Chemical biology news from across RSC Publishing.
Instant insight: True blue flowers
14 July 2009
What makes a purple pigment blue? The answer could lead to the elusive blue rose says Kumi Yoshida of Nagoya University, Japan
Anthocyanins are to be thanked for beautiful flower colours. These sugar-containing flavonoids differ from other plant pigments - such as green chlorophylls, yellow and orange carotenoids and purple betalains - by exhibiting a wider variety of colours. When anthocyanins are found in petals, dissolved in the petal cells' vacuoles (large sacs that make up over 90 per cent of the cells' volume), they are responsible for an assortment of reds, purples and blues.
Heavenly blue anthocyanin gives the petals of blue Morning glory their colour
Many years of research have focused on the development of flower colour, particularly how blues are created. Numerous efforts have been undertaken to resolve two major mysteries: how so few anthocyanin chromophores can produce so many colours and what makes these colours, usually unstable above pH 4, survive inside living cells.
An anthocyanin solution can change colour with pH in a similar fashion to litmus. For example, a flavilium cation with a red colour may form in strongly acidic conditions, an anhydrobase with a purple colour form at neutral pH, and an anhydrobase anion with a blue colour form in alkaline conditions. Therefore, from an organic chemistry perspective, it is very simple to say that flower colour comes about because anthocyanin chromophores - anthocyanidins - change their structure depending on the pH. Creating a blue-coloured flower involves understanding how to stabilise the blue anhydrobase anion form in a weakly acidic to pH neutral plant vacuole.
The most important technique in producing blue flower colouration involves the metalloanthocyanins - complexes of anthocyanins, flavones and metal ions which are found in blue dayflowers, cornflowers and salvias. These pigments use the metal-complexation and aromatic stacking strategies to generate their colour. For example, cornflowers contain the cyanidin chromophore, which is the same as in the red rose. They create blue colouration by binding the chromophore to paramagnetic Fe3+. The blue dayflower, on the other hand, also resorts to the oxidised chromophore strategy. It uses a delphinidin chromophore, which has one more hydroxyl group than the cornflower's cyanidin. When its components are mixed with Mg2+ ions, they rapidly produce a blue supramolecule.
So could the true blue rose be on the horizon? Its pursuit has persisted for decades. Molecular breeding techniques have led to advanced flower colour chemistry so that you can now see roses and carnations with bluer hues. But they are not yet truly blue in colour. For the true blue rose to be developed, a multilateral strategy is necessary. Clarification of petal-bluing mechanisms is a crucial step that should open the door to this long-sought flower.
Read more in the review 'Blue flower color development by anthocyanins: from chemical structure to cell physiology' in Natural Product Reports.
Link to journal article
Blue flower color development by anthocyanins: from chemical structure to cell physiology
Kumi Yoshida, Mihoko Mori and Tadao Kondo, Nat. Prod. Rep., 2009, 26, 884
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