Accessibility degradation

Examination of the pyrazino[2,3-rf]pyrimidine structure of pteridines reveals two principal pathways for the synthesis of this ring system, namely fusion of a pyrazine ring to a pyrimidine derivative, and annelation of a pyrimidine ring to a suitably substituted pyrazine derivative (equation 76). Since pyrimidines are more easily accessible the former pathway is of major importance. Less important methods include degradations of more complex substances and ring transformations of structurally related bicyclic nitrogen heterocycles. 

Whereas cellulose films are biodegradable, that is they are readily attacked by bacteria, films and packaging from synthetic polymers are normally attacked at a very low rate. This has led to methods of degrading polymers to a sufficiently low molecular mass (typically about 10000) which are then accessible to biodegradation. 

Later Goto and Shishido prepared di-3-ethoxy-5 6-dimethoxy-A -ethylnoraporphine ethiodide, m.p. 186-7°, and this, by the Hofmann degradation process, gave the ethiodide of the de-At-ethyl base, m.p. 194°, from which the dimethoxyethoxyvinylphenanthrene, m.p. 108°, was obtained, identical with that from natural Z-tuduranine. The latter is therefore 3-hydroxy-5 6-dimethoxy-A -H0)aporphine. A later paper (1941) also relating to tuduranine is not yet accessible. 

A recent communication described the cleavage of 17a-hydroperoxy-20-keto steroids with base to give 17-ketoandrostanes in good yield. Since such hydroperoxides are now accessible from 20-ketopregnanes in one step vide infra), this constitutes a convenient two-step degradation process. In practice, the intermediate hydroperoxide need not be isolated. Other enolizable... 

Nail sickness Nail sickness is chemical decay associated with corroded metals in marine situations. Chemical degradation of wood by the products of metal corrosion is brought about by bad workmanship or maintenance, or unsuitable (permeable) timber species, all of which permit electrolyte and oxygen access which promotes corrosion. Chemical decay of wood by alkali occurs in cathodic areas (metal exposed oxygen present). Softening and embrittlement of wood occurs in anodic areas (metal embedded oxygen absent) caused by mineral acid from hydrolysis of soluble iron corrosion products. 

At temperatures above Tm, chemical and enzymatic degradation of microbial exopolysaccharides is enhanced. The apparent enhanced stability of microbial exopolysaccharides in their ordered confirmation is thought to be due to the glycosidic bonds in the backbone of the polymer which raises the activation energy. This restricted movement would also restrict access of enzymes and chemicals to the backbone. 

In addition, any chemical aging (Section 23.5.3) could affect these mechanisms for instance, if an elastomer surface is degraded by contact with hostile chemicals, access to the interior may be facilitated and its permeation characteristics would change accordingly. 

In a classical neural pathway, such as that depicted in Fig. 1.3, neuron A must excite neuron B and at the same time inhibit neuron C in order to optimise the excitation of B. It could achieve this with one NT able to activate receptors linked to different events on B and C. Of course, neuron C would have other inputs, some of which would be excitatory and if the same NT was used it could activate the inhibitory mechanism on C as well. Also, the NT released from A might be able to stimulate as well as inhibit neuron C (Fig. 1.3(a)). Even the provision of separate receptors linked to excitation and inhibition would not overcome these problems since both would be accessible to the NT. One possible solution, used in the CNS, is to restrict the NT to the synapse at which it is released by structural barriers or rapid degradation. Also the inputs and receptors linked to excitation could be separated anatomically from those linked to inhibition and, in fact, there is electrophysiological and morphological evidence that excitatory synapses are mainly on dendrites and inhibitory ones on the soma of large neurons (Fig. 1.3(b)). Nevertheless, the problem of overlap would be eased if two NTs were released, one to activate only those receptors linked to excitation and another to evoke just inhibition, i.e. place the determinant of function partly back on the NT (Fig. 1.3(c)). This raises a different problem which has received much consideration. Can a neuron release more than one NT ... 

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