Characterization of two novel antioxidant tetrahydroxyxanthones from <i>Hypericum</i> seeds and scanning electronmicroscopic investigations of their testa

Abstract

Hypericum seeds have recently been identified as a natural source of different xanthone derivatives. Two main constituents, the tetrahydroxyxanthones THX-1 and -2, were identified in methanolic extracts of H. perforatum and H. tetrapterum by means of HPLC(DAD)-MSn methods. However, the exact configuration of both THX was unknown so far. For this reason, the two compounds were enriched by chromatography on polyamide and silica, and the corresponding fraction was investigated by 1D- and 2D-NMR techniques. Based on a tentative structural assignment a total synthesis of THX-1 and -2 was performed. Comparing the chromatographic and spectroscopic/spectrometric features of synthetic THX-1 and -2 with their natural counterparts their exact configuration was determined as 1,4,6,7-THX (THX-1) and 1,2,6,7-THX (THX-2), respectively. The novel compounds exhibited a threefold higher radical scavenging activity in the DPPH assay than the vitamin E derivative Trolox^®. Hence, they are strong antioxidants. Furthermore, scanning electron microscopy (SEM) provided insights into the two-layer structure of the testa (seed coat), with the thick lignified sclerenchyma layer presumably being the depository of the xanthones.