Proanthocyanidins from Quercus petraea and Q.robur heartwood:quantification and structures

摘要

In oak heartwood extracts the typical reaction of proanthocyanidin(PA)tannins occurs,resulting from the oxidative depoly-merization of PA into anthocyanidins.Analysis of the anthocyanidins allowed the identification of procyanidins and prodelphini-dins.Global quantitative methods allowed the level of oak heartwood PA content to be estimated.We have used different techniques to determine an average amount of soluble procyanidins of between 0.3 and 0.8 mg g~(-1)dry wood and between 0.8 and 2.7 mg g~(-1)for flavanols.Dimeric procyanidins were identified by HPLC after'collecting each peak followed by analysis of flavanols produced after thiolysis and desulfuration.The main procyanidins were:Bl,B2,B6 and B8.The major fraction of the PA wood extract exhibited a higher degree of polymerization(N >2)and required appropriate fractionation before structural analysis.We used low pressure chromatography on a LH20-column with two steps of elution:methanol/water for fraction I and acetone/water for fraction II.~(13)C NMR analysis of the former,revealed the presence of a signal corresponding to o-dihydroxylated PA,characteristic of procyanidins.For the latter,the presence of the characteristic signal of o-trihydroxylated PA allowed the identification of prodelphinidins.In addition,in the same way,we have demonstrated the oligomeric form of procyanidin in fraction I and the polymeric form of prodelphinidin in fraction II.