Reaction cycle model, hypothetical

Cycles that involve ligand exchange at one site are podal, and resemble those triligostats of the last section in which all reactions occur with retention of configuration. No ligand metathesis is possible in such cycles. Chart XIV outlines a hypothetical seven-reaction cycle of this type, only the first two reactions of which were completed 13>. The sequence is modeled after the actual, podal, triligostatic seven-reaction cycle also outlined in Chart XIV 14>. 

The neuraminidases together with gangliosides have been localized in the nerve ending structures (6, 7 ). Theoretically the sialylation and desialylation cycle may mediate a cyclic reaction at a very important locale in a nerve synaptic structure. This hypothetical involvement of sialic acid metabolism in synaptic transmission has gained support from several studies which have suggested a synaptic localization of the glycosyltransferases (8, 9,10,11) and from proposed theoretical models in which the sialo-glycolipids are considered an important constituent in the functional units of neuronal membranes (12,13,14). 

The last two models have been investigated extensively by Flory He considered networks forming part of a larger network with fixed jimctions outside the network under consideration. The crosslinking points of the inner network were assumed to fluctuate freely, or to have fixed positions in space. In both cases, the inner network can be considered as localized by boundary conditions. In particular, Flory considered a network formed in two hypothetical steps. First, a giant acyclic molecule is formed by joining all chains via the available multifunctional junctions such a tree-like molecule can be characterized by v -H 1 v junctions plus chain ends (i.e. number of labelled points). Figure 3 shows an example of the network after the first step. In the second step additional connections are formed by the reaction of 2 unreacted functionalities, which reduces the number of labelled points to approximately The number is called the cycle rank, which can be defined... 

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