Chiral objects are those that have the same shape, but cannot be superimposed. Chirality can be difficult to wrap your head around, but one example of chiral objects is right and left hands. Chirality is an important concept in chemistry, where sterioisomers are chiral molecules with the same chemical formula and series of atoms, but different three-dimensional structures. If two sterioisomers are mirror images of each other, they are known as enantiomers. If two stereoisomers are not mirror images, they are known as diastereomers. Diastereomers are not mirror images because they have more than one point where the structure can change, or stereocenter.
All of the naturally occurring amino acids, except glycine, are enantiomers. The two mirror images are called D- and L-amino acids. All the amino acids in proteins synthesized in ribosomes, in other words almost all proteins made by living things, are composed of L-amino acids. Not all proteins are formed in ribosomes and, although proteins containing D-amino acids are relatively rare, it is increasingly evident that they are present in most, if not all, life-forms and have biological functions. The role of D-amino acids is still being unraveled.
The Thalidomide Tragedy
Thalidomide is probably best known for the extreme birth defects it caused in the late 1950s and early 1960s, when it was prescribed as an aid for morning sickness. Thalidomide is an enantiomer, and the prescriptions given to the pregnant women included equal parts of each thalidomide configuration. When the birth defects were studied, it was learned that only one of the enantiomers was teratogenic, or caused defects in the womb. The other enantiomer is comparatively safe, although it can be converted to the teratogenic form in the body.
Glucose, Galactose and Mannose
Sugars are another example of biological molecules that have sterioisomers. The six-carbon sugars mannose and galactose are diastereomers of the sugar glucose. Glucose has four stereocenters, meaning a total of eight D-sugar and eight L-sugar stereoisomers exist. Most naturally occurring sugars are D-sugars. Apart from the four stereocenters, glucose molecules also form rings by joining the first sugar to the sixth sugar, creating yet another set of alternate conformations, in this case known as alpha and beta rings.
Not all stereoisomers are organic. Coordination compounds are compounds that have a central metal atom surrounded by nonmetal atoms, known as ligands. Coordination compounds are most often square, hexagonal or octahedral, depending on how many bonds they form, and can have enantiomers or diastereomers, depending on the bond arrangement. The dye Prussian blue is an example of a coordination compound with stereoisomers. One enantiomer of the dye is soluble in water, but the other is not.
- Introduction to Organic Chemistry: Stereochemistry
- Bridgewater State University: Stereochemistry of the Amino Acids
- Viva Origino: D-Amino Acids: A New Fontier in Amino Acids and Protein Research—Practical Methods and Protocols [PDF]
- News Medical: History of Thalidomide
- Rennselaer Polytechnic Institute: Carbohydrates—Sugars and Polysaccharides
- Encyclopaedia Britannica: Coordination Compounds
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