homogenates incubated with 1,25-dihydroxyergocalciferol and from chick kidney homogenates incubated with 24,25-dihydroxyergocalciferol. The identity was established by ultraviolet absorbance, sensitivity to periodate, nuclear magnetic resonance, and mass spectrometry. The new metabolite had an affinity equal to 1,24,25-trihydroxycholecalciferol for the bovine-thymus and chick-intestinal 1,25-dihydroxyvitamin D receptor and had an affinity twice that of 1,24,25-trihydroxycholecalciferol for the rat-intestinal receptor. It was 3- and 6-fold less competitive than either 1,25-dihydroxycholecalciferol or 1,24,25-trihydroxycholecalciferol, respectively, for the rat plasma vitamin D transport protein. 1,24,25-Trihydroxyergocalciferol was at least 10-fold less active than 1,25-dihydroxycholecalciferol, 1,25-dihydroxyergocalciferol, and 1,24,25-trihydroxycholecalciferol at stimulating intestinal-calcium transport and was also relatively ineffective at stimulating bone-calcium resorption in rats. Moreover, in rats, [3H]1,24,25-trihydroxyergocalciferol was cleared from plasma approximately 40% faster than [3H]1,24,25-trihydroxycholecalciferol. These data suggest that C-24 hydroxylation of 1,25-dihydroxyergocalciferol represents a significant in vivo deactivation step, whereas equivalent deactivation of 1,25-dihydroxycholecalciferol seems to involve metabolic steps subsequent to C-24 hydroxylation (C-24 ketonization). C-24 ketonization of 1,25-trihydroxyergocalciferol would not be anticipated due to the presence of the 24(S)-methyl group. These results reveal further dissimilarities between ergocalciferol and cholecalciferol metabolism in mammals and suggest a mechanism for the lesser tendency of ergocalciferol to cause hypercalcemia relative to cholecalciferol.

Stabilization of subunit enzyme by intersubunit crosslinking with bifunctional reagents: studies with glyceraldehyde-3-phosphate dehydrogenase

Enzyme subunits, the first step of the conformation deactivation of which is the reversible dissociation into catalytically inactive subunits, can be stabilized against dissociative deactivation by intramolecular intersubunit crosslinking. Maximal stabilization can be achieved by using bifunctional crosslinking reagents whose molecular length corresponds to the distance between the protein functional groups to be crosslinked on different subunits. Glyceraldehyde-3-phosphate dehydrogenase (E.G.1.2.1.12), consisting of four identical subunits, can be stabilized against deactivation by using different diacids of HOOC-(CH 2) n -COOH type which interact with protein amino groups after preliminary activation with water-soluble carbodiimide. Maximal stabilization was observed in the case of succinic acid. The presence of intersubunit crosslinkages was proved by the data from SDS-polyacrylamide gel electrophoresis under dissociative conditions, following the intensity of the bands corresponding to enzyme oligomers. The disappearance of the dissociation step in the two-step process of enzyme inactivation after subunit crosslinking was also revealed by kinetic experiments. New methods of ESR spectroscopy (free radical recombination) were used to study the mobility of native and crosslinked protein. It was shown that the decrease in intramolecular protein mobility correlates directly with the increase in its stability against dissocation.

Fall-off behavior of the separable exponential Kassel model for thermal unimolecular reactions

For thermal unimolecular reactions a fairly realistic model can be constructed by combining the separable exponential model for the collisional activation and deactivation steps with the Kassel model for the reactive steps. The resulting model admits of interesting results which can be expressed analytically. It is argued that the model has a well defined strong collision limit, and an equation for the efficiency factor β̄ for weak colliders is then derived. This factor is a function of the ratio of the rate coefficient κ to its high pressure limiting value κ∞. The model qualitatively, albeit not quantitatively, reproduces the experimental behavior of β̄(κ/κ∞) for methyl isocyanide isomerization in helium at 518 K. Experiments and previous computations both show β̄(κ/κ∞) to decrease at a decreasing rate. The separable exponential-Kassel model gives β̄’s which either increase or decrease with κ/κ∞, but increasing β̄’s occur only for physically unrealistic values of one of the model’s parameters. The model shows that the decreasing rate of decrease of β̄ is related to the small value of the Kassel parameter s for methyl isocyanide isomerization. It predicts that β̄ decreases at an increasing rate for reactions with large s.

Straightening of the ends of glass capillary columns

After more than 10 years of controversy, it has becomer accepted that a capillary column should be directly connected to the injector and detector without the user of intermediate tubing. Although this is feasible with fused silica columns, with glass capillary column it is necessary for the ends of the columns to be straightened. For most types of column, with moderate to high polarities, end straightening is simple and can be effected with a small flame followed by a Carbowax deactivation step, but for persilylated inert high-temperature columns this procedure leaves residual activity in the end sections. This activity, which cannot be eliminated, destroys the inertness of the whole column. A new electrical end-straightening device, which operates at far lower temperatures than those attained when flames are used, has been developed. At these lower temperatures no carbonization of a polar silcone stationary phases occurs. Consequently, there is no reactivation of the persilanized glass surface. Straightening of a new column is accomplished simply by pushing the end throuh the straightening device (no previous removal of the liquid phase and no deactivation after straightening are necessary). Full inertness, even of very thick film colums, is preserved. The design and use of the new device are discussed.

Chemically activated methylcyclobutane from the 337.1 nm photolysis of ketene in the presence of cyclobutane. Dependence of S/D on extent of photolysis

The 3371 A photolysis of ketene/cyclobutane/oxygen mixtures gave stabilization/decomposition ratios for chemically activated methylcyclobutane that varied with the extent of photolysis. Experiments designed to circumvent this complication gave a 10 ± 2 kcal/mole collisional deactivation step size for cyclobutane.

Photosensitized cis—trans isomerization of nitrostilbenes in solvents of different polarity

The triplet sensitized cis—trans isomerization of 4-nitro-4′-methoxystilbene (NMS) in non-polar and polar solvents has been studied using 13 selected sensitizers. The photostationary cis/trans ratio at various triplet energies of the sensitizer and photoisomerization quantum yields are given. The fraction α′ of NMS molecules in the planar and twisted triplet states decaying to the trans ground state is determined by two different methods. In the first α′ is calculated from the cis/trans ratios employing high-energy sensitizers. In the second method α′ is obtained from quenching measurements with ferrocene using naphthalene sensitized and directly excited photoisomerization conditions. For the latter method no assumptions about the rates of energy transfer from the sensitizer to the stilbene and about the yields of energy wastage are necessary. Results from both methods are in reasonable agreement: the value of α′ increases with increasing solvent polarity. The decrease of the quantum yield φ t→c for the direct trans → cis isomerization of NMS and of 4-nitro-4′-dimethylaminostilbene (NDMAS) with increasing solvent polarity is accounted for by an increase of α′ and by a radiationless deactivation step from the excited trans singlet state to the trans ground state bypassing the triplet state. Results for 4-nitrostilbene, 4,4′-dinitrostilbene, 4-cyano-4′-dimethylaminostilbene and 4-cyano-4′-methoxystilbene in solutions of cyclohexane, benzene and/or methanol are also presented. For these stilbenes, in contrast to NMS and NDMAS, α′ depends only slightly on substituents and solvents.

Multistep collisional deactivation of chemically activated ethylcyclobutane

The photolysis of ketene in the presence of methylcyclobutane, cyclohexane, and oxygen was investigated at pressures between 2 and 0.16 Torr at 298/sup 0/K. Chemically activated ethylcyclobutane was formed at energies approximately 50 kcal/mol in excess of the threshold energy for unimolecular decomposition to ethylene and butene-1 by the addition of CH/sub 2/(/sup 1/A/sub 1/) to the methyl group of methylcyclobutane. Cyclohexane, which was present as an internal standard, added CH/sub 2/(/sup 1/A/sub 1/) to yield methylcyclohexane which decomposed by at most 7 percent at the lowest pressures. Experimental evidence was obtained that the collisional deactivation of hot ethylcyclobutane occurs by a multistep process with methylcyclobutane as the principal collision partner. Theoretical calculations, using RRKM theory and a stepladder deactivation model, showed that the data could be fit by deactivation step sizes in the range from 1.5 to 4 kcal/mol. Possibilities for small values of ..delta..E are briefly mentioned.

Reaction of singlet methylene with methylenecyclopropane. Part 2.—High energy reaction pathways for chemically activated insertion products

The reaction of singlet methylene, formed by the photolysis of keten (λ≃ 313 nm) with methylenecyclopropane has been investigated over the pressure range, 1–300 Torr, but predominantly at the lower end. The products are ethylene, propylene, acetylene, allene, spiropentane, methylenecyclobutane, methylmethylenecyclopropane (MMCP), ethylidenecyclopropane (ECP), isoprene and various penta-1,2-, penta-1,3- and penta-1,4-dienes. Studies of the pressure dependences of acetylene, isoprene and the pentadienes indicate that they originate from the chemically activated insertion products MMCP and ECP. Detailed consideration of the pressure dependences of the acetylene and isoprene yields in conjunction with RRKM calculations, provides support for multistep collisional deactivation in this system. A stepladder model with a deactivation step size of ≃ 7.2 kcal mol–1 fits the data.Further RRKM calculations are used together with this model to provide approximate values for the critical energies of the pentadiene forming pathways.

Diffusion processes in the slow oxidation of isobutane

The slow oxidation of isobutane has been investigated in a static system at 300 °C and 330 °C in the presence of the diluents nitrogen and carbon dioxide. Product analysis by means of gas chromatography shows that the main primary products are ethylene, propylene, isobutene, acetaldehyde, propylene oxide, isobutene oxide, propionaldehyde, isobutyraldehyde, acetone, and t-butanol, with trace amounts of acrolein, methyl oxetane and methacrolein. A mechanism is proposed for the formation and distribution of products based on diffusional studies. The reaction scheme involves a deactivation step whereby vibrationally-excited butylperoxy radicals can undergo deactivation by collision with surrounding gas molecules.

Reaction of singlet methylene with methylenecyclopropane. Part 1.—Evidence for multistep collisional deactivation of chemically activated spiropentane

The reaction of singlet methylene, formed by the 325 nm photolysis of ketene, with methylene-cyclopropane has been investigated over a wide range of pressures. The products are ethylene, allene, spiropentane, methylenecyclobutane, methylmethylenecyclopropane, ethylidenecyclopropane and isoprene. Their formation was interpreted in terms of decomposition, rearrangement and partial stabilisation of the initially formed products of addition and insertion. Studies of the pressure dependence of the product distribution, despite its complexity lead to the unequivocal conclusion that chemically activated spiropentane molecules can not be characterised by a unique lifetime. RRKM calculations used in conjunction with a step-ladder model for collisional deactivation give a fit to the data for a deactivation step size of 7.20 kcal mol–1 at an energy of 105.5 kcal mol–1 for the initially formed spiropentane. This result is consistent with a figure of 101 kcal mol–1 for the heat of formation of methylene, although uncertainties, particularly in the entropy of activation for spiropentane isomerisation, mean that a lower value cannot be ruled out.

Photolysis of ketene in the presence of propylene: Multistep collisional deactivation of methylcyclopropane and the excess energy of singlet methylene

The photolysis of ketene in the presence of propylene-5% oxygen mixtures was investigated at average wavelengths of 3550, 3340, 3130, and 2680 Å, and ketene photolysis in the presence of propylene-n-pentane-5% O2 mixtures was investigated at average wavelengths of 3130 and 2680 Å. Reaction pressures ranged approximately between 10 and 760 torr. Chemically activated methylcyclopropane, arising from singlet methylene addition to propylene, isomerized to the four butene isomers at each wavelength. The rate of isomerization increased monotonically with decreasing wavelength, demonstrating that the methylcyclopropane average energy increased with increasing photon energy. Of the chemically activated butenes formed either by methylcyclopropane isomerization or by insertion of singlet methylene into the carbon-hydrogen bonds of propylene, butene-1 decomposed significantly at the lowest pressures. A multistep process was required to describe the collisional deactivation of chemically activated methylcyclopropane. Theoretical analysis of data at 3550 Å, where methylcyclopropane was considered to have an average energy of 93 kcal mole−1, gave a deactivation step size of 1.5 kcal mole−1 per collision for a stepladder model. Extension of the theoretical treatment to shorter wavelengths predicts that singlet methylene retains only 3 kcal mole−1 of the approximately 25 kcal mole−1 of excess photon energy available at 2680 Å when it adds to the propylene double bond.

Inter-Relations of Activation, Deactivation and Destruction Steps in Chemical Kinetics

Inter-Relations of Activation, Deactivation and Destruction Steps in Chemical Kinetics