The days are getting shorter and the weather has turned chilly. Whilst the seasonal change might give us grey days and force us to find those coats and scarves, hidden in the depths of our wardrobes, it also brings an amazing display of autumnal colour. These unmistakable transitions, happening in forests and parks around us, are due to molecular changes in the leaves of trees. In this post we take a look at the chemistry of leaves and the molecules that result in the vivid colours of the season.
1. The top image shows the leaves of heuchera, a plant often grown for its multicoloured foliage. Indeed the leaves in this photo show the breadth of pigments, from yellow, to brown, purple and red. The leaves are placed in test tubes, small glass tubes, which are often used by chemists when performing chromatography, to separate chemicals. These leaves demonstrate the various pigments which occur naturally in leaves. Of course, the colour we most commonly associate with leaves is green and this green colour is due to a very important family of molecules called chlorophyll (P May, Chlorophyll, School of Chemistry, University of Bristol). Plants, like all organisms, need energy to grow and reproduce, and the energy source they use is sugar. Whilst a few plants might get the food they need from eating, in the same way that humans do, the majority of plants get their sugar energy from sunlight, and it is chlorophylls that enable them to do this.
The process of converting sunlight into sugar is known as photosynthesis and a plant’s leaves are the factories where this process occurs. Photosynthesis uses carbon dioxide and water, which the plants absorb from the surrounding environment, and convert them into oxygen and sugar. This reaction is endothermic, which means that it absorbs energy from it’s surroundings. In this case the energy is light.
Visible light has a range of energies, with different energies relating to the different colour. You can see this range of energies during autumnal storms, in the formation of rainbows. Chlorophyll molecules absorb the blue and red parts of visible light; this also means they reflect green light, hence the colour of leaves. But chlorophyll isn’t a particularly stable molecule and plants which rely on photosynthesis have to constantly synthesise it. During the summer, chlorophylls enable the leaves to make sugar, which is transported around the plant for growth and storage. At the same time, a tree’s roots absorbs nutrients from the soil, which are transported to the leaves, and allow the plant to make more chlorophyll. So, in the height of summer the trees in our parks have leaves full of chlorophyll and so appear emerald green.
As the seasons change, the days shorten. This change in light levels triggers a response in the tree. A membrane forms between the leaves and the branches of the tree, which prohibits the flow of nutrients and slows the production of chlorophyll molecules, a process known as leaf senescence begins (Current Topics in Developmental Biology, 2005, 66, 135 – 160; Plant Physiology, 2005, 130, 1635 – 1648). At the same time, temperatures drop, which causes the remaining chlorophyll to break down. Chlorophyll is also sensitive to light levels and bright sunlight destroys chlorophyll. So clear, wintry skies further decrease the amount of chlorophyll present in the leaves, and the green colour fades. Advantageously, the products of chlorophyll degradation are also effective natural antioxidants (Angew. Chem. Int. Ed., 2007, 46, 8699 – 8702).
2. Not all plants have the same coloured leaves. This is because chlorophyll is not the only molecule, present in the leaves, which absorbs and reflects light. As chlorophyll only absorbs certain energies, or colours, of visible light, plants have evolved which incorporate other molecules that are able to absorb green light. Carotenoids, and specifically carotene, absorb blue-green light, resulting in the molecule appearing yellow. Yes, this is why carrots appear orange; the name is no coincidence. Birch leaves, for example, have high levels of carotene and chlorophyll, and hence birch leaves appear a brighter green than some other trees (www.scifun.org, The Chemistry of Autumn Colours, General Chemistry). Being more stable than chlorophyll, the carotenoid molecules tend to persist in the leaves of trees, even as the chlorophyll is decomposed, and slowly the leaves turn more yellow.
Another family of molecules responsible for the pigments in plants are the anthocyanins. We met these molecules in one of our first ever posts, exploring rhubarb. Anthocyanins also absorb green light, thus some trees have evolved higher levels of these red molecules, and striking examples are often found in maples and acers. In plants, anthocyanins are produced by the reaction of sugar and proteins, so sugar levels need to be high for the leaves to turn red. The synthesis of anthocyanins also requires light. A good example of this combination of high sugar content and light can be observed in ripening apples; the side of the apple which is exposed to the sun turns red as anthocyanins are produced, whilst the other side remains green.
Carotene and chlorophyll are really sizeable molecules, which are not soluble in the aqueous environment of the leaf cell sap. Instead, these molecules are attached to cell membranes. Anthocyanins, however, are soluble in water, and thus exist in solution in the cell sap of leaves. You can see the water solubility of anthocyanins when you boil red cabbage or rhubarb; the water becomes slightly coloured. In dry conditions, when there is less rainfall, there is less water available and the cell sap becomes concentrated. This results in a concentrated solution of sugar in the leaf, and more rapid anthocyanin production. So, for vivid, red autumnal reds you need dry, sunny days and cold, dry nights.
3. Chlorophylls have two distinct parts, the chlorin or porphyrin ring and the phytol, the long carbon chain. The porphyrin ring contains four nitrogen atoms which can chelate a metal ion in a square planar arrangement. In chlorphyll the metal is Mg2+ and, when the structure was elucidated in the early 1900s, it was the first evidence of Mg2+ in living tissue. There are many examples of this chelating porphyrin ring with metal ion, for instance heme, found in haemoglobin, has Fe2+ at the centre of the ring, whilst vitamn B12 contains a cobalt ion. The structural similarities has led to claims that chlorphyll can be used as a natural remedy for bodily odours, gum disease, skin conditions, to name a few. However, there is no firm proof that chlorophyll can aid in the treatment of these conditions (Quackwatch, Amazing Claims for Chlorophyll, 1987).
Although much of the structural elucidation work on the chlorophylls took place in the mid 1900s, chlorophyll f, which is responsible for photosynthesis in cyanobacteria and stramatolites, was only discovered in 2010 (Scientific American, 19th August 2010).
Chlorophylls facilitate photosynthesis by aiding charge separation (World of Molecules, Chlorophyll Molecule). The chlorophyll molecule undergoes a redox reaction; it is oxidised, to an excited state, by donating an electron, and subsequently reduced to it’s ground state by accepting an electron. The chlorophyll molecules are bound as part of a pigment-protein complex, essential for photosynthesis. Although well known, the process of photosynthesis is not well understood, and current efforts to resolve its mechanistic pathways are still under way (Advances in Protein Chemistry & Structural Biology, 2013, 93, 81 – 93). This has not perturbed research groups, keen on utilising the ability of chlorophyll to act as a light harvesting molecule. Chlorophyll, and similar molecules, are being tested, for their potential to increase the efficiency of photovoltaic cells (New Scientist, 19th August 2010; Int. J. Electrochem. Sci., 2013, 8, 8175 – 8190).
Contributors: Natalie Fey (photos and structures); Jenny Slaughter (photos, structures and words).