New skeleton found in Herbarium cupboard

Studying the chemistry of plants and fungi is an important part of the scientific research carried out at Kew. I work in a team led by Professor Monique Simmonds that studies plant and fungal chemistry in order to research known and potential uses of plants and fungi, authenticate traded materials and extracts, or describe the diversity of plant and fungal chemistry within a systematic context. While engaged in these activities, the team often shares in the discovery of compounds that are new to science. Discovering a new compound in a plant in the laboratory is our equivalent to a Kew botanist discovering a new species of plant on a field trip. 

Kew botanists describe between 200 and 300 new species each year. Although each one is a thrill to find, what really excites a Kew botanist is discovering a plant in the field that belongs to a new genus – that is, discovering a new species sufficiently different to any known species that it merits creating a new genus. 

We discover several new compounds in plants each year (last year we described 14), but finding one that shows fundamental differences to known compounds is possibly even more unusual than discovering a new genus of plants in the field - so rare, in fact, that I did not think I would be lucky enough to be involved in such a discovery. Last year, however, we chanced upon one such compound as part of collaborations with botanists who are working towards reclassifying the economically important legume (bean) family. The finding was published recently in the South African Journal of Botany (Kite et al., 2013). 

Our research was prompted by taxonomic studies of legume species traditionally placed in the genus Acosmium. Until recently, the 17 species of Acosmium were all thought to be members of a large group of legumes that typically contain compounds called quinolizidine alkaloids – a more familiar garden plant that is a member of this larger group is laburnum (Laburnum anagyroides). Some quinolizidine alkaloids, like those in laburnum, are poisonous to humans. 

Fortunately, the herbarium specimen of Guianodendron contained such high accumulation of this compound that, with a quick bit of chemical cleaning, enough could be obtained for Nigel Veitch, another member of our research group, to determine its chemical structure using NMR (nuclear magnetic resonance) spectroscopy. He realised that it represented a new type of quinolizidine alkaloid – our chemical equivalent to discovering a new genus (Rodrigues & Tozzi, 2007), and, rather satisfyingly, discovered in a relatively new genus of legume (Guianodendron was first described in 2006). 

In accordance with the genus name, we decided to call the new alkaloid guianodendrine. The structure of guianodendrine is shown below, for those readers who can decipher the chemical hieroglyphics. Other known quinolizidine alkaloids that have a 6-6-6-5 carbon ring system bear the oxygen at a different position, while other known quinolizidine alkaloids bearing oxygen at the same position as guianodendrine have a 6-6-6-6 ring system. This is why we consider guianodendrine to be a new type – although, as with classifying plants, some may disagree! 

One unfortunate outcome of the recent reclassification of the species we studied is that acosmine and other alkaloids bearing names derived from Acosmium do not now occur in Acosmium (in its restricted taxonomic sense). As chemists do not have a formal means for changing the common names given to compounds, you can see how this is one way in which the literature, even the scientific literature, on the chemistry of plants can become confusing. 

We uncovered a similar example to Acosmium a few years ago when we found that chemical review literature listing Cladrastis (as currently recognised) as another legume genus containing quinolizidine alkaloids was erroneous, due to it being based on a reports of alkaloids in a plant once named as a species of Cladrastis, but now placed in Maackia (Kite & Pennington, 2003).   

By giving the new compound discovered in Guianodendron the common name guianodendrine we hope we are not laying the foundation for future confusion. The compound was isolated from the single species, Guianodendron praeclarum, currently recognised in the genus, so for as long as Guianodendron is recognised as a separate genus, it must contain G. praeclarum, its so-called type species.