Classification and Mechanism

W. W. Pigman, Specificity, classification and mechanism of action of the glycosidases, Adv. Enzymol. Relat. Subj. Biochem., 4 (1944) 41—74. 

Fig. 16. Classification and mechanism of origin of mucous substances of the stomach. From Glass and Boyd (G27).

PHOTORESISTS, THEIR LITHOGRAPHIC PROPERTIES, CLASSIFICATION, AND MECHANISMS TO PRODUCE A RELIEF IMAGE... 

A. Classification and Mechanism of Action These drugs are described in detail in Chapter 10. All beta-blockers are effective in the prophylaxis of atherosclerotic angina attacks. 

Classification and Mechanism of Action of Skin Cleansing Procedures/Products... 

Pigman, William Ward, Specificity, Classification, and Mechanism... 

Two-phase suspension systems produce beaded products with broader particle-size distribution (e.g., 1-50 /rm). The microspherical particles usually need to be classified repeatedly to reduce the particle-size distribution in order to improve the resolution and efficiency in the separation for use in chromatography. The actual classification process depends on the size range involved, the nature of the beaded product, and its intended applications. Relatively large (>50 /rm) and mechanically stable particles can be sieved easily in the dry state, whereas small particles are processed more conveniently in the wet state. For very fine particles (<20 /rm), classification is accomplished by wet sedimentation, countflow setting, countflow centrifugation, or air classification. 

The notion of concurrent SnI and Sn2 reactions has been invoked to account for kinetic observations in the presence of an added nucleophile and for heat capacities of activation,but the hypothesis is not strongly supported. Interpretations of borderline reactions in terms of one mechanism rather than two have been more widely accepted. Winstein et al. have proposed a classification of mechanisms according to the covalent participation by the solvent in the transition state of the rate-determining step. If such covalent interaction occurs, the reaction is assigned to the nucleophilic (N) class if covalent interaction is absent, the reaction is in the limiting (Lim) class. At their extremes these categories become equivalent to Sn and Sn , respectively, but the dividing line between Sn and Sn does not coincide with that between N and Lim. For example, a mass-law effect, which is evidence of an intermediate and therefore of the SnI mechanism, can be observed for some isopropyl compounds, but these appear to be in the N class in aqueous media. 

No unifying theoretical concepts have been recognized which can be used to provide satisfactory criteria for the comprehensive classification of the kinetics and mechanisms of reactions involving solids. Thus the scope and treatment of the subject cannot be entirely systematic. General problems encountered during any attempt to review the field include the following. 

Inevitably, there is a certain degree of overlapping between these two general classifications. For instance, CVD optical applications are found as both coatings and fibers while fibers are used in optics as well as in structural and mechanical applications. These relationships will be reviewed in the several chapters on applications. 

Of course, while the identification of these distinct systems may be useful there are many neural pathways that would not fit easily into one of them. Thus some inhibitory pathways, such as that from the caudate nucleus to substantia nigra, utilising GABA, are not intrinsic neurons. The dopamine pathway from the substantia nigra to striatum may start from a small nucleus but unlike other monoamine pathways it shows little ramification beyond its influence on the striatum. The object of the above classification is not to fit all neural pathways and mechanisms into a restricted number of functional categories but again to demonstrate that there are different forms of neurotransmission. 

The vast number of thermodynamically possible reactions obtained by permuting oxidants and reductants within the scope of this review present major problems of classification and selection. To only a limited extent is the modernity or detail of a paper indicative of its relevance, some of the definitive papers having been published before 1950. Discussion has been concentrated, therefore, at points where a kinetic investigation of a reaction has resulted in a real advance in our understanding both of its mechanism and of those of related reactions, and work which has been more of a confirmatory nature will not receive comparable consideration. Detailed reference to products, spectra, etc. will be made only when the kinetics produce real ambiguities. 

Snyder, S.H. Faillace, L. and Weingartner, H. A new psychotropic agent Psychological and physiological effects of 2,5 demethoxy-4-ethyl amphetamine (DOET) in man. Arch Gen Psychiatry 21 95-101. 1969. Timmermans, P.B.M.W.M., and Van Zwieten, P.A. Alpha, adrenoreceptors Classification, localization mechanisms and targets for drugs. J Med Chem 25 1389-1401, 1982. 

Ennis, B. J., Tardos, G. I., and Pfeffer, R. A Microlevel-Based Classification of Granulation Regimes and Mechanisms, Proceedings of the Second World Congress on Powder Technology, p. 409, Kyoto, Japan (1990a)... 

The classification procedures used are based on either density of population or distance between members. These methods can serve to generate a basis for the classification of large numbers of dissimilar variables such as behavioral observations and compounds with distinct but related structures and mechanisms (Gad, 1984 Gad et al., 1985), or to separate tumor patterns caused by treatment from those caused by old age (Hammond et al., 1978). 

The discussion and classification of reagents is masterful in identifying Ingold s new nomenclature and principles with more widely known oxidation-reduction and acid-base theory. The 1953 lectures at Cornell University, published as Structure and Mechanism in Organic Chemistry, follow this same strategy, showing how old classification schemes overlap with each other and how apparent inconsistencies disappear as old schemes are incorporated into the new one. Nineteenth-century Berzelian electrochemical dualism, revived by Lapworth and Robinson in the cationic/anionic schema, disappears into the electrophilic/nucleophilic language. 

Other terms that he invented include the system of classification for mechanisms of aromatic and aliphatic substitution and elimination reactions, designated SN1, SN2, El, and E2. "S" and "E" refer to substitution and elimination, respectively, "N" to nucleophilic, and "1" and "2" to "molecularity," or the number of molecules involved in a reaction step (not kinetic order, having to do with the equation for reaction rate and the concentration of reactants). Ingold first introduced some of these ideas in 1928 in a... 

The peptidases were separated into catalytic types according to the chemical nature of the group responsible for catalysis. The major catalytic types are, thus, Serine (and the related Threonine), Cysteine, Aspartic, Metallo, and As-Yet-Unclassified. An in-depth presentation of catalytic sites and mechanisms, based on this classification, is the subject of Chapt. 3. 

The previous chapter offered a broad overview of peptidases and esterases in terms of their classification, localization, and some physiological roles. Mention was made of the classification of hydrolases based on a characteristic functionality in their catalytic site, namely serine hydrolases, cysteine hydrolases, aspartic hydrolases, and metallopeptidases. What was left for the present chapter, however, is a detailed presentation of their catalytic site and mechanisms. As such, this chapter serves as a logical link between the preceding overview and the following chapters, whose focus is on metabolic reactions. 

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