1. Free Radicals in Medicine. I. Chemical Nature and Biologic Reactions*
PETER A. SOUTHORN, M.B., B.S., GARTH POWIS, D. Phil. 
Mayo Clinic Proceedings 
Volume 63, Issue 4, Pages (April 1988)
DOI: /S (12)
Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

2. Fig. 1
Diagram of heterolytic and homolytic covalent bond fission. Each component in compound AB donates an electron (×) to create a double electron “covalent” bond. In upper equation, heterolytic asymmetric cleavage of this bond produces negatively charged A ion and positively charged B ion. In lower equation, symmetric homolytic cleavage of bond produces free radicals A· and B·, each of these components possessing one unpaired electron.
Mayo Clinic Proceedings  , DOI: ( /S (12) )
Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

3. Fig. 2
Sequential univalent reduction of molecular oxygen. (See text for discussion.)
Mayo Clinic Proceedings  , DOI: ( /S (12) )
Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

4. Fig. 3
Arrangement of electrons in outermost occupied orbitals of molecular oxygen and delta and sigma singlet oxygen. Arrows indicate direction of electron spin.
(Data from Halliwell and Gutteridge.8)
Mayo Clinic Proceedings  , DOI: ( /S (12) )
Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

5. Fig. 4
Formation of superoxide anion radical by oxidation-reduction cycling of paraquat pyridinyl cation. The following reactions are involved in paraquat toxicity. A, Pentose phosphate pathway provides reducing equivalents in the form of reduced nicotinamide-adenine dinucleotide phosphate (NADPH). B, Paraquat pyridinyl cation (PQ++) is reduced to cation radical (PQ ). This reaction proceeds continuously, being catalyzed by NADPH-cytochrome P450 reductase. C, Oxidation-reduction cycling of pyridinyl cation radical reacting with molecular oxygen forms superoxide anion radical (O2 ) and pyridinyl cation.
Mayo Clinic Proceedings  , DOI: ( /S (12) )
Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

6. Fig. 5
Lipid peroxidation initiated by hydroxyl radical. PUFA = polyunsaturated fatty acids.
Mayo Clinic Proceedings  , DOI: ( /S (12) )
Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions

7. Fig. 6
Diagram of enzyme systems to bypass reactive intermediates being released during sequential univalent reduction of molecular oxygen.
Mayo Clinic Proceedings  , DOI: ( /S (12) )
Copyright © 1988 Mayo Foundation for Medical Education and Research Terms and Conditions