Anthocyanins, from the Greek words for “dark blue flower”, are natural coloring agents found in divserse plant parts such as blueberries, wine grapes, and even autumn leaves. They belong to a broader class of molecules called flavonoids, which have been studied for their possible health benefits.
Callistephin is an anthocyanin that was first identified in purple-red asters (Richard Willstätter and Ch. L. Burdick, 1917). Subsequently it was extracted from strawberries, pomegranates, and blue corn. In 1928, British chemists Alexander Robertson and Nobel laureate Robert Robinson determined callistephin’s structure and synthesized it in the laboratory.
Callistephin is the 3-O-glycoside of pelargonidin, another plant pigment. Both molecules are cationic and normally exist as their chloride salts.
Today, many food and cosmetic manufacturers prefer to use natural colorants in their products; and anthocyanins are foremost among them. But these pigments are difficult and expensive to remove from plants and to purify. Last month, Mattheos A. G. Koffas at Rensselaer Polytechnic Institute (Troy, NY) and colleagues reported a biochemical reaction sequence that synthesizes anthocyanins such as callistephin from glucose.
The researchers used genetic engineering to create four strains of Escherichia coli bacteria—three to produce the synthetic intermediates and one for the final product (see “Four Microbes Make Callistephin”).
So far, methods like this have made anthocyanins only in the mg/L concentration range; but some intermediates have been produced in g/L. If the chemists can boost the production of the final products to g/L, the process will be suitable for commercialization.
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