Attack cavity

Stannous fluoride is used in toothpastes and dental rinses to protect tooth enamel from attack by bacteria—cavities (also known as dental caries). It was the first fluoride used for that purpose, in the toothpaste Crest. 

Ad(ii) On catalysts with pores and cavities of molecular dimensions, exemplified by mordenite and ZSM-5, shape selectivity provides constraints of the transition state on the S 2 path in either preventing axial attack as that of methyl oxonium by isobutanol in mordenite that has to "turn the comer" when switching the direction of fli t through the main channel to the perpendicular attack of methyl oxonium in the side-pocket, or singling out a selective approach from several possible ones as in the chiral inversion in ethanol/2-pentanol coupling in HZSM-5 (14). Both of these types of spatial constraints result in superior selectivities to similar reactions in solutions. 

High-temperature/low-pressure inorganic digestions are an area of application that has benefited from recent advances in vessel and sensor design. The inert properties of Teflon and its resistance to acid attack make it the material of choice for microwave pressure-vessel construction. Improved commercial systems offer additional safety precautions and improved facilities for pressure and/or temperature control. Also, the distribution of microwave radiation inside the oven cavity is fairly homogeneous. Low-pressure systems allow decomposition temperatures of about 180 °C. However, for many matrices, such temperatures are not sufficient to guarantee the complete ashing of thermoresistant sample components. 

Matsue et al. [43] attempted to study the molecular rocket reaction in a ruthenocene-/ -cyclodextrin inclusion compound using the I00Ru y, p) "raTc reaction. They noticed a parallel relationship between chemical processes and nuclear-recoil-induced processes in the non-included ruthenocene compound, as shown in Fig. 9. In the nuclear-recoil-induced processes no dimerization can be observed because of the extremely low concentration of the product, whereas in the chemical processes dimerization is possible, as demonstrated by Apostolidis et al. [48]. When ruthenocene included in /J-cyclodextrin is irradiated with y-rays, a part of the ruthenocene molecule is converted to [TcCp2-] which escapes from the jS-cyclodextrin cavity. The [TcCp2] rocket thus produced can attack neighboring inclusion compounds so as to extract the enclosed ruthenocene molecules and abstract H or Cp (Cp cyclopentadienyl radical). This process is shown schematically in Fig. 10. 

As a simple model for the enzyme penicillinase, Tutt and Schwartz (1970, 1971) investigated the effect of cycloheptaamylose on the hydrolysis of a series of penicillins. As illustrated in Scheme III, the alkaline hydrolysis of penicillins is first-order in both substrate and hydroxide ion and proceeds with cleavage of the /3-lactam ring to produce penicilloic acid. In the presence of an excess of cycloheptaamylose, the rate of disappearance of penicillin follows saturation kinetics as the cycloheptaamylose concentration is varied. By analogy to the hydrolysis of the phenyl acetates, this saturation behavior may be explained by inclusion of the penicillin side chain (the R group) within the cycloheptaamylose cavity prior to nucleophilic attack by a cycloheptaamylose alkoxide ion at the /3-lactam carbonyl. The presence of a covalent intermediate on the reaction pathway, although not isolated, was implicated by the observation that the rate of disappearance of penicillin is always greater than the rate of appearance of free penicilloic acid. 

An additional example is the observed moderate acceleration in the cleavage of particular phenyl esters in the presence of a cyclodextrin. In such cases, the bound ester is attacked by an hydroxyl group on the cyclodextrin to yield a new ester. There was found to be a significant enhancement of phenol release from meta-substituted phenyl acetate on interaction with cyclodextrin (relative to other esters which do not fit the cavity so well) (Van Etten, Clowes, Sebastian Bender, 1967). During the reaction, the acyl moiety transfers to an hydroxyl group on the... 

Examples of dramatic reductions in rates of proton removal are found if the acidic proton is protected within a molecular cavity and if an unfavourable conformational change is required to expose the proton to attack by base. This type of complex proton transfer is described in Section 5. 

Based on the three-dimensional structure of CHS, we proposed that the initiation/elongation/cyclization cavity serves as a structural template that selectively stabilizes a particular folded conformation of the linear tetraketide, allowing the Claisen condensation to proceed from C6 to Cl of the reaction intermediate.14 In contrast, CTAL formation can occur either in solution or alternatively while sequestered in the enzyme active site. In either case, enolization of the C5 ketone followed by nucleophilic attack on the Cl ketone with either a hydroxyl group (in solution) or the cysteine thiolate (enzyme bound) as the leaving group results in CTAL. Similar lactones are commonly formed as by-products of in vitro reactions in other PKS systems.36 38... 

However, this hypothesis does not explain the higher distribution of the cyclobutyl bromide as compared to cyclopropylcarbinyl bromide, since a distribution near 1 1 would be expected, if nucleophilic attack to the bicyclobutonium occurs in the same way as in solution. The different distribution, favoring the cyclobutyl bromide, may suggest that the bromide ion is not uniformly dispersed on the zeolite cavity, preferentially occupying certain positions on the zeolite surface, where it can better attack the bicyclobutonium at one of the three positions. 

The laboratory-scale fluoride transport tests revealed severe attack on the Pt plating of the electrodes. AEA is now seeking resolution of this problem with electrode manufacturers. Pt and Ti released in these attacks may plug electrode cavities or impact membrane performance. 

Blowfly maggots that infest or blow the wool of sheep are especially common in warm humid areas, but occur wherever sheep are raised. Several species are involved, but the most troublesome species in Africa is Lucilia cuprina. The flies are attracted either to open wounds or to fermenting wool, the latter a condition that arises when sheep in full fleece are subjected to frequent rains during warm weather. While Lucilia cuprina apparently confines its attack mostly to the wool and the serous fluid that exudes from the skin of infested sheep, some of the associated species may attack the skin and flesh with serious or fatal result. Lucilia sericata has been observed to penetrate the abdominal wall and enter the peritoneal cavity. 

The one-electron oxidation of iV-benzylphenothiazine by nitric acid occurs in the presence of /i-cyclodextrin, which stabilizes the radical cation by incorporation into its cavity. The reaction is inhibited by adamantane, which preferentially occupies the cavity. Novel Pummerer-type rearrangements of / -sulfinylphenyl derivatives, yielding /7-quinones and protected dihydroquinones, and highly enantioselective Pummerer-type rearrangements of chiral, non-racemic sulfoxides have been reviewed. A comprehensive study has demonstrated that the redox potential for 7- and 8-substituted flavins is linearly correlated with Hammett a values. DFT calculations in [3.3.n]pro-pellanes highlight low ionization potentials that favour SET oxidative cleavage of the strained central C-C bond rather than direct C-H or C-C bond attack. Oxidations and reductions in water have been reviewed. ... 

Fats and other lipids are poorly soluble in water. The larger the accessible surface is—i. e., the better the fat is emulsified—the easier it is for enzymes to hydrolyze it (see p. 270). Due to the special properties of milk, milk fats already reach the gastrointestinal tract in emulsified form. Digestion of them therefore already starts in the oral cavity and stomach, where lipases in the saliva and gastric juice are available. Lipids that are less accessible—e.g., from roast pork—are emulsified in the small intestine by bile salts and bile phospholipids. Only then are they capable of being attacked by pancreatic lipase [4] (see p. 270). 

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