Introduction

Stereochemistry may seem like a trivial subject because there are no major differences between stereoisomers. In nature however, especially in the biological system like a human body, these minor changes may have severe consequences.

Explanation

In society, many drugs are composed of a single stereoisomer of a compound, and while one stereoisomer may have positive effects on the body, another stereoisomer may be toxic. Due to this, one of the key roles of organic chemists consists of synthesizing compounds consisting of a single stereoisomer.

In some instances, toxicity has been linked to one member of a pair of isomers. This pair need not be the active isomer for the substance to be toxic. For example, granulocytopenia is related to the d-isomer of levodopa and vomiting is caused by the d-isomer of levamisole.

Shown below is another example – the binding of Ibuprofen, a common pain reliever. While one stereoisomer of the compound has the right three-dimensional shape to bind to the protein receptor, the other does not and can not bind, and is therefore ineffective as a pain reliever.

One of the most common examples of the impact of stereochemistry cites back to the thalidomide disaster. Thalidomide is a pharmaceutical drug, first prepared in 1957 in Germany, prescribed for treating morning sickness in pregnant women. The drug was discovered to be teratogenic (able to disturb the growth and development of an embryo or fetus), causing serious genetic damage to early embryonic growth and development, leading to limb deformities in babies. In the human body however, thalidomide undergoes racemization: even if only one of the two enantiomers is administered as a drug, the other enantiomer is produced as a result of metabolism. Thus, it would be incorrect to state that one of the stereoisomer is safe while the other is teratogenic. Thalidomide is currently used for the treatment of other diseases, notably cancer and leprosy. Strict regulations and controls have been enabled to avoid its use by pregnant women and prevent developmental deformations. This disaster was a driving force behind requiring strict testing of drugs before making them available to the public.

Therefore, the importance of stereochemistry in biological systems extends to more than just drugs and medicines. The human body, for example, can only create and digest carbohydrates and amino acids of a certain stereochemistry. Thus, all of our proteins that make up our hair, skin, organs, brain, and tissues, are composed of a single stereoisomer of amino acids. Moreover, our bodies can make and digest starch (found in potatoes and bread) but not cellulose (found in wood and plant fibers), even though both are just polymers of glucose of different stereochemistry.

References:

http://www.chemhelper.com/biostereo.html

http://en.wikipedia.org/wiki/Stereochemistry