Half-lifetime

The reasons of this behaviour were soon discovered by Schulz team29). One was purely technical. Under the conditions prevailing in the earlier experiments of Schulz and Lohr the polymerization was too slow for employment of the flow technique adopted by the authors in their earlier investigation, but too fast for the conventional batch technique. Development of a stirred reactor allowing studies of reactions with half-lifetime as short as 2 sec eliminated this difficulty 30). 

Lozano and co-workers reported an interesting stabilization effect of IL for lipase-catalyzed reaction the authors discovered that the presence of an appropriate substrate was essential for stabilization of enzyme in an IL solvent. The half lifetime of native CAL was only 3.2 h in [emim][PFg] solvent, while it lengthened remarkably to 7500 h in the presence of the substrate. The authors succeeded in demonstrating an efficient lipase-recyclable use system based on SCCO2 solvent (Fig. 9). - ... 

Isotope Energy Eg (keV) Half-lifetime (ns) Observed by various methods ... 

The texture of the catalysts also controls the half-lifetime of charge carriers. This can be easily obtained from microwave absorption experiments using a time-resolved microwave conductivity... 

Assuming that lipid hydroperoxides are generated easily in cell membranes, these considerations point to a potential role of O2 in the cytotoxicity mechanism of LAOOH. Taking into account the longer half-lifetime of peroxyl radicals (7 seconds) compared to those of other reactive oxygen species (which for LAO", LA", 02" , O2, HO" are 1 X 10, 1 X 10 , 1 X 10 , 1 X 10, 1 X lO seconds, respectively) , this allows for the migration of a fatty acid peroxyl radical to other sites, where it can combine with another peroxyl radical and generate O2. 

Nevertheless, the one-state model can be handled as a special case with either Aj = 0 or ki = k2. While, e.g. Upases show good stabilities in several organic reaction media [ 14], alcohol dehydrogenases are less stable, having half-lifetimes in microemulsions of only a few hours [34,103,104] or even only minutes [105]. The stability depends on the microemulsion composition and additionaUy on the components of the reaction mixture, like salts, buffers or impurities of the surfactant. 

The reciprocal of the half-lifetime yielded a value for the dye regeneration by Co (dbbip)22+ of about 5 x 105 s-1 against 1.7 x 106s 1 observed in the case of 0.1 M tetrabutylammonium iodide. Contrary to the case of iodide,56 a cooperative effect of adsorbed cations was not observed in the case of cobalt, for which an addition of Li+... 

Several new facets of the chemical and physical behavior of mutagens isolated from food and pyrolysates have been noted recently. Trp-P-1, Trp-P-2, and Glu-P-1 are rapidly deaminated upon incubation with nitrite at acid pH (54). At pH 1.6, in 50 yM nitrite, the half lifetime of Trp-P-1 and Trp-P-2 is approximately 100 min, but less than 5 min for Glu-P-1. AdC is also deaminated in 1 mM sodium nitrite at pH less than four with the difference that longer incubation, for 1.5 h, leads to the formation of a directly mutagenic nitroso derivative (55). These reaction conditions approach those in the stomach (pH 1-2, 0-10 yM nitrite), but careful kinetic studies in vivo will be required... 

The most active catalysts for halogenation with perhalomethane are, evidently, quaternary ammonium fluoride salts. One mol% of tetra- -butylammonium fluoride trihydrate (TBAF) can quantitatively transform a malonate ester to an a-chloro derivative at 25 °C with a half-lifetime of 10 minutes (equation 77)628,629 ... 

At 30°C in the absence of Arg, the ferrous-oxi complex transforms very slowly to the ferric state. In the presence of substrate and H4B, a new species with the 12-nm shifted Sorey band is detected. A decay of this species is accompanied by the formation ofN -hydroxy-L-arginine. Because the presence of HUB is necessary for these reactions, the main function of this compound is to be a reducing agent. This suggestion is supported by experiments on the stabilizing effect of ascorbic acid on the chemical stabilization of tetrahydropterin in the endothelial nitric oxide synthesis (Heller et al., 2001). At the same time, a significant increase in the half lifetime of H4B in solution is demonstrated. As is shown (Wei et al., 2001), a ferrous-dioxy intermediate in iNOS forms for 53 s 1 and then is transformed to the [S-Fe(IV)=0] state. The rate of the [S-Fe(IV)=0] decay is equal to the rate ofH4B radical formation and the rate of Arg hydroxylation. In contrast,... 

Pig. The plot of rate versus concentration for a zero-order reaction The reaction rate does not vary with concentration of reactant. Reaction and Half-lifetime... 

In order to characterise the rate at which a chemical reaction may proceed it is customary to introduce a convenient parameter called the half-lifetime of the reaction. It is defined as the time required for the reaction to be half completed and is denoted by the symbol, f1/2 It can be related to the corresponding rate constant. 

Example The rate-constant for a first-order reaction is 1.54 10 3 s 1. Calculate its half-lifetime. 

The bottleneck of very short lifetimes of resonace states (10 14s) becomes less severe once one assumes that the primary role of resonance states is to provide doorways to bound valence anionic states, with lifetimes determined by kinetics of the following chemical reactions [36], The reactions might proceed on these regions of potential energy surfaces, at which valence anions are bound with respect to the neutral species. The rates of these chemical transformations, e.g., the SSB formation, do not have to compete with short lifetimes of resonance states. It is worth noting that even for a kinetic barrier of ca. 20 kcal/mol, the half lifetime amounts (at 298 K) to about 30 seconds. Hence, if the kinetic barrier for SSB formation were lower than 20-23 kcal/mol, all nucleotides that could form stable anions would have enough time to cleave the C-O bond on the timescale of the electrophoretic assay of DNA damage. 

Characterize the structure and dynamics of active sites in enzymes and the correlated motions of secondary and tertiary structures. Measure half-lifetimes of individual steps of electron- and ion-transport during catalytic cycles. Synthesize ligands for metal centers and functionalize inorganic pores to attain enzyme-like activity and selectivity with inorganic-like robustness. 

Unfortunately, the dependence of the rates on pH cannot be measured by our method since acidimetric titrations are not possible in buffered solutions. However, in weakly acidic and neutral solutions, there cannot be much influence because otherwise no rate constants could have been obtained. In alkaline solution, on the other hand, a base induced fragmentation occurs, which is faster by some powers of 10 than the solvent fragmentation. Thus, in 0.025N methanolic NaOH at 0°C, the half lifetime of stilbene ozonide is only 2 minutes. 

Eem values are inherently dependent on the initial concentration of the pollutant or parameter to be removed. This is because the half lifetime ti/2 of a zero... 

Carbenium ions with a-aromatic substituents (e.g., styrene derivatives) absorb at very different UV-visible wavelengths compared to the corresponding monomers and polymer chains. In such cases, UV-visible spectroscopy can be used to quantitatively determine the concentration of growing carbenium ions. Because the extinction coefficients of these carbenium ions are typically e = 10,000 to 30,000 moI l L cm 1, they can be detected only at concentrations higher than [C + ] = 10-6 mol, -L. In some of the systems discussed in this chapter, the concentration of carbenium ions are below the detection limit of UV. On the other hand, those systems with carbenium ion concentrations [C+] = 10-5 mol-, L will propagate very rapidly because kp 105 mol" Lsec-. The resulting half-lifetimes 1 sec) will require rapid detection techniques, such as... 

The equilibrium constants of ionization are relatively low in most polymerization systems, resulting in very small proportions of ionic species. The proportion of ionic species may be estimated from the overall polymerization rates by assuming that covalent species are inactive. In polymerization systems with a half-lifetime of monomer in the range of r 1=5 20 min to 3 hr, the concentration of propagating ionic species should... 

Rate of polymerization should be adjusted to a range convenient for synthetic manipulations (half-lifetimes in monomer consumption minutes to hours)... 

Table 10.3 Field intensity half-lifetime and practical use of antistatic finished textiles ...

Figure 16. Time course of the transient absorbance changes measured upon laser excitation of cis-[Ru"(dcbpy)2(NCS)2] dye adsorbed on nanocrystalline Ti02 films. Bleaching signals were measured at A = 520 nm in anhydrous propylene carbonate, a) without electrolyte, and in the presence of b) 0.1 M TBAI, c) 0.1 M Lil, and d) 0.05 m Mgl2. Insert, dependence of the half-lifetime t /2 of the dye ground-state absorbance recovery upon the concentration of Li+ cations. Iodide concentration of [I ] = 0.1 M was kept constant while pLi = -log[Li+] was varied.

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