Why grass is green

The Colours of Life
January 2, 1998

Grass is green and blood is red because they both contain pigments from the porphyrin family. What is more, unlike the colours that give flowers their beauty, these pigments have been a major driving force in the evolution of life and they are essential to the continuing existence of the organisms that contain them. Lionel Milgrom, who has led a colourful life as a porphyrin chemist for more than 20 years, has written a comprehensive account of these fascinating molecules that is intended to appeal to a general audience.

Porphyrins (the word comes from the ancient Greek word for purple) are large, flat molecules whose unique electronic structure enables them to interact with light in a way that gives them their characteristic colours and their remarkable biochemical properties.

Chlorophyll, the stuff that makes plants green, allows photosynthesis to take place against a thermodynamic gradient. In other words, high-energy carbohydrates can be built up in plants from carbon dioxide and water, which are low-energy molecules, because chlorophyll can trap the energy of sunlight and transform it into the chemical energy that drives the process.

Photosynthesis produces oxygen, from the water it uses, as a waste product. When free oxygen from photosynthetic bacteria began to accumulate in the atmosphere at the end of the Archean eon, it would have been a powerful toxin for some primitive microbes. At the same time, it allowed the development of more advanced life forms by providing such organisms with a more efficient means of generating biochemical energy. Later, large, fast-moving vertebrates (including ourselves) needed a transport system to move oxygen around their bodies. Enter haemoglobin, the oxygen transport protein whose red colour comes from an iron-containing porphyrin called haem.

Milgrom captures the elegant chemistry of haemoglobin with this question. "Iron, in the presence of oxygen and water, turns to rust, the highly soluble hydrated form of ferric oxide. Anyone who has owned a car for a long period of time will know that a constant battle has to be fought against rusting bodywork. Yet our bodies are full of air, water and iron-containing proteins, so how come our arteries and cells are not clogged with thick, gelatinous, hydrated ferric oxide?" The answer lies in the specific way in which the iron atom is chemically bound within the haemoglobin structure, which allows it to transport oxygen molecules to all the tissues of the body without combining with it to form rust.

The remarkable properties of these colourful molecules have great potential for exploitation in fields as diverse as energy supply, medicine, electronics and materials. Milgrom is especially good on reviewing the future of porphyrins. For instance, when porphyrins absorb light they can pass the energy on to oxygen molecules, thereby creating a highly reactive form of oxygen called singlet oxygen. Although this is highly damaging to tissue in the porphyrias -Ja group of inherited diseases characterised by defects in porphyrin metabolism -it can be harnessed to fight disease. In photodynamic therapy, which has been tested with good results in a range of cancers, a porphyrin is injected into a tumour and the area then irradiated with light. Singlet oxygen thus formed destroys the cancer cells.

There is also the possibility that photosynthetic porphyrins could harvest sunlight and use it to split water into hydrogen and oxygen. Hydrogen has great potential as a cheap and clean energy source. And finally, stacked arrays of porphyrin molecules have much promise as "molecular metals", which could be used in a variety of electronic and computer applications.

If this book has a fault, it is that Milgrom does not focus clearly enough on his audience. When talking about the chemistry of oxygen, for instance, the style is easy yet authoritative (just what you would expect from an author who has developed a parallel career as a science journalist). But even the most dedicated non-specialist will find the chapters on the classification of porphyrins and the theory of their electronic properties very hard going. There are far too many diagrams of chemical structures, several of which are not even referred to in the text.

Nevertheless, Milgrom's enthusiasm for his porphyrins is never far away, and it more than makes up for his excursions into the technical and esoteric aspects of the subject. There are few enough books around that try to communicate the importance and excitement of chemistry to the general reader. The Colours of Life is worth a try for this reason alone.

Susan Aldridge is a freelance journalist and writer, who was formerly a research chemist at the Medical Research Council.

The Colours of Life: An Introduction to the Chemistry of Porphyrins and Related Compounds

Author - Lionel R. Milgrom
ISBN - 0 19 855380 3 and 855962 3
Publisher - Oxford University Press
Price - £49.50 and £22.50
Pages - 249

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