Preferential site

Another approach, the so-called seeding technique, provides preferential sites for the nucleation of scale, which permits the heat-transfer surfaces to remain clean of scale. Extensive studies of this technique have been conducted, and field use was reported ia the former USSR as early as the mid-1960s (42). The use of ion-exchange methods is another possible approach. Eor calcium, the exchange can be represented as... 

The exact site of anthocyanin absorption is not fully known recently it was suggested that the stomach is one of the preferential sites for the process in humans. Interestingly, anthocyanin glycosides are efhciently absorbed in rats (up to 25%) from both stomach " and small intestine, suggesting that anthocyanins have more than one site of absorption along the intestinal tract. 

Blat Y, Kleckner N 1999 Cohesins bind to preferential sites along yeast chromosome III, with differential regulation along arms versus the centric region. Cell 98 249-259... 

The presence of suspended solid materials increases the extent of LAS biodegradation [13,28], but the rate of the process remains invariable. The influence of the particulate material is due specifically to the increased density of the microbiota associated with sediments. However, suspended solids may also reduce the bioavailability of IAS as a result of its sorption onto preferential sites (e.g. clays, humic acids), although this is a secondary effect due to the reversibility of the sorption process. Salinity does not affect IAS degradation directly, but could also reduce LAS bioavailability by reducing the solubility of this molecule [5], Another relevant factor to be taken into account is that biodegradation processes in the marine environment could be limited by the concentration of nutrients, especially of phosphorus and nitrogen [34],... 

In humans, also, preferential sites exist for the formation of metastasis from various primary tumors [reviewed in Zetter (Zl)]. Thus, bone is a preferred site for metastasis from primary malignancies in breast, prostate, and kidney, while liver is a frequent metastatic site for tumors originating in the colon. Different types of leukemias vary widely in their ability to spread to liver, lymph, bone, and spleen. Some organs, however, are rarely colonized by metastatic growth. These resistant sites include skeletal muscle, heart, and skin. 

According to an analysis performed by Ferro and Cacciamani (2002), the preferential site occupancies observed in MgCu2-type phases are summarized in Table 3.8. The atoms preferentially present in each site type are indicated. 

Table 3.8. cF24-MgCu2 type Laves phases, (preferential site occupancies). 

Fig. 1.16 Particle size and grain size in a nanostructured powder. Black points depict hydrogen molecules ( ) in pores between particles, and hydrogen atoms ( ) diffusing through a grain boundary network and gradually entering the grain interior black triangles point to triple points that are preferential sites for hydrogen atom accumulation...

S.4.3.4 Order-disorder transformations. The previous examples considered strict site preference for the components in sublattice phases. For example, in the (Cr, Fe)2B compound, B is not considered to mix on the metal sublattice, nor are Cr and Fe considered to mix on the B sublattice. This strict limitation on occupancy does not always occur. Some phases, which have preferential site occupation of elements on different sublattices at low temperatures, can disorder at higher temperatures with all elements mixing randomly on all sublattices. 

Backbone chain scission degradation can be divided as occurring via depolymerization, random chain breakage, weak-link or preferential site degradation, or some combination of these general routes. In depolymerization, monomer is split off from an activated end group. This is the opposite of the addition polymerization and is often referred to as unzipping. ... 

Heteroatom (O and N) attachment to the C8-site of dG to form 8-oxo-dG and C8-arylamine adducts lowers the oxidation potential relative to dG. The oxidation potential of 8-oxo-dG is 0.74 V versus NHE. Consequently, 8-oxo-dG can act as a deep radical cation trap within duplex DNA. Depending on the DNA sequence, an 8-oxo-dG lesion will be the preferential site of further oxidation and will protect isolated Gs and GG steps from oxidation the oxidation of 8-oxo-dG by G(—H) occurs with a rate of 4.6 x 10 /M/s. Thus, there is speculation that GC-rich domains outside the coding regions of genes serve to protect the genome from mutagenesis by oxidation. ... 

For conjugated structures thermolytic reactions with loss of sulfur have been studied since 1980 and afford various types of cyclic and acyclic products (Section 4.11.5.1). Nucleophilic attack at ring carbon is very characteristic of 1,2,3-dithiazoles (Section 4.11.5.4). This type of reaction was especially prolific with Appel s salt, studied mostly by Appel, Rees and their co-workers. The preferential site of attack is C(5) but nucleophilic substitution may occur at C(4) when the C(5) site is blocked by poorly leaving substituents, see the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) for examples of 1,2,3-dithiazoles <84CHEC-I(6)924> and 1,2,3-oxathiazoles <84CHEC-l(6)930>. Nucleophilic attack at ring sulfur in 1,2,3-dithiazoles occurs on S(2) (Section 4.11.5.5), see... 

The stereochemistry of liganding of metal ions in proteins is now known for several proteins (see Armstrong, 1988). Some selected examples follow with data derived from the Protein Data Bank (Bernstein et al., 1977). In the cases in which two different metals are bound, information can be obtained on preferential sites for each metal in the presence of the other. 

All of the above discussion is strictly applicable only to homogeneous gas phase reactions. Usually the above considerations do apply reasonably well to non-polar liquids and nonpolar solutions, although normal Z values may be an order of magnitude less than for gas reactions. Reactions in solids are often much more complex, since they are usually heterogeneous, involve catalytic effects, reactions at preferential sites (dislocations, etc), and nucleation phenomena. These complicated processes are quite beyond the scope of the present article. For some description of these phenomena, and further references, the reader should consult Refs 9, 10 11... 

The central diazepine ring of the condensed indole derivative in 5.19. was built up by the electrophilic attack of the pyridylpalladium intermediate on the indole system. Since the preferential site of the attack, the 2-position of indole was blocked through substitution, the ring closure was directed into the pen-position, giving rise to the formation of a seven membered ring.18... 

V-Methylinudazole, when heated in DMF with bromobenzene in the presence of a palladium-triphenylphosphine catalyst and potassium carbonate furnished two products (6.87.) the 5-phenyl and the 2,5-diphenyl derivative. The product distribution suggests that the preferential site of the arylation is the more electron-rich 5-position.118 Prolonged heating in a polar, high boiling solvent in the presence of base is characteristic of such transformations. 

Molecular orbital calculations indicate that the lowest triplet state of benzene and its derivatives should be appreciably more basic than the ground states of the molecules.211 Furthermore, it is predicted that the preferential sites of protonation may be different in the excited states than they are in the ground states. 

Metal oxidation is a heterogeneous solid state reaction and starts in the same way as other heterogeneous reactions with nucleation and initial growth. This was discussed in Chapter 6. A time-dependent nucleation rate may dominate the overall growth kinetics of thin Films. Even under an optical microscope (i.e., in macroscopic dimensions), preferential sites of growth can still be discerned [J. Benard (1971)). This indicates that lateral transport on the surface (e.g., at sites where screw dislocations emerge) can possibly be more important for the initial reactive growth than transport across thin oxide layers. 

The areas where a test piece is attached to clamps and cut edges are preferential sites for cracking. It is generally good practice to coat clamped areas with an ozone resistant paint (which does not affect the rubber in any way) but cut edges are best left. For most purposes a Hypalon-based paint is satisfactory. Clamps, even when made of material such as aluminium, should be soaked in ozone prior to use. Any pattern or flaws on the test piece surface will also tend to act as stress raisers and show preferential cracking. 

The islands are formed partly by reaction of further molecules of sensitizer adsorbed around the edges of planar nuclei and partly by surface desorption and readsorption of Ag2S molecules. Vacancies around the edges of and within these monolayers provide preferential sites for the separation of silver atoms and mol-... 

When OR group is located inside the fluorocarbon chain, the corresponding vinyl ketone is the only product [172], and the CF(OCH3)-moiety is the preferential site for demethylation [171] ... 

Scheme 1. Mechanism of telomerisation of fluoroalkene F2C = CRR with RpOF (in order to generalise the formulae, the expression C2F2RR has been prefered to CF2CRR or CRR CF2 since the nature of R and R (which can be either electron-withdrawing or electron-donating groups) influences the sense of addition of RfO to a preferential site of the fluoroolefins).

In a similar way, Iwasaki et al. [145] modified the surface of polyamide 6 films or fibers after ozonization. These authors grafted in a second step hydrophilic monomers such as acrylamide, and also vinyl acetate or methyl methacrylate. Once more, oxidation mechanisms and preferential sites of attack of ozone have not been clearly described in the literature. 

The simple model predicts that the highly electronegative CF3- radical should add preferentially to CH2 in vinyl fluoride and CHF in trifluoroethylene. For the weakly electropositive methyl radical, either weak thermodynamic control or even contrathermodynamic control is predicted. The ab initio minimal basis set calculations fall in line with these qualitative predictions, except that for the attack of CH3 on trifluoroethylene no preferential site of attack is found. The activation energy differences calculated by Salem are compared with the experimental results in Table 21. Although the absolute values of the activation energies are too large, the differences reproduce qualitatively the experimental trend. 

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