Identification of prenylated pterocarpans and other isoflavonoids in Rhizopus spp. elicited soya bean seedlings by electrospray ionisation mass spectrometry

Phytoalexins from soya are mainly characterised as prenylated pterocarpans, the glyceollins. Extracts of non-soaked and soaked soya beans, as well as that of soya seedlings, grown in the presence of Rhizopus microsporus var. oryzae, were screened for the presence of prenylated flavonoids with a liquid chromatography/mass spectrometry (LC/MS)-based screening method. The glyceollins I-III and glyceollidins I-II, belonging to the isoflavonoid subclass of the pterocarpans, were tentatively assigned. The formation of these prenylated pterocarpans was accompanied by that of other prenylated isoflavonoids of the subclasses of the isoflavones and the coumestans. It was estimated that approx. 40% of the total isoflavonoid content in Rhizopus-challenged soya bean seedlings were prenylated pterocarpans, whereas 7% comprised prenylated isoflavones and prenylated coumestans. The site of prenylation (A-ring or B-ring) of the prenylated isoflavones was tentatively annotated using positive-ion mode MS by comparing the (1,3) A(+) retro-Diels-Alder (RDA) fragments of prenylated and non-prenylated isoflavones. Furthermore, the fragmentation pathways of the five pterocarpans in negative-ion (NI) mode were proposed, which involved the cleavage of the C-ring and/or D-ring. The absence of the ring-closed prenyl (pyran or furan) gave exclusively -H(2) O(x,y) RDA fragments, whereas its presence gave predominantly the common RDA fragments.