Structure and antioxidant activity of diarylheptanoids extracted from bark of grey alder (Alnus incana) and potential of biorefinery-based bark processing of European trees.
The bark of trees has a big potential as a source of green chemicals. The aim of the present work was to valorise the potential of deciduous tree species with this regard. Three widely spread trees in Europe (grey alder, ash tree, aspen) were in focus as a source of polyphenols, and the yields of polyphenolic compound in the extracts were considered as evaluation criteria. The highest yields of hydrophilic extractives were found in barks of grey alder and aspen (36.8 and 22.9%, respectively). In the former, the highest antioxidant activity was found towards free radicals (DPPH. and ABTS.+) and superoxide anion radical. Open chain diarylheptanoids, mainly oregonin, were identified as the major constituents of the grey alder hydrophilic extract. In addition to oregonin, the presence of 2 linear diarylheptanoids [platyphylloside and 1,7-bis-(3,4-dihydroxyphenyl)-heptane-5-O-β-D-glucopyranoside] was confirmed. For the first time, the compounds 1,7-bis-(3,4-dihydroxyphenyl)-3-hydroxyheptane-5-O-β-D-xylopyranoside and 1,7-bis-(3,4-dihydroxyphenyl)-heptane-3-one-5-O-β-D-glucopyranoside were detected in grey alder bark. The results of experiments in vitro and in vivo have shown the high potential for diarylheptanoids-containing extracts in prophylaxis and/or treatment of diseases due to the metabolic disorders and ageing. The biological activity of grey alder extract was confirmed in in vitro experiments by incubation of human blood samples. In vivo experiments with rats also showed positive results. The conclusion is that grey alder extracts have a high potential for prevention of ageing related pathologies. Besides diarylheptanoids, the bark contains condensed tannins in commercially available quantity (12.5%). Eco-friendly wood adhesives were obtained on a tannins basis. The bark left after polyphenols isolation can be used in soil melioration and as a sorbent for the removal of oil products from water surface. The investigation of the phenolic pool of grey alder could contribute to cluster technologies within the biorefinery-based bark processing.