Resorcinol-pattern proanthocyanidins

Proanthocyanidins are plant phenolic biopolymers that consist of flavanoid monomer units. Two major classes of proanthocyanidins occur those that possess a resorcinol-pattern A-ring (Figure 1) and those that possess a phloroglucinol-pattern A-ring. The latter are by far the most common, occurring in a high proportion of monocotyledonous and dicotyledonous plants (1,2). The resorcinol-pattern proanthocyanidins are confined to a few genera of tropical or subtropical hardwoods and associated shrubby species (2), but are economically important, since the internationally commercially predominant wattle (3,4) and quebracho... 

The resorcinol-pattern proanthocyanidins are widely used not only for leather tanning, but also for a range of other commercial products, particularly as adhesives for plywood and fiberboard (6,7). Wattle tannin is produced from sustained-yield forests of Acacia mearnsii. largely in southern Africa (6,8). Some of the impetus at least to develop other uses for wattle tannin, apart from... 

R2 = R2= OH. Prodelphinidins. Resorcinol-pattern A-ring proanthocyanidins R1 = R2 = H. Profisetinidins. 

The condensation reaction may take place at position 6 or 8. Position 6 of the flavan skeleton is favored because it is usually less sterically hindered than position 8. In contrast to the proanthocyanidin assay the condensation reaction runs without depolymerization of the proanthocyanidins [135]. Consequently monomers also react [116] and a multitude of different dyes are produced depending on the complexity of the sample. The maxima of the chromophores of the resulting dyes are not influenced by the B-ring hydroxylation pattern (i.e. procyanidin-prodelphinidin ratio) like in the proanthocyanidin assay [116]. Absorbances are however influenced by the substitution pattern on the ring at which condensation takes place. Condensation products of phloroglucinol nucleis show absorbance maxima at about 500 nm, whereas resorcinol and pyrogallol nucleis exhibit absorbance maxima around 520 nm [77]. 

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