Chiral compounds have mirror-image molecules, with the same chemical parts in opposite arrangements, unlike achiral molecules, which have mirror-image molecules that are identical. Chiral molecules cannot be rotated to match each other, but they have identical chemical properties in most situations. Chiral molecules are important in areas of biology in which cellular machinery typically uses only one version of the molecules.
The different mirror-image molecules in chiral compounds are known as isomers. Molecules in biology are often very large and complex, and they have three-dimensional structures that act in specific ways on their environment. Their size means that each molecule often has different regions with different chemical activity. Despite the fact that different isomers have identical physical and chemical properties, such as boiling points, polarities and densities, they interact with other biological molecules differently. Sugars, DNA and proteins are all examples of biological molecules that are chiral.
Despite the fact that the isomers of chiral compounds behave identically in many situations, different isomers can be distinguished using light. When light shines through a chiral isomer, it bends a certain amount of that light based on the properties of the compound. When light shines through the opposite isomer, it bends the same amount of light but in the opposite direction.