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New compounds from Old Lions

During Kew’s 250th anniversary year, researchers at Kew studied the chemistry of two of Kew’s ‘Old Lions’ and discovered nine natural substances new to science.

Styphnolobium japonicum, one of Kew’s ‘Old Lions’ (Image: Geoffrey Kite)

Pagoda tree, Styphnolobium japonicum

The legume Styphnolobium japonicum (pagoda tree) is an attractive species commonly grown in streets, parks and gardens. At Kew, a specimen planted in 1762 is one of only a handful of trees (fondly referred to as the ‘Old Lions’) that survive from the original botanic garden started by Princess Augusta.

The pagoda tree has been the subject of numerous chemical investigations, as the fruits and flowers are a rich source of flavonoids (one of the most important groups of naturally occurring phenolic substances in plants) and are important in traditional Chinese medicine. Thus, when the leaves and fruits of S. japonicum were analysed at Kew, it came as a surprise to discover four flavonoid glycosides and a maltol derivative that were new to science and eight flavonoid glycosides that had not been reported previously from this species. Some of the new flavonoid glycosides were unusual because they contained four sugar residues; three residues or fewer are more typical.

The research on the chemistry of S. japonicum has been useful in developing a method for the chemical authentication of the herbal ingredient Fructus Sophorae, obtained from the fruits of the pagoda tree. Confusion has arisen between the previous scientific name of the pagoda tree (Sophora japonica) and that of Sophora flavescens, which is the source of another herbal ingredient.

Robinia pseudoacacia, another of Kew’s Old Lions (Image: Geoffrey Kite)

Black locust tree (or false acacia), Robinia pseudoacacia

Another of Kew’s Old Lions still surviving from Princess Augusta’s original botanic garden and planted at the same time as the pagoda tree is a specimen of Robinia pseudoacacia (black locust tree or false acacia). This species is also grown widely as an ornamental tree and was also considered to be well known in terms of its flavonoid chemistry. Indeed, investigations on this species initiated in the nineteenth century made an important contribution to the development of research into flavonoids, as the first preparations of robinin and acacetin were obtained from the flowers.

When the team at Kew analysed the leaves of this species, they were again surprised to detect four flavonoids that had not been described previously in the scientific literature. These compounds were glycosides of acacetin and three related flavones in which the sugar part was particularly interesting as its structure was novel.

The Kew team also examined the flavonoids in the flowers of R. pseudoacacia and were able to confirm the exact structures of some compounds that have been suggested as markers for honey obtained from this source. This is often referred to acacia honey, but it is not to be confused with honey originating from species of Acacia.

These two studies of Kew’s Old Lions, undertaken in part to celebrate Kew’s 250th anniversary, show how modern analytical techniques can uncover the presence of novel substances in widely accessible and previously well-studied species.

Almost two and a half centuries since they were planted, these fine old trees continue to be a source of inspiration for today’s researchers, and reflect the scientific value of Kew’s collections.

Item from Dr Geoffrey Kite and Dr Nigel Veitch (Phytochemists in the Sustainable Uses Group, RBG Kew)

Article references

Kite, G.C., Stoneham, C.A., Veitch, N.C. (2007). Flavonol tetraglycosides and other constituents from leaves of Styphnolobium japonicum (Leguminosae) and related taxa. Phytochemistry 68: 1407-1416.

Kite, G.C., Veitch, N.C., Boalch, M.E., Lewis, G.P., Leon, C.J., Simmonds, M.S.J. (2009). Flavonol tetraglycosides from fruits of Styphnolobium japonicum (Leguminosae) and the authentication of Fructus Sophorae and Flos Sophorae. Phytochemistry 70: 785-794.

Veitch, N.C., Elliott, P.C., Kite, G.C., Lewis, G.P. (2010). Flavonoid glycosides of the black locust tree, Robinia pseudoacacia (Leguminosae). Phytochemistry 71: 479-486.

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