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Recognising unrecognised patterns

S Ananthanarayanan | New Delhi |

The progress of science has been marked by extraordinary people who recognised patterns, which others could not. That capacity was not entirely a “gift” but came from scholarship in the sciences and facility in abstract thinking, as in mathematics.

Acuity of perception is also believed to be sharpened by practice of music, dance or the visual arts. Joseph Gal, professor at the School of Medicine, University of Colorado at Aurora, Colorado, US, describes in the journal, Nature Chemistry, how proficiency in drawing portraits perhaps helped the young Louis Pasteur to spot some structural features in crystals, which were missed by capable scientists before him.

Pasteur is best known for discovering the principles of vaccination, microbial fermentation and pasteurisation. His contribution, however, is also formidable in chemistry, and his discoveries about the structure of tartaric acid crystals laid the bases of understanding the structure of organic compounds. Gal, in his article, suggests that Pasteur’s proficiency in art helped him visualise the geometry of crystals and identify the two forms — looking alike but distinct — that tartaric acid could take.

The background of the work with tartaric acid is the different effects that transparent materials have on light that passes through them. One effect, of course, is colour — the material allows some colours but not others. And another is the speed of light during its passage through the material. It is because light is a little slower when it passes through glass, than through air, that we have lenses, prisms, telescopes and microscopes. And because the speed of light through a material is different for different colours of light, a prism splits light into rainbow colours. Indeed, it is for this reason that droplets of water in the sky bring about the rainbow.

An important aspect of the nature of light is that it is a wave, a lot like the waves in the sea or ripples on the surface of a pond. We can readily see that these waves are not flows of water but are the regular up-down movements of particles of water that stay essentially at the same place. The light wave also consists of electric and magnetic effects that vary not in the direction of the beam of light but in the transverse plane. Unlike particles of water, which move only up and down, however, electromagnetic effects can vary up-down, right- left, or in any other direction, except along the direction of the beam itself.

And then, there are materials in which light has a different speed for the waves whose components move up-down, as against right-left, with respect to an axis of the material. Different parts of the light falling on a sheet of such a material would hence bend by different angles and a beam could be split into two beams, each with different axes of vibration. Each such beam, with a single axis of vibration, is known as a polarised beam, as opposed to normal light, which has light vibrating along all axes.

When a beam that has been polarised, or consists of waves along a specific plane, is passed through a class of “optically active” materials, the electric and magnetic parts get slowed down to different extents. The result is that the two kinds of vibration fall out of step and the axis of polarisation is turned by an angle, which depends on the material and the thickness of the sheet of the material. And then, depending on the material, the change in the axis of polarisation can be to the left or right. Materials that change the axis to the right are called “dextrorotatory” and those that turn the axis to the left are “laevorotatory”.

Tartaric acid or TA is an organic acid, which is to say that it is based on carbon, like methane, benzene, and living things. Thus TA naturally occurs in plants and is found in wine lees. Jean Baptiste Biot had discovered in 1832 that tartaric acid solutions were optically active and dextrorotatory. A puzzling feature, however, was that TA, derived from natural sources like from wine lees, showed activity but TA synthesised in the laboratory did not show any. Pasteur, then just 25 years old, took the work forward and began to examine the crystals of tartaric acid and its salts. He found that the crystals were not completely symmetric, but displayed “handedness”. This is like our right and left hands, which are built in the same way and match when they are placed against each other, but not if placed one on the other. This is to say that one hand cannot be re-oriented, or turned around, so that it appears to be the same as its mirror image. Pasteur found that synthetically formed crystals of tartrate salts could be distinguished as two distinct types, and one crystal type was the mirror image of the other. Painstaking separation of the crystals and testing their effect on light that was passed through their solutions revealed that one kind was dextrorotatory, the other kind was laevorotatory, and a mixture of the two, which had equal portions of the two kinds of tartrate, was not optically active!

The subtle property of the two forms of crystals of the same substance — that they formed in the same way — but were still non-super imposable, except through a mirror, led Pasteur to the conclusion that molecules themselves had two forms — not symmetrical but mirror images of each other. This was in 1849. A whole 25 years later, in 1874, van’t Hoff and Le Bel proposed a mechanism and the structure of the two molecular forms is now well known.

The science of these alternate forms of molecules is now an important field of stereochemistry and is applied in designing drugs or reagents, which retain the properties of one form but avoid shortcomings, like side effects because these cannot arise in the alternate form. Today, we have sensitive methods to measure wavelengths of infra-red light that molecules absorb or emit, or to investigate crystals by interference of X rays. But Pasteur worked out the basics of asymmetric molecules from just visual examination of crystals, “with tweezers and a magnifying glass,” at a time when modern methods were not there. And almost as soon as he entered the field of study, in which others had been long immersed, he observed a feature that had eluded them. Gal has studied Pasteur’s work in detail and has written extensively on the subject. He says in the paper that Pasteur was as much an artist as he was a scientist. Till the age of 20, Pasteur was a committed artist who created portraits and lithographs. While creating an accurate likeness sensitises the creator to symmetries, shades and proportions, creating prints using lithography calls for the original impression on the lithograph to be a mirror image of the subject. The practice surely enabled Pasteur to visualise shapes and structures and to see how a detail of asymmetry could be the reason for differences in optical activity.

The well-known example of the artist-scientist is Leonardo da Vinci, with interests in painting, sculpting, architecture, literature, anatomy, cartography, history and writing, apart from the sciences. And there are more instances — Albert Einstein was trained in the violin since the age of five.

The writer can be contacted at [email protected]