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Mirrored molecules

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Symmetry can be tricky, especially when you’re a chemist. Grab some plasticine and discover why a mirror doesn’t always make a perfect copy.

You will need

• Plasticine – you need at least 5 different colours
• Toothpicks

What to do

1. Break off pieces of plasticine and roll them into balls. You will need at least 8 of red plasticine, and 2 of every other colour.
2. Take 2 balls of plasticine of the same colour (not red). Stick 4 toothpicks into each ball, so that they form a pyramid shape.
3. Stick balls of red plasticine on the ends of the toothpicks. You should have used up all 8 red balls.
4. Look at your models. They should be the same. Place them next to each other, as if one was the reflection of the other.
5. Remove a red ball from one of the toothpicks, and replace it with one of the other colours. Remove the corresponding ball from the other model and replace it with the new colour. Your models should now have 3 red and 1 each of 2 other colours. Are your models still identical?
6. Remove another red ball from each model, and replace it with a fourth colour. You should now have 2 of one colour, and 1 each of 3 other colours. Make sure they are mirror images of each other. Are they identical?
7. Remove and replace another red ball from each model with the final colour, so you now have 5 differently coloured balls. Make sure your models are mirror-images of each other. Take one model, and try to arrange it so that it is identical to the other. Can you do it?

What’s happening?

The 2 models might look the same, but they’re slightly different. Even though they are made of the same components, they are not identical. This property is called chirality, from the Greek word for ‘hand’. This is because hands show a similar property – they are mirror-images of each other, but, keeping the palms facing the same direction, they can’t be placed exactly over each other. To make these models, you need to have 4 different balls arranged around the centre ball. If any 2 balls around the centre are the same colour, the models are identical mirror-images.

Applications

In chemistry, some molecules come as chiral pairs that are mirror images of each other. As they have the same number and types of atoms, chiral molecules share most of the same chemical properties. However, they have some differences.

The geometry of a molecule is important. Receptors in the mouth and nose are themselves chiral. They can interact with some molecules, provided the shapes of the receptor and molecule can fit together. These interactions give rise to the sensations of taste and smell.

Think of receptors as being like your hands, and chiral molecules as gloves. You can fit your right hand into a both a right- and left-handed glove. The interactions between your hand (the receptor) and the different gloves (the molecules) produce different responses – each glove fits, but feels different.

The slight difference in the geometry of chiral molecules means they can taste or smell different from each other. For example, with amino acids (the building blocks of proteins), one form will tend to taste sweet, while its mirror-image may be bitter.

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