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Sunday, August 26, 2012

Chirality revisited

As a break from our harsh Canadian winters we ‘went south’ to the Caribbean islands for many years. I love beach combing and as a result, I have baskets full of sea shells at the cottage that I have picked up and enjoy handling.

When I read that spiral shells were a good example of the invisible chirality [left or right handedness] that many molecules in living systems possess, I lined up some of my spiral shells with their earliest growing tips down and took the picture on the left. When you look at the picture, you can’t help but see that the open ‘mouth’ of the shell is always on the left.

Try it out yourself the next time you have any spiral shells at hand, and you may be lucky enough to find a shell that breaks the rule and opens on the right. They are rare, but exceptions do exist, and therefore the shell's outward chirality must be for another reason, because our body’s chiral molecules are always only one-handed without exception! For snails, the reason for the shell coiling happens to be sex. Snails make coiled shells because their sexual organs are usually twisted, and it is difficult with snails of opposite handedness to mate. Picture on the right is the Nautilus shell.

The fact that our body’s DNA, enzyme and sugar molecules, etc., are chiral was not generally known until a widespread medical tragedy occurred. In the late 1950’s, a drug called ‘thalidomide’was developed by a pharmaceutical company as a sedative and a treatment for morning sickness in early pregnancy. When tested on animals it had few or no side effects, and so was used in many countries, including the UK and Canada starting in 1958. It was not until 1961 that doctors fully realized the drug was the cause of severe birth defects – many children developing flippers instead of arms.  It was withdrawn but too late for kids shown on the left .

When thalidomide was investigated, the results knocked organic chemists for a loop. It was discovered that the molecule was chiral, and when synthesized in the lab, equal amounts of two isomers, one left-handed and the other right-handed, were produced. Because molecules involved in our body chemistry are also chiral, the two isomers reacted differently, with one isomer causing the birth defects. Fortunately, under normal circumstances, if we ingest a molecule with the wrong handedness, our bodies recognize it and - unlike the unfortunate thalidomide case where a fetus was in an early stage of development - it is sent to the liver where most toxins are dealt with.

The development of one-handedness in our body chemistry probably happened just by chance when life first formed on our planet. Should we ever travel to another planet many light years away, it is possible that life started there with its molecules having the opposite handedness, and we could eat as much of their food as we wanted, but we would starve to death! Rie


  1. Do you have any proof of that 10% dextro shells?
    what is the source of the chirality?
    two theories pop up..
    one. from molecules to macro structures:
    "dextro sugars builds chitin in one way only, and the macro molecules are destined to coil X not Y."
    this happens in DNA where L-acids create a D coil.

    (I think that a combination of the two theories might be the reason.)
    two. the earth is spinning, the water in my sink spins D while down south it spins L :
    this would result in geographic preferences,
    10% of the shelled gastropods live in one half of the earth's waters and 90% in the other..
    (any info on that?)