DESI-MSI applications for direct (spatial) biomolecular analysis of South African natural medicinal products

In pharmacognosy, conventional analytical methods provide important chemical information on metabolite identities and relative quantities. However, in situ analysis and the mapping of the spatial distribution of metabolites remain underexplored. Mass spectrometry (MS) imaging techniques, specifically desorption electrospray ionization-MS imaging (DESI-MSI), enables the in situ analysis and visualization of the spatial distribution of metabolites in different plant tissues. This study aimed to demonstrate the application of DESI-MSI for the in situ analysis and visualization of the spatial distribution of metabolites in selected South African medicinal plants and natural products.

DESI-MSI was used to putatively assign antimicrobial compounds from propolis via HPTLC-bioautography assay and to visualize the spatial distribution of various compounds, including coumarins in the roots of Pelargonium reniforme and Pelargonium sidoides, pyrrolizidine alkaloids in the stems of Lobostemon fruticosus, and aspalathin in the leaves of Aspalathus linearis.

The antimicrobial compounds of South African propolis were putatively assigned as chrysin, pinocembrin, galangin, pinobanksin and its derivatives. The technique enabled the differentiation between P. sidoides and P. reniforme based on the location and spatial distribution of umckalin, dihydroxy-dimethoxycoumarin and isofraxidin sulphite in the root samples. Furthermore, the spatial distribution of pyrrolizidine alkaloids (PAs) was mapped in the pith, epidermis, and cortex regions of L. fruticosus stems, while aspalathin was mapped in the margins and tips (upper and lower epidermal regions) of A. linearis leaf samples.

This study demonstrates the feasibility of using DESI-MSI for in situ chemical profiling of commercially important South African medicinal plants and natural products, supporting their identification and differentiation for quality control.