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New skeleton found in Herbarium cupboard

Geoffrey Kite
16 May 2014
Blog team: 

Research phytochemist Geoffrey Kite describes the discovery of a quinolizidine alkaloid with a novel carbon skeleton in a Kew herbarium specimen.

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).

 Photo of a liquid chromatograph-mass spectrometer

Some of the equipment used at Kew to analyse plant extracts. This instrument is a liquid chromatograph-mass spectrometer.

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.

Initially two Brazilian botanists, Rodrigo Rodrigues and Ana Maria Tozzi, realised that species of Acosmium were not all closely related to each other and reclassified them into three genera (Leptolobium, Guianodendron and Acosmium) (Rodrigues & Tozzi, 2007), then a team led by another Brazilian botanist, Domingos Cardoso, analysed DNA sequence data and found that three species (those remaining in Acosmium) were not only unrelated to the others, but also not members of the quinolizidine alkaloid-containing group of legumes (Cardoso et al., 2012).

Photo of Leptolobium dasycarpum

Leptolobium dasycarpum, formerly Acosmium dasycarpum (Photo: William Milliken)

This was a potential source of confusion because the scientific literature contains several studies on the alkaloids of species of Acosmium (as traditionally defined). Indeed this genus was known to contain a particular type of quinolizidine alkaloid, one example being acosmine, named after the genus. However, chemists had never actually examined the three species that remained in Acosmium following the reclassification, and so it was important to investigate the chemical profiles of these species. Although Acosmium is a South American genus, Kew’s Herbarium contains over 7 million herbarium specimens from across the globe and we were able to utilise this resource to begin the investigation.

The legume collection in the Kew Herbarium is one of the most comprehensive in the world, holding about 750,000 specimens all arranged systematically in line with the latest molecular data. Working with Gwilym Lewis (who leads legume research at Kew) and Domingos Cardoso, we were able to find specimens of the three Acosmium species in the herbarium cupboards of the legume collections and chemically analyse small fragments of excess leaf material from the specimens. The analyses revealed that these three species did not contain quinolizidine alkaloids, supporting the hypothesis that they were not members of this alkaloid-containing legume group.

The wider investigation revealed that herbarium material of species now placed in the reinstated genus Leptolobium contained the expected acosmine-like quinolizidine alkaloids. More exciting, though, was the discovery that leaf material of the one known species of Guianodendron contained another type of quinolizidine alkaloid, and one which was not immediately recognisable.

Image of herbarium specimen of Guianodendron praeclarum

Herbarium specimen of Guianodendron praeclarum containing the new compound. The blue label shows that the specimen has been sampled chemically.

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!

Image of the molecular structure of guianodendrine

Molecular structure of guianodendrine

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.

- Geoff -


References

Cardoso, D., Lima, H.C. de, Rodrigues, R.S., Queiroz, L.P. de, Pennington, R.T. & Lavin, M. (2012). The realignment of Acosmium sensu stricto with the dalbergioid clade (Leguminosae: Papilionoideae) reveals a proneness for independent evolution of radial floral symmetry among early-branching papilionoid legumes. Taxon 6: 1057 – 1073. Available online

Kite, G.C., Cardoso, D., Veitch, N.C. & Lewis, G.P. (2013). Quinolizidine alkaloid status of Acosmium s.s., Guianodendron and Leptolobium, the segregate genera of Acosmium s.l. South African Journal of Botany 89: 176–180. Available online

Kite, G.C. & Pennington, R.T. (2003). Quinolizidine alkaloid status of Styphnolobium and Cladrastis (Leguminosae). Biochemical Systematics and Ecology 31: 1409-1416. Available online

Rodrigues R.S & Tozzi, A.M.G.A. (2007).  Guianodendron, a new genus of Leguminosae (Papilionoideae) from South America. Novon 16: 129-132. Available online

Rodrigues, R.S. & Tozzi, A.M.G.A. (2007). Morphological analysis and re-examination of the taxonomic circumscription of Acosmium (Leguminosae, Papilionoideae, Sophoreae). Taxon 56: 439–452. Available online


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