Subject rapid equilibrium mechanisms

Each monomer contains one pyridoxal phosphate and a highly conserved primary structure in the vicinity of the cofactor binding site. The enzymes follow the same catalytic mechanism, a rapid equilibrium random Bi Bi mechanism. However, unlike animal phosphorylases, the microbial and plant enzymes are not subjected to covalent or allosteric control. Within the group of higher plant phosphorylases two types of enzyme have been distinguished which differ in monomer size, peptide pattern, glucan specificity, and intracellular location (1-3). Based on immunochemical studies on leaf tissues (4-5), one enzyme form has been localized in the cytosol whereas the other one resides in the chloroplast. Thus, the two plant phosphorylase types represent non-interconvertible proteins which presumably have entirely different metabolic functions. 

The mechanism of this reaction and the structure of the intermediate carbocation were once the subject of intense controversy. When the double bond of norbornene is protonated, a carbocation is produced (Eq. 10.25). The question of whether this cation is 54a and is in rapid equilibrium with the isomeric cation 54b, or whether 54a and 54b are simply two contributing resonance structures to the resonance hybrid 55, has been the focus of the debate. Note that the contributing structures 54a and 54b dijfer in the location of an cr-bond, not in the location of Tr-bonds as is more commonly encountered in resonance structures. The delocalized structure 55 is thus referred to as a nonclassical carbocation, and most chemists now accept this formulation as the more likely representation of this intermediate. It is evident from empirical results that the sterically more accessible side of the carbocation is the one away from or exo to the bridging carbon atom as shown. Nucleophilic attack of water solely from this side leads to cxo-norborneol (53) rather than endo-norborneol (56). 

The key feature of the mechanism of RAF polymerization with thiocarbonylthio compounds as proposed in our first communication on the subject is the sequence of addition-fragmentation equilibria shown in Scheme 7. Initiation and radical-radical termination occur as in conventional radical polymerization. In the early stages of the polymerization, addition of a propagating radical (Pn ) to the thiocarbonylthio compound [RSC(Z)=S (7)] followed by fragmentation of the intermediate radical provides a polymeric thiocarbonylthio compound [PnSC(Z)=S (9)] and a new radical (R ). Reaction of this radical (R ) with monomer forms a new propagating radical ((Pm ))- Rapid equilibrium... 

Dissolution kinetics of a simple component close to saturation and the mechanism of the backward precipitation reaction are still subject to controversy. Minerals such as calcite and aragonite are known to reach rapidly a dissolution equilibrium when placed in closed aqueous systems. According to simple and classical thermodynamical concepts, this requires that each forward... 

In recent years, the stopped-flow rapid-mixing kinetic technique and, to a lesser extent, the temperature perturbation of equilibrium (temperature-jump) technique have been used by a number of workers (68, 72, 79—86) to investigate individual steps in the above reaction scheme. Much of this work has been directed toward the investigation of the relationship between catalytic mechanism and subunit function in the dimeric enzyme. Since the scope of this review is limited to consideration of the involvement of zinc ion in the mechanism of the chemical transformation, no discussion of subunit function is presented here. Those readers who wish to pursue this aspect of LADH catalysis are referred to the original literature (65, 72, 79—86) and to the excellent reviews which have recently appeared on this subject (87, 88). 

One means of reducing the time scale of moisture absorption is to subject the strac-ture to rapid thermal excursions. Because the original structure of the cured epoxy resin is in a non-equilibrium state, thermal spiking can lead to more rapid moisture absorption. Relaxation of the matrix stmcture will occiu as a result of these moisture and thermal events. Other mechanisms which lead to enhanced moismre concentrations are discussed in the context of matrix resin and fibre selection. 

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