Ibuprofen and Acetaminophen
Organic chemical compounds contain a vast number of isomers, molecules with the same molecular formula but different atomic arrangement, of which there are three types: structural, geometric and optical. The most closely-linked of isomers are optical isomers, which differ only by the three-dimensional placement of the molecule’s attachments, which renders its mirror images to be non-superimposable. The presence of optical isomerism within a molecule is determined by the existence of a chiral centre—a carbon atom with four different groups (see Appendix, figure 1). Pure optical isomers have identical physical properties such as melting point, boiling point and density, as well as identical chemical properties; thus there are only two ways that they can be distinguished: their interaction with other chiral substances and their interaction with plane polarized light. An enantiomer (one out of a pair of optical isomers) rotates plane polarized light in the opposite direction of which the other enantiomer rotates the plane polarized light, of equal magnitude (see Appendix A, figure 2). An enantiomer that shifts plane polarized light to the left is given the prefix “L” (levorotatory) or “S”, while an enantiomer that shifts plane polarized light to the right is given the prefix “D” (dextrorotatory) or “R”.
Despite their physical and chemical similarities, optical isomers are known to have substantially different behaviours within the human body. This is due to the fact that enzymes and receptors in the body are stereospecific, meaning that they can interact with one enantiomer of certain molecules and not the other. For instance, the human body can only break down D-glucose (dextrose) for energy but not L-glucose, and can only utilize L-amino acids rather than D-amino acids. While one form of a stereoisomer may be beneficial, the other may be ineffective or even harmful, in some cases. Thus, it is crucial that optical isomerism is taken into account during medicinal drug development and usage. For example, the sedative thalidomide was available in Europe in the 1960s for purposes of alleviating morning sickness in pregnant women. However, the drug was sold as a racemic mixture, which contains equal portions of both enantiomers (this would not shift plane polarized light as the two enantiomers shift in different directions), and while R-thalidomide (see Appendix A, figure 3) works effectively as a sedative; S-thalidomide can cause genetic damage leading to mutation of the fetus. Consequently, 12 000 infants were born worldwide with malformation of the limbs.
To further illustrate the importance of optical isomers in drug action, the drug ibuprofen can be explored. Ibuprofen is a drug used for anti-inflammatory purposes such as pain relief, fever and swelling reduction, and is classified as a nonsteroidal anti-inflammatory drug (NSAID). Derived from propanoic acid in the 1960s by a pharmacy chain called Boot’s UK Limited, ibuprofen was initially launched as a treatment for rheumatoid arthritis, and was awarded the Queen's Award for Technical Achievement in 1987. It is currently available under a variety of trademarks such as Advil, Motrin, Nurofen, and Brufen, among others. Ibuprofen works by inhibiting the enzymatic action of cyclooxygenase (COX1 and COX2) within the body, which catalyzes the conversion of a compound called arachidonic acid into prostaglandins. Prostaglandins are locally-acting hormones that cause swelling, heat, loss of function, fever and pain, collectively known as inflammation, at a site of injury through the accumulation of white blood cells. Through inhibiting this reaction, painful symptoms can be reduced or eliminated.
The IUPAC name for ibuprofen is 2-(4-(2-methylpropyl) phenyl) propanoic acid. It is a carboxylic acid which also contains a phenyl group (see Appendix A, figure 5). Due to its chirality, ibuprofen has two enantiomers (see Appendix A, figure 6):...
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