D.8.1 Describe the importance of geometrical isomerism in drug action

Geometrical Isomerism -- molecules with a carbon-carbon double bond which prevent the rotation around a bond so that the position of the atoms can change.

There are 2 important isomers - trans and cis isomers.

Cis isomer
  • two functional groups will be on the same side of the double bond and bonded to two different carbon atoms.

Trans isomer
  • two functional groups bonded to opposite sides of the double bond.
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Physical properties of cis and trans isomers
Usually the trans isomer will have a higher melting point because the molecules are able to pack more closely together causing the forces of attaction betwen the molecules to increase. Therefore more energy needs to be absorbed in order to break them, increasing the melting point.

Cis and trans diamminedichloroplatium (II)
  • Both isomers exhibit different properties.
  • The cis isomer called cisplatin is an effective treatment of testicular and ovarian cancer while transplatin is not.
  • Both molecules have no overall charge which enables them to diffuse through the cancer cell membrane.
  • Once inside the cell, the two isomers will lose a chlorine molecule and gain a molecule of water to form the complex ion [Pt(NH3)2Cl(H2O)]+.
  • This complex ion will then enter the cancer cells' nucleus but due to the orientation of the two different isomers, only the cis isomer will be able to bind to two guanine