The Chemistry of Roses

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Roses are the flower most closely associated with Valentine’s Day – and we’ve got chemistry to thank for both their colour and aroma! In this post we take a closer look at the chemical compounds involved.

Colour

Though traditional red roses are usually the standard on Valentine’s Day, they come in an array of other colours too. The exact colour of a rose is dependent on the particular pigment molecules present in its petals, which fall into one of two classes: carotenoids and anthocyanins.

Carotenoids are colour-causing compounds which tend to gives shades of yellow and orange. Perhaps the most well-known member is beta-carotene, the compound that gives carrots their orange colour, but they also cause colour in a range of other plants, including tomatoes and peppers. In roses, a large number of different carotenoids can be found – a paper written in 1991 identified at least 75 different carotenoids across 40 different yellow rose varieties. 

This diversity means that different roses likely contain a number of different carotenoid compounds contributing to their colour. Some of these carotenoids are unique to roses, while others have previously been found in other plants or animals. One of the most interesting cases is that of galloxanthin, found in some roses and also previously isolated from the retina of birds – suggesting that both roses and birds possess a similar enzyme capable of acting on carotenoids.

When it comes to red roses, the picture is a little simpler. Though the anthocyanin compounds that give red colours can still be varied, their diversity is surprisingly limited compared to the carotenoids. Some of the most common compounds that contribute to the red shades are cyanin and pelargonin. Interactions with other colourless compounds in rose petals, a phenomenon known as ‘copigmentation’, can also influence the exact shade of red that we see.

Aroma

The aroma of roses showcases a similar chemical complexity to their colours. Again it is not one compound but many which contribute to a rose’s sweet scent. These compounds are often present in quite low amounts, but this low concentration belies their contribution to the aroma; many of them have very low odour thresholds, the amount that needs to be present for the human nose to be able to detect their smell. 

Some of the key contributors have names related to their presence in the flower. (-)-cis-rose oxide contributes a typical floral rose fragrance, and is detectable by our nose at very low concentrations in air – as little as 5 parts per billion. To give this some perspective, one part per billion is equivalent to around half a teaspoon of water in an Olympic-sized swimming pool.

Other compounds which contribute are rose ketones, are class of compounds including beta-damascenone, beta-damascone and beta-ionone. Terpene compounds also add to the scent; these include geraniol, nerol, citronellol, and farnesol. These compounds aren’t unique to roses, as they crop up in the aromas of other flowers too!

Forgoing roses for chocolate for Valentine’s Day? We’ve got you covered on that front too, with this graphic in C&EN.

 

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References & Further Reading