The Chemistry of Autumn Colours

Autumn officially began 3 days ago. The days have started getting darker, the wind has got colder and the leaves have started changing colours – and chemistry is here to explain how.

Aside from pumpkin-spiced lattés and bonfire nights, the colours of autumn leaves are one of the favourite things about this interim season. The transformations in leaf pigments – from brilliant greens to fiery oranges and deep purples – are caused by a range of chemical compounds that have been hiding under our noses all year round.

3 Types of Pigments in Autumn

There are several chemical compounds found in leaves and they are each responsible for the array of colours we see in autumn. These compounds are able to make us see colours because they contain conjugated systems.

In colour-causing compounds found in leaves, conjugated systems are characterised by alternating double and single bonds. When a lot of conjugation takes place inside a compound, the molecules are able to absorb and reflect different wavelengths from the visible light spectrum. The changes in light and temperature that take place in autumn bring these wavelengths to life through leaves, and there are three main pigments involved in this: chlorophyll, carotenoids and anthocyanins.

1. Chlorophyll

The chlorophyll pigment is found in the chloroplasts of nearly every leaf cell, and it is the reason why leaves appear green. But chlorophyll is more than just a pigment; it is a workhorse compound that plays a crucial role in photosynthesis by allowing plants to convert light energy into chemical energy.

With the chemical formula C₅₅H₇₂O₅N₄Mg, chlorophyll causes leaves to look green because its molecules absorb sunlight in the red and blue spectrum. This means that, since there are diminished levels of these two colours, the leaves reflect light in the green spectrum.

The light that chlorophyll absorbs is a form of energy that powers the chemical reactions necessary for photosynthesis to take place. These reactions involve transforming carbon dioxide and water into oxygen and carbohydrates, or sugars.

For plants to synthesise chlorophyll, they need 2 things: sunlight and warmth. This is why, during summer, leaves are brilliantly green – because chlorophyll is constantly being regenerated. It is also why, as we enter autumn, the production of chlorophyll in plants begins to decline and eventually decompose.

But this gives rise to a spectacular display of colours because, when chlorophyll starts to decompose, we start to see other pigments that have been hiding beneath our very noses.

2. Carotenoids

One category of leaf pigments that have been hiding in plain sight all year round is carotenoids. This large family of chemical compounds absorbs different wavelengths of sunlight than chlorophyll. By absorbing light in the blue and blue-green spectrum, carotenoids reflect orange-yellow instead of green light.

Because they absorb different light spectrums, one function of carotenoids during summer is to transfer this new light energy to chlorophyll so that it can continue to fuel photosynthesis.

Like chlorophyll, carotenoids are very large molecules that can be found in the chloroplasts of the leaves. The reason we don’t often see their bright yellow pigment is because, especially during summer, there is much more chlorophyll contained in leaves and these higher levels overpower the presence of carotenoids.

However, carotenoids are much more stable than chlorophyll because they take longer to decompose. So, when autumn rears its head and chlorophyll levels start to deplete, the orange and yellow hues of carotenoids start to appear.

This group of pigments give colour to a range of things other than leaves, including:

• Canaries
• Daffodils
• Bananas
• Corn
• Egg Yolks
• Buttercups

An important compound in carotenoids is beta-carotene (C₄₀H₃₆). This is a hydrocarbon that is responsible for the more orange hues in leaves, as well as the colour of carrots. Lutein (C₄₀H₅₆O₂), another carotenoid compound that is classed as a xanthophyll, is what contributes to the yellow colours exhibited by this pigment.

3. Anthocyanins

Anthocyanins belong to the flavonoid class of compounds in leaves. They can be seen flashing their characteristic colours in trees like red maples, red oaks and sumacs. Anthocyanins reflect pigmented hues of red, scarlet and deep purple because they absorb sunlight in the regions of blue-green, blue and green. This red pigment is also found in:

• Roses
• Black grapes
• Most red fruits

Unlike their relatives, anthocyanins aren’t hiding in leaves throughout the year because they aren’t attached to the cell membranes. Instead, they are synthesised in the leaf cell by an elusive process that depends on a high sugar concentration in the leaf, as well as sunlight.

Because sunlight is specifically required to produce the pigment that made anthocyanins famous, the side of a tree that faces the sun often has a stronger colour than the other side. This is also why some organic apples are only red on one side.

Anthocyanins are the most elusive of these 3 pigments because not a lot is known about how they function in the leaf. However, it has been suggested that they help protect the leaves from light damage in order to put-off shedding for as long as possible.

The transformative powers of leaves during autumn are just one example of how chemistry has impacted the world as we know, or see it. While autumn colours may seem like a simple enough concept, when you look closely at them, they’re one of the most striking natural phenomena!