432 5. QUINONES 



Some quinhydrone exists in all mixtures of p-benzoqiiinone and p-hen- 

 zohydroquinone but under most experimental conditions its concentration 

 is probably less than 10% of the other components and, especially as it 

 appears to be unreactive, is not important in enzyme studies. The structure 

 and properties of quinhydrone are well summarized by Janz and Ives 

 (1961). 



The kinetics of quinone oxidation-reduction reactions are usually quite 

 complex and here we shall discuss briefly only a few examples to illustrate 

 that various species of molecule, ion, and free radical are involved. The 

 oxidation of p-benzohydroquinone by Fe+++ and the reverse reaction have 

 been studied in detail. The rate expressions were given by Porret (1934) as: 



Reduction of p-Q, v = k{¥e++r'~ (p-Q) (H+y'^ 



Oxidation of p-QH, v = k'(Fe+++Y'~ (p-QH^) (H+)-' 



The reaction mechanism was established by Baxendale et al. (1951) to be: 



p-QH- + Fe+++ ^ 39-QH- + Fe++ 



p-QH- ±1^ P-Q-- + H+ 

 P-Q-- + Fe+++ -^ p-Q + Fe++ 



but other reactions of the semiquinone usually complicate the kinetics. 

 The ratio k'jk is 430 for p-benzoquinone and 7400 for toluquinone (Bax- 

 endale and Hardy, 1954). The rates of oxidation of substances by the 

 quinones are often slow, although the free energy change may be large, 

 due to various kinetic factors and the low concentrations of certain inter- 

 mediates. 



The oxidation of hydroquinones by oxygen can also be kinetically com- 

 plex, as shown by the extensive work of Weissberger and his colleagues at 

 the Kodak laboratories. With tetrasubstituted hydroquinones, such as 

 durohydroquinone, the products are the respective quinones and hydrogen 

 peroxide: 



DQHa + O2 -► DQ + H2O2 



but if ring positions are free they may be attacked by the peroxide to form 

 hydroxyquinones and eventually humic acids (James and Weissberger, 

 1938). The rate of autoxidation increases with the number of methyl 

 groups, this possibly being related to the steady-state concentrations of 

 the semiquinones (James et al., 1938). The reaction is often autocatalyzed 

 by the quinone so that the rate increases after an initial lag period (Weiss- 

 berger, 1948). Metal ions such as Cu++ and Mn++ exert quite marked ca- 

 talytic effects, while cysteine and thioglycolate are inhibitory, the latter 

 effect probably being due to reaction of the thiols with the autocatalytic 

 quinones formed. In the absence of metal ions, autoxidation of ^^-benzo- 



