5 min read
Magnolias, molecules and memories
Compounds new to science have been discovered in magnolia flowers thanks to Kew's small molecule analysis facilities, as Analytical Methods manager Geoffrey Kite explains.
Magnolias in the Gardens
Kew's magnolias produce one of the more majestic mass flowering events in the Gardens. In early spring, numerous large goblet or star-shaped pink or white flowers decorate the leafless branches of these large trees or shrubs. The distant glimpse of flowers adorning the tree tops is particularly alluring when the low spring sun illuminates the flowers against a clear blue sky, enticing the visitor to take a closer look.
Botanists have been taking a closer look at magnolia flowers for a long time. One of the classical theories on the evolution of the first flowering plants was modelled on the large flowers of Magnolia, although magnolia flowers are now regarded as being relatively specialised. Despite this botanical fascination, there have been relatively few studies on the chemistry of magnolia flowers beyond the analysis of their scents and essential oils. Instead, scientists studying plant chemistry have focused their attention on the parts of magnolia used in traditional medicine; namely the bark, closed flower buds and seed cones.
Magnolia-based medicines
In traditional Japanese Kampo medicine, the closed flower buds of Magnolia salicifolia (willow-leaved magnolia) are used to treat rhinitis, and investigation into this anti-allergenic effect has found compounds called phenylethanoid glycosides among the active constituents. This information was of interest to scientists at Kew investigating potential new botanical extracts for skin-care products as part of a project supported by the UK Engineering and Physical Sciences Research Council in a strategic partnership with Procter & Gamble UK. Botanical extracts containing phenylethanoid glycosides have aroused interest in the skin-care industry due to the anti-inflammatory and other effects of these compounds. However, there is a consumer preference for colourless extracts in skin-care products, so an extract made from the green flower buds may require some processing. The Kew team thought an extract prepared from the white flowers might be more suitable, but this prompted the question: do the tepals (undifferentiated petals and sepals) of Magnolia salicifolia also contain phenylethanoid glycosides?
Analysing molecules
To answer this question, the project team needed to use the full range of small molecule analytical equipment available in the Analytical Methods laboratories at Kew. Firstly, an extract was prepared from the tepals of Magnolia salicifolia and analysed using a high resolution liquid chromatograph-mass spectrometer (LC-MS). In this instrument, the liquid chromatograph separates the compounds in the extract and passes them to a mass spectrometer which measures their molecular masses. The mass spectrometer determines the molecular mass so accurately that the molecular formula of the compound can be calculated - this sounds rather straightforward when stated glibly, but when you consider that the mass of a sugar (sucrose) molecule is about 0.000000000000000000000568 g this is indeed a very impressive weighing machine!
The analysis revealed that the tepals did contain phenylethanoid glycosides, and they were among the most abundant compounds detected in the tepal extract. More excitingly, the molecular mass data indicated that at least one of the phenylethanoid glycosides must be new to science, as it had a molecular mass greater than any known phenylethanoid glycoside.
To determine the molecular structures of the new compound and the other main phenylethanoid glycosides they had to be purified from the extract. To do this, the separation of the compounds achieved with the LC-MS was scaled up using a similar liquid chromatograph, but instead of the compounds being passed to a mass spectrometer they were collected in tubes in a pure state for subsequent analysis using a nuclear magnetic resonance spectrometer (NMR). This instrument records how the atoms of a molecule resonate when given a pulse of radio waves in a strong magnetic field, and from the complex data acquired by the instrument an experienced NMR specialist can reconstruct the structure of the molecule.
The NMR analysis confirmed that the phenylethanoid glycoside with the high molecular mass was new to science. Indeed it proved to be the first known phenylethanoid glycoside that incorporated five sugar units into its structure - previously described phenylethanoid glycosides had one to four sugar units. Furthermore, another two of the phenylethanoid glycosides isolated from tepals of Magnolia salicifolia proved to be new compounds.
Was it just the flowers of Magnolia salicifolia that contained these compounds or were they widespread in the genus? To answer this, tepals were gathered from 19 species of Magnolia growing in Kew and extracts of the tepals were analysed by LC-MS. Phenylethanoid glycosides were abundant in the majority of the species but the large phenylethanoid glycoside and one of the other new compounds (which was structurally related to it) only occurred in species belonging to the subgenus Yulania. To indicate this systematic occurrence, these two new compounds were named yulanoside A and yulanoside B (the third new compound was given the name 2'-rhamnoechinacoside – a derivative name of a known compound).
Undiscovered compounds
It seems hard to believe that these compounds lay undiscovered in such iconic flowers for so long. I had studied plant chemistry at Kew for 28 years and had admired Kew's magnolias each spring without realising they could possibly contain compounds new to science. I had even gathered some fallen tepals of Magnolia salicifolia to use as a natural confetti at my wedding! It makes me wonder just how many other new compounds remain to be discovered in Kew's living collections.
The paper describing the new magnolia compounds has just been published in the September 2015 issue of Phytochemistry. The molecular structures were determined by my long-term colleague and NMR specialist, Nigel Veitch, and this was one of the last papers I co-wrote with him before his untimely death in September 2014. So the sight of the fallen tepals of Magnolia salicifolia this spring brought a mixture of memories; the marvels of nature’s molecules; the happiness of my wedding day, and the sadness of a tragic loss.
References
Porter, E.A., Kite, G.C., Veitch, N.C., Geoghegan, I.A., Larsson, S. & Simmonds, M.S.J. (2015). Phenylethanoid glycosides in tepals of Magnolia salicifolia and their occurrence in flowers of Magnoliaceae. Phytochemistry 117: 185-193. Available online