CHEMICAL PROPERTIES 429 



what greater, parhaps 0.45-0.49 v, for the naphthoquinones. Occasionally 

 insufficient attention has been given to the variations in the redox poten- 

 tial with the pH in metabolic studies; for example, it is possible, for the 

 potential of a quinone within a cell to be 60-100 mv greater than in the 

 external medium. If oxidation of enzyme groups is indeed responsible for 

 some of the effects observed, it is clear that the pH would be an important 

 factor. Oxidation-reduction potentials of quinones commonly used in met- 

 abolic studies are summarized in Table 5-1. 



The redox potentials of the 1 ,2- or o-quinones are usually about 90 mv 

 higher than those of the corresponding 1,4- or p-quinones, and thus have 

 been said to have a higher energy content. However, possibly the cause 

 is the hydrogen bonding in the o-hydroquinones, this stabilizing the re- 

 duced forms (Badger, 1954). Introduction of groups on the quinone ring 

 alters the redox potentials by inductive effects; electronegative groups 

 reduce the potentials and electropositive groups raise the potentials, the 

 effects being more marked in the naphthoquinones than in the benzoqui- 

 nones. The effects of substitution on the values of Eq in the physiological 

 range of pH will also depend occasionally on alterations of the piii^'s. 

 Recent discussions of the relation of redox potential to structure involve 

 molecular orbital calculations of the lowering of the 7r-electron energy 

 upon reduction (Basu, 1956), but this is only another way of expressing 

 changes in resonance energy. Particularly valuable reviews of the theories 

 of quinone redox potentials are those of Preisler (1939), Branch and Calvin 

 (1941, p. 305), and Evans and De Heer (1950). 



Although the values of Eq would indicate most of the quinones to be 

 rather potent oxidants, the potentials at physiological pH's are in an inter- 

 mediate range, except for o- and p-benzoquinone, being somewhere between 

 the cytochromes {E^ for cytochrome b is + 0.077 and for cytochrome 

 c is + 0.25 v) and the flavins {EJ values around — 0.21 v) and pyridine 

 nucleotides {Eq for NAD is — 0.32 v), so that certain quinones and naph- 

 thoquinones are perhaps ideally poised for participation in electron trans- 

 port from the flavoproteins to the cytochromes. The potentials for most 

 benzoquinones are certainly higher than for the majority of thiols {E^ 

 values between and — 0.35 v; see page 11-656), so that the equilibrium 



Q + 2 R— SH ±5 QH2 + R— S— S— R 



would generally lie far to the right. However, certain other considerations 

 must be taken into account: (1) the rate of oxidation may be slow for 

 various reasons and equilibrium unattained, (2) certain naphthoquinones 

 have reasonably low potentials at pH 7 and would not oxidize some SH 

 groups, and (3) since we know very little about the redox potentials of 

 enzyme SH groups it is difficult to predict the effects on these groups. 

 Oxidation-reduction reactions of quinones usually involve an interme- 



