Solid-phase extraction and HPLC-MS profiling of pyrrolizidine alkaloids and theirN-oxides: a case study ofEchium plantagineum

Abstract

Pyrrolizidine alkaloids and their N-oxides can be extracted from the dried methanolic extracts of plant material using dilute aqueous acid. The subsequent integration of solid-phase extraction (with a strong cation exchanger) of the alkaloids and N-oxides from the aqueous acid solution, together with analysis using HPLC-ESI/MS, provides a method for the simultaneous profiling of the pyrrolizidine alkaloids and their N-oxides in plant samples and the collection of useful structural data as an aid in their identification. The N-oxide character of the analytes may be confirmed by treating analytical samples with a redox resin and observing the formation of the corresponding parent pyrrolizidine alkaloids. The present case study of Echium plantagineum highlighted a higher ratio of N-oxides to the parent tertiary bases than has been previously reported. Furthermore, a higher proportion of acetylated pyrrolizidine-N-oxides was observed in the flower heads relative to the leaves. Six pyrrolizidine alkaloids or pyrrolizidine-N-oxides, not previously reported from E. plantagineum, were tentatively identified on the basis of MS and biogenetic considerations. Three of these, 3'-O-acetylintermedine/lycopsamine, leptanthine-N-oxide and 9-O-angelylretronecine-N-oxide, have been reported elsewhere, whilst three others, 3'-O-acetylechiumine-N-oxide, echimiplatine-N-oxide and echiuplatine-N-oxide, appear unreported from any other source.