Reversible Trapping

The time lag for membrane permeation can again be determined by extrapolating the anodic charge-time curve to the time axis, but when reversible trapping is present, is given by 

Oriani also derived an expression for the apparent diffusivity, assuming from the outset that the hydrogen atoms in traps and in normal lattice sites are in local equilibrium. The apparent diffusivity was obtained as a function of the concentration of mobile hydrogen. 

When the traps near saturation ( - 1, so a and are large), the apparent diffusivity increases with concentration and approaches 

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Azide ion is a modest leaving group in An + Dn nucleophilic substitution reactions, and at the same time a potent nucleophile for addition to the carbocation reaction intermediate. Consequently, ring-substituted benzaldehyde g m-diazides (X-2-N3) undergo solvolysis in water in reactions that are subject to strong common-ion inhibition by added azide ion from reversible trapping of an o -azido carbocation intermediate (X-2 ) by diffusion controlled addition of azide anion (Scheme... 

Sialic acid seems to be involved not only in regulation of the lifetime of soluble, serum glycoproteins but also of mammalian blood-cells. It was observed by Woodruff and Gesner474 that desialylated lymphocytes are reversibly trapped in liver they recirculate to the blood stream after about 24 h. This phenomenon was confinned with Listeria-specific, mouse T lymphocytes, which accumulated in the liver for one day, in contrast to the control cells.60 Reappearance of these cells in the circulation after one day may be explained by re-sialylation of their membrane glycoconjugates. This time period is in the range observed for the turnover of sialic acid in cell membranes, lasting, for example, for 33 h in rat-liver hepatocytes.475... 

Scheme 10.7 Reversible trapping of the cumyl radical by the nitroxide, 1,1,3,3-tetraethylisoindoline-2-oxyl.

Nitroxide mediated polymerization (NMP) is another type of controlled radical polymerization technique used to synthesize polymer hybrids. It relies on the reversible trapping of growing macro-radicals by nitroxide to form dormant species in which the C-ON covalent bond is thermally cleavaged (Fig. 19). At a polymerization temperature, the equilibrium between dormant and active species is strongly shifted to the dormant side, which Emits the irreversible chain termination reaction. 

Ochiai B, Yokota K, Fujii A et al (2008) Reversible trap—release of C02 by polymers bearing DBU and DBN moieties. Macromolecules 41(4) 1229-1236... 

In the presence of reversible trapping, according to the altered conductivity expression in Eq. (91), there can now be a chemical diffusion of oxygen even if creon = 0, simply by a neutral oxygen flux but also by a counterflux of CF and 20 . The modifications referring to the d//0/dc0 term have been shown to have extremely... 

Kinetic modeling was used to estimate the depth of the fast, reversible traps in PQT-12. We assume that the square law observed during trapping is indicative of a trap state made of two correlated holes (i.e. a bipolaron, see Section 5.3.4). Making use of detailed balance we obtain ... 

The recovery rate of the reversible traps determines the duty-cycle cut-off below which only long-lived traps contribute to bias stress. At higher duty cycle, the current decay is the result of the simultaneous interplay of fast and slow trapping. 

These structural characteristics mean that the dendrimer porphyrin can be used to mimic the function of the heme protein - its ability to bind to oxygen. A dendrimer porphyrin with an Fe(II) ion can stably trap oxygen via coordination with imidazole ligands. The oxygen was reversibly trapped within the dendrimer, and it can be released when the oxygen in the surrounding solvent was removed. The dendrimer sphere shields the porphyrin part from the outer environment. Therefore, side effects such as irreversible oxidation of the porphyrin by water and dimerization of the oxygen-bound porphyrins can be suppressed. 

Fig. 7. Reversible trapping of synaptic vesicle membrane in the plasma membrane in reticulospinal synapses. (A) Electron micrograph of a lamprey reticulospinal synapse stimulated with action potentials at 20 Hz for 20 min and then incubated for 90 min in Ca " -free solution with 10 mM EGTA. Note the reduction in the number of synaptic vesicles and the presence of large membrane expansions compared to an unstimulated synapse (inset). (B) Activation of clathrin-mediated endocytosis in reticulospinal synapses by addition of Ca +-containing extracellular solution. Spinal cord preparations were stimulated at 20 Hz for 20 min, incubated for 90 min in Ca -free solution, and then incubated in Ca -containing solution (2.6 mM) for 120 s. Electron micrograph of a synapse shows the appearance of coated pits (arrows) lateral to the active zone. Designations as in Fig. 1. Scale bar, 0.2 p.m. Modified from Gad et al. (1998) Neuron 21 601-616, with permission copyright is held by Cell Press.

Another carbon dioxide insertion reaction is observed with Al (TPP)(OMe) This compound readily and reversibly traps carbon dioxide at room temperature in the presence of 1-methylimidazole. The trapped carbon dioxide is sufficiently activated to react with an epoxide at room temperature, thus producing the corresponding alkylcarbonate. As illustrated in Scheme 11, the cyclic carbonate is considered to be formed - at least partly - by nucleophilic attack on a linear intermediate. Thus the alkoxide aluminum porphyrin-methylimidazole system would be a good catalyst for synthesis of alkylene carbonates from carbon dioxide and epoxides under mild conditions. 

A general model for hydrogen diffusion with reversible trapping was formulated by McNabb and Foster [103]. The general diffusion equation can be written as... 

The time lag for membrane permeation with reversible trapping is given by... 

The effect of both irreversible and reversible traps can be represented by [104]... 

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