Subject linkage isomers

Thiocyanato (M—SCN) and isothiocyanato (M—NCS) complexes constitute another group of well-known linkage isomers and their properties are the subject of a detailed review by Burmeister.61 Often the energy difference between the isomers is small and it is apparently... 

There is no final consensus on whether procyanidin biosynthesis is controlled thermodynamically or enzymatically. In either case proanthocyanidins are synthesized through sequential addition of flavan-3,4-diol units (in their reactive forms as carbocations or quinone methides) to a flavan-3-ol monomer [218]. Based on the latest findings there is some evidence that different condensation enzymes might exist which are specific for each type of flavan-3,4-diol [64] and that polymer synthesis would be subject to a very complex regulatory mechanism [63]. But so far, no enzyme synthetase systems have been isolated and enzymatic conversion of flavanols to proanthocyanidins could not be demonstrated in vitro [219]. If biosynthesis was thermodynamically controlled, the variation in proanthocyanidin composition could be explained by synthesis at different times or in different compartments [64], The hypothesis of a thermodynamically controlled biosynthesis is based on the fact that naturally and chemically synthesized procyanidin dimers occur as a mixture of 4—>8 and 4—>6 linked isomers in approximate ratios of 3-4 1 [220]. Porter [164] found analogous ratios of 4—>8 and 4—>6 linkages in proanthocyanidin polymers. 

The azo Unkage is not subject to the polarization forces present inj the azomethine linkage. Geometrical isomers of azo compounds, therefore, are more stable and more readily isolated. Azobenzene is j known to exist in both the cts and the considerably more stable] trans forms The almost coplanar structure of the trans isomer. ... 

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