Class II behavior

Fig. 1.3 A proposed mechanism map that distinguishes Class I and Class II behavior.

The preceding expressions can be solved provided the composition or the exit stream is known. In many ins lances, it is acceptable to assuma that the composition corresponds to saturation conditions sysiems in which this occurs are said to exhibit fast-growth or Class II behavior. Should growth kinetics be too slow to use essentially all the supeisaturation (i.e., the solution concentration is gretrter than that at saturation). the system is said to exhibit slow-growth hahavior and is classified as a Cless I system. 

In Fig. 7, the critical phase behavior of binary aqueous solutions of several selected hydrocarbons and additionally fluorobenzene is shown, most of them having been measured in our laboratory [3]. The dashed curve is the vapor pressure curve of pure water, and the solid lines are parts of the branches of the binary critical p T) curves that start from the critical endpoint llg (systems 9 and 10) or from the critical point of pure water CP(H20). Whereas naphthalene H- water (system 9) and biphenyl -f water (system 10) show class-II behavior, all other systems belong to class III according to the classification of van Konynenburg and Scott, and thus exhibit gas-gas equilibria of the second kind. The consequence is that naphthalene and biphenyl are completely miscible with water already at quite low pressures near the vapor pressure curve of pure water. This behavior is of interest for measurements in mixed solvents and for separations. 

Classification of Binary Phase Diagrams. Figure 2 shows a classification of binary phase diagrams suggested by Scott and Van Konynenburg (19), with examples of each class. This classification is based on the presence or absence of three-phase lines and the way critical lines connect with these this is best seen on a P,T projection. In classes I, II and VI the two components have similar critical temperatures, and the gas-liquid critical line passes continuously between the pure component critical points class II and VI mixtures differ from class I in that they are more nonideal and show liquid-liquid immiscibillty. Class II behavior is common, whereas class VI, in which closed solubility... 

Figure 4. Class II behavior for a polar-nonpolar mixture (argon-krypton reference system) shown as a P,T projection...

Table 1 summarizes the classes of phase behavior found for these polar/nonpolar systems, using an argon-krypton reference system, and compares it with the behavior for simple nonpolar Lennard-Jones systems. An important difference between the two types of systems is that the Lennard-Jones mixtures do not form azeotropes, and appear to exhibit class II behavior only when the components have very different vapor pressures and critical temperatures (T j /Ta > 2). In practice, the liquid ranges of the two components would not overlap in such cases, so that liquid-liquid immiscibility (and hence class II behavior) would not be observed in Lennard-Jones mixtures (the only exception to this statement seems to be when the unlike pair Interaction is improbably weak). Thus, the use of theories based on the Lennard-Jones or other Isotropic potential models cannot be expected to give good results for systems of class II, and will probably give poor results for most systems of classes III, IV and V also. 

Some cephalosporins can be both substrates and inhibitors of /3-lactamases. The acyl-enzyme intermediate can undergo either rapid deacylation (Fig. 5.4, Pathway a) or elimination of the leaving group at the 3 -position to yield a second acyl-enzyme derivative (Fig. 5.4, Pathway b), which hydrolyzes very slowly [35][53], Thus, cephalosporins inactivate /3-lactamases by a mechanism similar to that described above for class-II inhibitors. It has been hypothesized that differences in the rate of deacylation of the acyl-enzyme intermediates derive from their different abilities to form H-bonds. A H-bond to NH in Fig. 5.4, Pathway a, may be necessary to assure a catalytically essential conformation of the enzyme, whereas the presence of a H-bond acceptor in Fig. 5.4, Pathway b, may drive the enzyme to an unproductive conformation. The ratio between hydrolysis and elimination, and, consequently, the relative importance of substrate and inhibitor behaviors of cephalosporins, is determined by the nature of the leaving group at C(3 ). An appropriate substitution at C(3 ) of cephalosporins may, therefore, increase the /3-lactamase inhibitory properties and yield potentially better antibiotics [53]. 

Fig. 6. Qualitative pressure—temperature diagrams depicting critical curves for the six types of phase behaviors for binary systems, where Ca or C corresponds to pure component critical point G, vapor L-, liquid U, upper critical end point and U, lower critical end point. Dashed curves are critical lines or phase boundaries (5). (a) Class I, the Ar—Kr system (b) Class II, the C02—C8H18 system (c) Class III, where the dashed lines A, B, C, and D correspond to the H2-CO, CH4-H2S, He-H2, and He-CH4 system, respectively (d) Class IV, the CH4 C6H16 system (e) Class V, the C2H6 C2H5OH...

Class II oenocytes produce hydrocarbons in grasshoppers (Diehl, 1975) other than its involvement in waterproofing the cuticle and the ootheca, the role of hydrocarbons in the sexual behavior of the grasshopper is not known, unlike in some arctiids, cockroaches, and flies (Schal et al., 1998 a or b). Similarly, while the biochemical processes involved in the cycling of hydrocarbons between oenocytes and target tissues has been described (e.g. Schal et al., 1998), little information is available on such processes in grasshoppers nor is information available on their ultrastructure. 

A sex pheromone has been implicated in the courtship behavior of the ichneumonid wasp, Campoletis sonorensis (Vinson, 1972). Although the exact site of production of the sex pheromone is not identified, Vinson suggested that the sex pheromone gland in this insect is associated with the cuticle. In another ichneumonid, Eriborus terebrans, one of the sex pheromone components is a hydrocarbon (Shu and Jones, 1993). We speculate that in ichneumonids the pheromone is synthesized in Class II oenocytes, similar to the hydrocarbon secretions produced in other insects (Schal et al., 1998). 

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