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Diffusion and hydrolysis

Onchidal and fasciculins are natural toxins, which produce their toxicity in mammalian systems by virtue of primarily acetylcholinesterase (AChE) inhibition. AChE hydrolyzes and inactivates acetylcholine, thereby regidating the concentration of the transmitter at the synapse. Termination of activation is normally dependent on dissociation of acetylcholine from the receptor and its subsequent diffusion and hydrolysis, except in diseases where acetylcholine levels are limiting or under AChE inhibition, conditions that increase the duration of receptor activation (Silver, 1963). [Pg.143]

It was also noted that at sufficiently high temperatures diffusion and hydrolysis may occur simultaneously (Abeysinghe et al. 1982) and that hydrolysis lowers Tg (Zhou and Lucas 1999). A theoretical model that employs probabilistic considerations for chain scissions by chemically reacted water, and thereby weight loss, had successfully predicted such combined weight gain/weight loss data (Xiao and Shanahan 1997). Those data correspond to curve D in Fig. 4.2. [Pg.15]

Table 4.1 Chapter 5). Wool fibres have been optimized to allow maximum recovery when unpacked and set. Also, their dissolution in vitro has been of utmost importance since the problems generated by asbestos (see water diffusion and hydrolysis of siloxane bonds Chapters 5 and 7). Biodegradable glass fibres have been developed accordingly (Chattier, 1997). [Pg.82]

Chromium(III) Chemistry. The most characteristic reactions of Cr(III) in aqueous solution at >4 pH, eg, in the intestine and blood, and hydrolysis and olation (147). As a consequence, inorganic polymeric molecules form that probably are not able to diffuse through membranes. This may be prevented by ligands capable of competing for coordination sites on Cr(III) (see Coordination compounds) (147). Thus any large fraction of ingested Cr(III) should be absorbed. Chromium (ITT) in the form of GTF may be more efficiendy absorbed. [Pg.387]

Although the reaction scheme shows a complete hydrolysis before condensation begins, this is likely not correct as stated earlier. The relative rates and extents of these two reactions will particularly depend on the amount of water added and the acidity of the system (10,11). The high functionality of the triethoxysilane endcapped PTMO oligomer should enhance the incorporation of PTMO molecules into the TEOS network. It was also assumed that the reactivities would be the same between silanol groups from silicic acid and endcapped PTMO. Therefore, no preferential condensation was expected and the deciding factors for which type of condensation (self- or co-) took place would be the diffusivities and local concentrations. [Pg.357]

Membrane deterioration may be merely caused by decrease of acetyl content(C ) in the active surface layer as a result of hydrolysis or oxidation, not by structure change. Analysis was carried out based on solution-diffusion model proposed by Lonsdale etal( ), using their measured values of solute diffusivity and partition coefficient in homogeneous membrnaes of various degree of acetyl content and also using those values of asymmetric membranes heat treated at various temperatures measured by Glueckauf(x) ... [Pg.123]

The sensing microzone of the flow-through sensor depicted in Fig. 5.9.B1 integrates gas-diffusion and detection with two analytical reactions [28], viz. (a) the urease-catalysed formation of ammonium ion by hydrolysis of urea (the analyte), which takes places on a hydrophilic enzyme membrane in contact with the sample-donor stream, which contains a gel where the enzyme is covalently bound and (b) an acid-b reaction that takes place at the microzone on the other side of the diffusion membrane and involves Bromothymol Blue as indicator. This is a sandwich-type sensor including a hydrophilic and a hydrophobic membrane across which the sample stream is circulated —whence it is formally similar to some enzyme electrodes. Since the enzymatic conversion of the analyte must be as efficient as possible, deteetion (based on fibre optics) is performed after the donor and acceptor streams have passed through the sensor. Unlike the previous sensor (Fig. 5.9.A), this does not rely on the wall-jet approach in addition, each stream has its own outlet and the system includes two sensing microzones... [Pg.273]

In some cases, e.g. the oxidation of hydrocarbons by potassium permanganate (Meyer and Saam, Ber. xxx. 1935, 1897), the hydrolysis of emulsions of esters in water CGoldschmidt, Zeit. Phys. Ghem. xxxi. 235, 1899), and the dissmution of arsenious oxide (Drucker, Zeit. Phys. Ghem. xxxvi. 693, 1901), the actual chemical reaction between solvent and solute appears to be slower than the process of diffusion, and thus the rate of chemical action is independent of the diffusion coefficient. [Pg.193]

Effect of diffusion and adsorption in methyl and ethyl acetate hydrolysis in water at 40°C... [Pg.378]

The studies cited do not clarify what factors determine rates of sulfate reduction in lake sediments. The absence of seasonal trends in reduction rates suggests that temperature is not a limiting factor. Rates of sulfate reduction are not proportional to such crude estimates of carbon availability as sediment carbon content or carbon sedimentation rate, although net reduction and storage of reduced sulfur in sediments often does increase with increasing sediment carbon content. Measured rates of sulfate reduction are not proportional to lake sulfate concentrations, and the relative rates of sulfate reduction and methanogenesis in a variety of lakes do not indicate that sulfate diffusion becomes limiting in eutrophic lakes. Direct comparison of diffusion and reduction rates indicates that diffusion of sulfate into sediments cannot supply sulfate at the rates at which it is reduced. Neither hydrolysis of sulfate... [Pg.336]


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See also in sourсe #XX -- [ Pg.377 , Pg.379 , Pg.381 , Pg.383 , Pg.385 ]




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