Fruity chemistry

	Raspberries rely on sugared versions of pigments to show their true colours.

The colours of flowers and fruits have always had a fascination for humans from the primitive need to find tasty and nutritious food to the sophisticated search for interesting natural products with potential medicinal and other applications.

Anthocyanins are the ubiquitous water-soluble pigments found in flowers and fruits, and are responsible for impressive red and blue colours. Fernando Pina of the New University of Lisbon, Portugal, and colleagues have been tracking the use of these compounds in art by the Romans and the ancient Mayan civilisation.

Richard Willstätter, of course, carried out the pioneering work on plant pigments including chlorophyll and anthocyanins in the early part of the twentieth century for which he won the 1915 Nobel Prize for chemistry and subsequent research has begun to reveal the patterns in these colourful molecules. Pina and his colleagues point out that acidity and the presence of co-pigments is very much a crucial part of giving these compounds their particular colour in a plant.

Now, Pina and his team have demonstrated that there is an alternative way some plants can produce intense colours based on exploiting sugary attachments to the pigment molecules and avoiding the need for co-pigmentation at acidic pH.

They have confirmed the isolation of four main anthocyanin pigments from the raspberry (Rubusidaeus a fruit with a very intense red colour, using high-performance liquid chromatography (HPLC): intensely red cyanidin 3-glucosylrutinoside, cyanidin 3-sophoroside, cyanidin 3-rutinoside and cyanidin 3-glucoside. "The existence of cyanidin exhibiting 3-substituted sugars suggests a simple strategy to obtain colour", say the researchers.

Reference:
Tetrahedron Lett., 2000, 41(12)