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Components of Cells
The Macromolecules
Symmetry

Handedness

A lot of the tools we use, and many of the objects we see around us every day, are designed for use either by a right handed person, or a left handed person. You can buy and use a pair of scissors, or a set of golf clubs, that are specifically made for a "left handed" or "right handed" user.

Similarly, shoes and gloves are specifically made in two versions; a "left" shoe or glove, and a "right" shoe or glove.

These are what are called chiral objects, meaning that they have the property of "handedness", or two versions that are not identical in all respects, even though they may be designed to do the same job or basically have the same properties (like the golf clubs).

A good test of a chiral object is to see if you can superimpose the original object on its mirror image. If you cannot superimpose the object on its mirror image, it is a chiral object (like a hand or a shoe).

Alternatively we also use a lot of tools and objects that are the same design for both left-handed and right-handed use. A tea cup, an electric drill or a baseball bat are made in exactly the same form for both kinds of users. These are achiral objects, meaning that they are superimposeable on their mirror images.

Symmetry

Wether an object has "chirality" or not, depends on the property of symmetry.

In its simplest form, an object has symmetry if it can be cut in half along a point, plane or line, and produce two "halves" that are identical reflections of each other. If a baseball bat is cut along its length (into two equal halves) you could hold either half up to a mirror and reproduce the intact, original bat once more.

A symmetry element is the point, plane or line along or about which a reflection (or a rotation) leaves the object looking exactly the same as the original.

For example, you can rotate a baseball bat around its long axis as much as you like, and it always looks like the same bat. The symmetry element is the long axis, and the baseball bat is a symmetrical object.

Achiral objects, like baseball bats, all have some type of symmetry element, whereas chiral objects, like shoes, can NEVER have any kind of symmetry. You cannot cut a glove in half along any plane or line and produce to halves that are equivalent reflections of one another.

Asymmetric Carbon atoms

If a carbon atom is bonded to four hydrogen atoms to produce the molecule methane, that methane molecule is symmetrical and achiral. But if a carbon atom is bonded to four different functional groups (such as in the molecule glyceraldehyde), then the resulting molecule can never have any plane of symmetry - it is asymmetric - and the resulting molecule is always chiral.

Any molecule in which there is a single carbon atom bonded to four different functional groups (or parts of the larger molecule) there is asymmetry and the molecule is chiral. This carbon atom is sometimes called the asymmetric carbon or the chiral center.

If a molecule, like glyceraldehyde, has such a "chiral center" then it can have two different structural forms, or stereoisomers. The more chiral centers a molecule has the more forms are possible and the more stereoisomers there will be.

This kind of property, where there are differing stereoisomers caused by one or more chiral center is often called enantiomorphism, and the pair of mirror-image stereoisomers are often called enantiomers.


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© 2004, Professor John Blamire