Let’s celebrate the biological side of Stokes too!

One of Ireland's greatest scientists, George Gabriel Stokes, made major contributions to the fields of physics, fluidics, optics and mathematics, but as we mark the bicentenary of his birth this year, far less attention has been given to his immense contributions to biology.

With his interests in optics, he was intrigued by coloured inorganic and organic compounds and described for the first time what he called fluorescence – a phenomenon whereby a substance absorbs light of one colour, or wavelength, and then emits light at a longer wavelength. The difference (shift) in wavelength between absorbed and emitted light is known as the Stokes shift. Compounds with different coloured fluorescence are widely used in biology, especially immunology.

In experiments described in a paper entitled "On the reduction and oxidation of the colouring matter of the blood", published in 1864 in the Proceedings of the Royal Society of London, Stokes investigated changes in the colour of blood.

Compared with the jargon-filled writing of modern scientific publications, the writing style of the 19th century makes for delightful reading. Thus, to quote Stokes: “The observation is perfectly simple, since nothing more is required than to place the solution to be tried, which may be contained in a test tube, behind a slit, and view it through a prism applied to the eye.”

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Using this simple equipment, he observed, and made woodcut drawings, of the changes in the spectral lines seen when a beam of sunlight passed through a tube of blood maintained in different conditions. “I had no sooner looked at the spectrum, than the extreme sharpness and beauty of the absorption-bands of blood excited a lively interest in my mind and I proceeded to try the effect of various reagents.”

It was known that the colour of arterial and venous blood was different, and that oxygen entered the blood upon passing through the lungs. However, it was unclear if the oxygen was simply dissolved in the blood or was combined with a substance in the blood. Therefore, Stokes’ pioneering experiments showed for the first time that changes in the colour of blood were rapidly and repeatedly reversible by variation in oxygen availability.

Haemoglobin molecule

We now realise that what Stokes was describing were the changes in the configuration of the haemoglobin molecule. The word haemoglobin was used for the first time in 1864 by Hoppe-Seyler, a German scientist, and at the time of his writing, Stokes was unaware of this term; he therefore used his own term “cruorine” for this substance.

However, with his remarkable biological knowledge, he concluded: “Now it has been shown in this paper that we have in cruorine a substance capable of undergoing reduction and oxidation, more especially oxidation, so that we have all that is necessary to account for the absorption and chemical combination of the inspired oxygen.”

In addition to blood, Stokes also carried out and published investigations into the chloropylls of plants and seaweeds and in the process disproved the notion, current at the time, that chlorophyll and biliverdin – a green pigment found in bile – were identical.

Now we know haemoglobin, one of nature’s most remarkable molecules, is not oxidised or reduced upon exchange of oxygen molecules. One molecule of haemoglobin, made up of four subunits, can carry four molecules of oxygen and the more oxygen it carries, the greater its strength of binding oxygen.

It was only over a century after Stokes' original observations that Max Perutz, who had worked obsessively for decades on the structure of haemoglobin, determined the detailed molecular changes that occur in haemoglobin when oxygen binds.

Although Stokes is primarily regarded in the broadest sense as a mathematician/physicist, his pioneering biological experiments nevertheless made a significant contribution to what eventually became known as the field of biophysics, an aspect of biology that dominated the first half of the 20th century.

Considerable prominence

In Horace Freeland Judson's remarkable book "The Eighth Day of Creation: the makers of the revolution in biology", Stokes is indeed given considerable prominence for his contribution to biology.

Initially, scientists such as Stokes used only visible light to illuminate biological substances, but following their discovery at the end of the 19th century, X-rays replaced visible light. Because the very short wavelength of X-rays approximates that of inter-atomic distances, the internal structure of inorganic crystals, and when they became available, crystallised biological molecules, is revealed by the scattering of X-rays that pass through them, a phenomenon called X-ray diffraction. In the first half of the 20th century, X-ray diffraction became a major experimental tool for the elucidation of the structure of proteins, including haemoglobin – and in the hands of Rosalind Franklin, that of DNA.

One is left in awe at the remarkable intellect and delightful writing skill of this man from Skreen (An Scrín – meaning the shrine in Irish), Co Sligo. He was the youngest of eight children but science was in his bloodline. His father, the local rector, also Gabriel Stokes, was from a family with notable mathematician, physician and engineering ancestors.

Skreen is now bypassed by the N59, but for those interested in the history of science, visiting the village to see the plaque in front of the rectory commemorating George Gabriel Stokes is well worth the detour.

Rhodri Ceredig is a former SFI Stokes professor of immunology at

NUI Galway