Slow sites

TPD studies tend to depopulate the molecular state sites during the Initial evacuation or carrier gas sweep. Thus, they are only sampling the slow sites which may well be dissociative as we have not examined this frequency region. The presence of an Intermediate molecular adsorbate would allow us to easily rationalize the possible change In observed molecularlty of the FRC results as the temperature was raised In that the dissociative sites would become accessible at the higher temperatures. 

For NOx storage, a model with two types of the NOx storage sites (the fast and slow sites) was also considered, however, the average NOx storage site... 

We have focused on the mobility as the main parameter characterizing charge transport. The photocurrent transients, however, typically extend well past the nominal transit time (ft), so that the nominal mobility extracted from ft underestimates the time required for charge to be transported across a photoreceptor film. A proper characterization of a material, therefore, requires some quantity indicating the width of the tail relative to ft [72a], As discussed previously, the tail is believed to arise because individual carriers sample various numbers of slow sites ,... 

In five sifes, TBA biodegradation was very slow (site 10) or not confirmed affer a second addition of TBA (sites 5, 6, 12, 13). The case of sites 5 and 6 is interesting because the same behavior was observed in the presence of MTBE and a similar explanation involving a cometabolic process could be proposed (see above, under MTBE biodegradability). 

The in vivo stability of a natural polyelectrolyte complex membrane, such as is formed between alginate and polylysine, (or even between synthetic polyelectrolytes) must never be assumed due to slow site-by-site displacement reactions which may occur with high molecular weight polymers (proteins, etc.) present in body fluids, and to processes of hydrolysis, enzymatically promoted or otherwise which may disrupt the membrane. 

There are two types of receptor, termed fast and slow sites [21]. The fast responses (detectable in <5 min) appear to be brought about by membrane-mediated phenomena, while the slow responses, which involve protein synthesis, are not detectable within the first half hour. The receptor types have different molecular requirements, and the fast reaction is not a pre-requisite for the slow. Growth assays employed to assess activity include Avena coleoptile, lettuce hypocotyl, rice seedling, and bean axis. Other types include lettuce seed and wheat embryo germination, transpiration assays, leaf disk senescence, and more recently, a-amylase production. Stomatal closing using epidermal strips is an assay for the fast receptor. 

One possible reason for the activity of R-ABA at a slow site, but not at the fast type, is that there is interference with receptor essential volume for the fast type, but not for the slow. This is shown in Milborrow s model [18] (Fig. 5), which has the side-chain equatorially oriented. With the side-chain anchored, inversion of configuration causes the 7 -methyl and the 6 -methyls to be on opposite sides, as shown. The axial methyl of R-ABA interferes in the fast site but not the slow, which is postulated as being more accommodating. Because it is now known that PA has significant physiological activity... 

The sensitivity of two isoschizomers to these resistant or slow sites can also be different. For instance, certain CTCGAG sites are totally refractory to PaeR7, whereas they can be cut by Xhol (16). 

C. It is secreted along with noradrenaline by the adrenal medulla, from which it may be obtained. It may be synthesized from catechol. It is used as the acid tartrate in the treatment of allergic reactions and circulatory collapse. It is included in some local anaesthetic injections in order to constrict blood vessels locally and slow the disappearance of anaesthetic from the site of injection. Ultimately it induces cellular activation of phosphorylase which promotes catabolism of glycogen to glucose. 

The sequence of events in a surface-catalyzed reaction comprises (1) diffusion of reactants to the surface (usually considered to be fast) (2) adsorption of the reactants on the surface (slow if activated) (3) surface diffusion of reactants to active sites (if the adsorption is mobile) (4) reaction of the adsorbed species (often rate-determining) (5) desorption of the reaction products (often slow) and (6) diffusion of the products away from the surface. Processes 1 and 6 may be rate-determining where one is dealing with a porous catalyst [197]. The situation is illustrated in Fig. XVIII-22 (see also Ref. 198 notice in the figure the variety of processes that may be present). 

Single molecules also have promise as probes for local stmcture when doped into materials tliat are tliemselves nonfluorescent. Rlrodamine dyes in botli silicate and polymer tliin films exliibit a distribution of fluorescence maxima indicative of considerable heterogeneity in local environments, particularly for the silicate material [159]. A bimodal distribution of fluorescence intensities observed for single molecules of crystal violet in a PMMA film has been suggested to result from high and low viscosity local sites witliin tire polymer tliat give rise to slow and fast internal conversion, respectively [160]. 

The Ru surface is one of the simplest known, but, like virtually all surfaces, it includes defects, evident as a step in figure C2.7.6. The observations show that the sites where the NO dissociates (active sites) are such steps. The evidence for this conclusion is the locations of the N and O atoms there are gradients in the surface concentrations of these elements, indicating that the transport (diffusion) of the O atoms is more rapid than that of the N atoms thus, the slow-moving N atoms are markers for the sites where the dissociation reaction must have occurred, where their surface concentrations are highest. 

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

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