How do you know it’s autumn? Well, there are a few telltale signs. Firstly, it gets colder, secondly it gets wetter. School has started again and the days get shorter. Perhaps the most obvious sign of Autumn is in the trees. Lush, green leaves suddenly turning into golden orange, deep reds and bright yellows. Eventually, the leaves fall, creating piles to wade through and kick about. So what causes those beautiful autumn leaf colours? Let’s find out with this month’s molecules – the anthocyanins.
Leaf It Out
Have you ever wondered why and how leaves change colour in the Autumn? Well, there are three things we can think about. The molecules that go, the molecules that stay, and the molecules produced.
The molecule responsible for the green colour in leaves is chlorophyll. As the days get colder and darker, trees stop making chlorophyll. The chlorophyll already in the leaves breaks down and stops being replace. Without chlorophyll, the molecules quickly lose their green colour.
Orange and Yellow
The chlorophyll might disappear, but two different types of molecule remain. These are xanthophylls and carotenoids. These molecules are responsible for the yellow and orange colour. Usually, the green colour chlorophyll overpowers the colour from the xanthophylls and carotenoids. So, with the chlorophyll gone, the yellow and orange colour wins. But, this yellow colour makes the leaves very attractive to insects.
The plant then creates a new set of molecules called anthocyanins. These anthocyanins give a leaf its deep red or purple autumn colours, and are also responsible for the colour of many fruits.
These molecules not only hide the leaves from the bugs, but also protect it from sunlight that might damage the leaves.
Anthocyanin is not a single molecule, but is the name for a group of very similar molecules. The general structure of anthocyanins is shown below.
Each label R can be a hydrogen, an hydroxy (O-H) or a methoxy (OCH3) group. As a result, there are lots of molecules with different combinations of R. Each molecule is responsible for a different colour.
Molecules Of Many Colours
Let’s take delphinidin as an example. The molecule’s structure is shown to the left against a blue background. In this molecule, R6 is a hydrogen atom and all the other R groups are -OH groups. Delphinidin is responsible for the blue colour of certain flowers and is also found in cranberries and some grapes.
In aurantinidin, all the R groups are -OH groups, except for R3 and R6 that are hydrogen atoms. The molecule’s structure is shown to the right against a red background. This molecule is typically responsible for a red-purple colour of many red berries. This includes grapes, blackberries, cranberries, and raspberries.
A Change of Colour
Another amazing thing about anthocyanins is they change colour depending on how acidic or basic their environment is. We can use the pH value to determine whether something is acidic or basic. In general, a pH less than 7 is acidic, greater than 7 is basic and exactly 7 is neutral.
Low Red, High Blue
Remember delphinidin from earlier? We said this molecule is responsible for the blue colour of flowers. Well, this blue colour only occurs with a high pH. When the pH is the colour of delphinidin changes to red.
A similar colour change occurs for another anthocyanin called cyanidin.
Let Me Fetch My Cabbage
The really cool thing is that anthocyanins occur in red cabbage. This makes red cabbage a great indicator to tell us how acidic or basic something is. In acid, the red cabbage is, well, red! But, in bases the colour changes to a more green/colour.
You can find some more information about red cabbage indicators at Engineer Inside – a page made by Phil (who makes the graphics here) during his day job at the University of Warwick. A Ph.D. student at the University made pH indicators from red cabbage to create a rainbow. You can watch Evé Wheeler-Jones’ video at the link above.
Cooking With Anthocyanins
We have seen the importance of anthocyanins in leaves and fruit, but can we use it? Yes, many food dyes contain anthocyanins and are safe to for humans to use and eat!
Another possible use for anthocyanins is in a specific type of solar cell. Solar cells are devices that can transform light energy into electrical energy.
A Splash of Colour
Autumn is full of red, orange, yellow colours, from a leaf turning from green to golden, to the fruits and berries. A lot of this colour comes from the anthocyanins with a very similar structure. However, small changes in structure, causes big changes in colour, but it’s not just the structure.
The pH can also change the colour of the anthocyanins. These molecules can be one colour in acids, or a different colour in bases.
A lot of science is about numbers, graphs or looking for something very small or very large. But sometimes all you need to look at is the colour!