Nature’s Palette

Harnessing the colours of creation for a brighter future
“A red sun rises. Blood has been spilt this night” The ominous words of the elf Legolas in Tolkien’s ‘The Two Towers’ conjures up a […]

Art by Kate Pocklington.

“A red sun rises. Blood has been spilt this night”

The ominous words of the elf Legolas in Tolkien’s ‘The Two Towers’ conjures up a vivid scene in the mind, one which would lose much of its evocative nature if our blood was actually green, or yellow, or baby blue. Likewise a faint mauve environment would no doubt have provoked quite different poetical musings from the likes of Cowper and Keats, than the verdant one in which we live.

The palette with which nature was painted is a clever chemical one. Plants are able to convert carbon dioxide and water to sugars during photosynthesis at an unimaginable rate because they contain the green dye chlorophyll. What’s more, haem—part of the molecule haemoglobin—has a structure which is perfectly designed to transport oxygen in the body to where it is needed most. It is this haem unit which is responsible for the redness of blood.

White light, emitted by the sun, is made up of many colours (as you can see if you allow light to pass through a prism, or indeed raindrops to create rainbows). Pigments and dyes are coloured because they absorb some of this light, but not all of it.  The rest is reflected as the colour we see.
Haem and chlorophyll are close cousins based on a family of ring shaped molecules called porphyrins. Indeed the word porphyrin comes from the Greek word ‘porphyra’ meaning purple pigment.

Porphyrins are brightly coloured compounds made up of carbon and nitrogen atoms arranged in a fairly rigid ring, often with a metal ion bound in the hole in the middle (such as iron in the case of haem). Lots of electrons whizz around the ring like cars on a Scalextric track. These electrons interact with light to gain energy, absorbing only the colours of light which match to the energy they need. Many subtle factors may affect the exact colours absorbed, for example the binding of oxygen to haem turns the molecule bright red, as you can see if you compare the colour of blood in your veins (scarlet) to that of your arteries (dark red-brown). Faulty production of haemoglobin causes the disorder porphyria which can cause horrific symptoms including sensitivity to sunlight, skin darkening and even increased hair growth-leading many to believe that porphyria is behind ancient tales of vampires and werewolves.

Nature has chosen her dyes well and this is something mankind is keen to exploit. Porphyrins can be synthesised in the lab and hence harnessed as potential candidates for artificial light harvesting compounds. Just as plants use chlorophyll to fix the sun’s energy as the chemical fuel glucose, so chemists are developing similar compounds to turn the sun’s rays into electrical energy. In this new solar technology, a porphyrin is attached to a surface and exposed to light. This molecule absorbs the light and, in doing so, throws one of its electrons out from its ring. The electron enters the surface material and flows through a circuit before eventually recombining with the porphyrin, generating an electrical current in the process. These new devices are called dye sensitised  solar cells (DSSC). By chemically altering the outside of the porphyrin ring, this process can be made more efficient, offering cheap, accessible alternatives to the expensive solar cells currently in use.

The properties of porphyrins have also been exploited medically, and are being used in cancer therapy trials as part of ‘photodynamic therapy’- using light to heal the body. A big problem in cancer therapy is how to specifically target and kill cancer cells without harming healthy tissue. Porphyrins can achieve this by passing the energy they gained from absorbing light to oxygen. Oxygen is very good at accepting this energy, and in doing so, the oxygen becomes toxic to cells. Modification of the porphyrin molecule, by attaching water soluble regions, may allow it to be introduced into the body and taken up by cells. Specific cells may be killed by shining light through the skin onto the target area to generate the toxic oxygen. The areas left dark are not harmed because the porphyrin dye itself is not toxic.

Porphyrins are not just bright and beautiful, but are prime examples of how nature, with its love of method and efficiency, has given us inspirational tools towards forging a cleaner, healthier future for ourselves.

Nicola Davis is a 3rd year DPhil in Chemistry working on the synthesis and properties of porphyrin systems at Worcester College.

About Nicola Davis