The Standing Invitation

Posts Tagged ‘Chirality

An Unsolved Problem

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You are walking in the countryside, William Paley-style, and you encounter a penny, lying on the ground, with heads facing up, tails facing down. It is reasonable to assume it just happened to fall that way ­– it could just as easily have fallen tails-up ­– and you would think nothing of it. But if you found thousands of thousands of coins, all with heads up, that would require an explanation.

There are some things that have a property called chirality, or handedness. If a thing is chiral, it has a mirror image version of itself that is not identical with itself. Gloves are chiral: a right glove and a left glove weigh the same amount, are made of the same material, have the same number and arrangement of fingers and thumbs. Importantly, they take the same amount of effort to make. But a left glove is not a right glove; they are non-superimposable mirror-images.

Chirality is a hugely important part of chemistry, because many molecules are chiral: they have mirror-image versions of themselves. The two mirror images of a molecule will have the same properties, the same weights and compositions. They take the same amount of energy to make, and when they are made they are made in equal amounts. Just as a coin tossed in the air is equally likely to land on either face, a molecule that is deciding which handedness to become in a chemical reaction will have no preference ­– resulting in equal amounts being formed.

So far, so good.

But we find that, in their interactions with living things, chiral molecules behave in rather odd ways. We have already agreed that a left-handed molecule has the same properties as its right-handed twin. But why does one molecule taste, to us, of lemons, and its mirror image of oranges? If one is molecule the active ingredient in cough syrup, why is its mirror image a drug a hundred times stronger than morphine?

The answer is that we are chemically chiral. And, just as a left and right shoe will fit differently onto a left foot, left-handed and right-handed flavour molecules will fit differently onto our left-handed taste receptors, producing different flavours, or will fit differently into our left-handed drug metabolism pathways, resulting in different medical effects.

But yes. I did just say left-handed. I did not say right-handed or an equal mixture of the two. Because, even though left-handed and right-handed things are equally probable, equally easy to make, living things are made from only left-handed molecules.

Every sugar molecule in your body – in all bodies everywhere ­– every amino acid and every protein and every DNA spiral has a possible mirror image form of itself, but these are nowhere to be found. In a world of symmetry, living things, surprisingly, are built of only one kind of chiral building block.

This is called biological homochirality, and how it came to be the case is one of the biggest unsolved problems in modern science.

Written by The S I

July 26, 2011 at 8:30 pm