Proanthocyanidins of mountain birch leaves: quantification and properties

Proanthocyanidins (PAs; condensed tannins) are present in mountain birch leaves in soluble and cell wall-bound forms. Crude preparations of soluble PAs were isolated from birch leaves and purified by chromatography on a Sephadex LH-20 column with a yield of about 7% of leaf dry mass. Some chemical characteristics were elucidated with 13C-NMR and HPLC-ECI-MS. Birch leaf PAs were mainly delphinidin type oligo- and polymers with average molecular mass of about 3000. In order to quantify PAs, the method involving heating PA-containing materials in 1-butanol:hydrochloric acid (95:5, v/v), and spectrophotometric determination of the anthocyanidin monomers so formed was modified and optimised. Mature leaves were characterised by a relatively high content of PAs: mean values for soluble and bound PAs were 103 and 40 mg/g dry mass, respectively. In mature leaves the soluble PAs determined the total protein precipitation capacity (PPC) of extracts. In young leaves, the contribution of PAs to the total content of phenolics and the total PPC of tannins was about 20-25% only.