Biochemistry Online: An Approach Based on Chemical Logic

Biochemistry Online





Learning Goals/Objectives for Chapter 8A:  After class and this reading, students will be able to

  • explain why oxidation reactions with ground state dioxygen have a high enough activation energy to make the reactions, although thermodynamically favored, kinetically slow
  • explain, using molecular orbital diagrams the difference between triplet and singlet dioxygen
  • using molecular orbital diagrams and Lewis structures, describe the chemical properties of the reduction products of dioxygen (superoxide, peroxide, and water)
  • explain the ways that biological systems use to enhance dioxygen activity and reduce the effects of reactive oxygen species (ROS) such as superoxide and peroxide
  •  write chemical reactions and mechanisms when appropriate for some reactions of triplet and singlet dioxygen, superoxide, peroxide and the hydroxy free radical
  • describe typical reaction of ROS with lipids, proteins, and nucleic acids and data to support the involvement of ROS in complex diseases and aging.
  • Briefly contrast the production and biological activities of ROS and reactive nitrogen intermediates (RNIs)

A3.  The Reactions of Dioxygen and its Reduction Products

1. Triplet O2 - Ground State:

Figure:  Triplet O2 - Ground State

Figure: unsaturated fatty acids are extra reactive at the methylene C that separate the double bonds

2. Single O2 - Excited State.

Figure:  Single O2 - Excited State

It can be made from triplet oxygen by photoexcitation.  Alternatively, it can be made from triplet oxygen through collision with an excited molecule which relaxes to the ground state after a radiationless transfer of energy to triplet oxygen to form reactive singlet oxygen. (This later process accounts for photobleaching of colored clothes when the conjugated dye molecules absorb UV and Vis light, relax by transferring energy to triplet oxygen to form singlet oxygen, which then chemically reacts with the conjugated double bonds in the dye. )

3. Superoxide

Figure:  Superoxide

4. Peroxide

Figure:  Peroxide

In contrast to dioxygen which contains multiple bonds between the O atoms, peroxide has only one bond.  In fact, it is quite weak and requires only 38 kcal/mol to break it. Remember, bonds can be broken in a heterolytic way (both electrons in a bond go to one of the atoms, or in a homolytic fashion, in which the one electron goes to each atom.

5. Hydroxy free radical:

Figure:  Hydroxy free radical

As mentioned above this species is extremely reactive. It will react with any molecule it encounters and does so immediately. It can abstract a H atom leaving another free radical.  For example, the hydroxy free radical could extract a hydrogen atom from a polyunsaturated fatty acid to from a carbon-centered radical.   A particularly nasty reaction is the insertion of the hydroxy radical into bases in DNA, as shown in the diagram.


Return to Chapter 8A:  The Chemistry of Dioxygen Sections

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Archived version of full Chapter 8A:  The Chemistry of Dioxygen


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