48 OXIDATION-REDUCTION POTENTIALS 



In view of tlie high bond strength of the S.S covalent bond Waters (1948) considers 

 the existence of a stable RS radical is unlikely. Pauling attributes to the SH bond 

 the energy of 87,000 calories. 



Michaelis and Flexner (1928) sought for an explanation of the anomalies of the 

 thiol electrode in the catalytic effect of iron and other metals on oxidation of thiol 

 compounds. It is possible therefore that the discrepancies in results might be due 

 to the electrodes being not " unattackable " but participating in the system. For 

 a discussion on thiol-metal complexes Mchaelis (1933) should be consulted. 



On kinetic evidence Preisler (1930) concludes that the reduction of cysteine 

 resembles a reaction of the second order and is not truly reversible. Other papers 

 on the electrode potential of the sulphydryl system are : — Kendall and Ort (1926), 

 Kendall and Nord (1926), Nord (1927), Dixon and Tunnicliffe (1927), Kendall and 

 Loewen (1928, 1, 2), Harrison and Quastel (1928), Williams and Drissen (1930), 

 Ghosh, Raychandhm-i, and Ganguli (1932), Ghosh and Ganguli (1933, 1935), Green 

 (1933), Larsson (1933). 



It seems therefore that the sulphydryl systems do not fall readily into line with 

 classical examples of reversible oxidation-reduction reactions. 



On the basis of thermal data Borsook, Ellis and Hoffman (1937), deduce a rather 

 lower potential than other workers. ( — 0-39 v. at pH 7-0.) 



The oxidation-reduction potentials of cysteine and related compounds have been 

 studied by Ryklan and Schmidt (1944). For molar aqueous solutions they arrive at 

 the following values for the normal oxidation-reduction potentials : 



Thioglycollic acid 0-27 volt 



Cysteine 0-27 ,, 



Thiolhistidine 0-32 „ 



Monothioethyleneglycol ... 0-35 ,, 



Ergothioneine ... ... ... 0-36 ;,, 



Glutathione ... ... ... 0-45 ,, 



and in 70 per cent, ethanol : 



Thiophenol 0-11 „ 



o-Thiocresol 0-30 ,, 



The catalytic waves observed with the polarograph in the case of cysteine and 

 proteins containing cysteine are described in the chapter on Polarography. 



HAEMOGLOBIN SYSTEMS 



The hsemoglobin-oxyhsemoglobin system is not a true oxidation-reduction 

 process, but merely an addition of molecular oxygen to haemoglobin. The system, 

 therefore, cannot be studied potentiometrically (Conant, 1923). The hsemoglobin- 

 methsemoglobin system has been studied electrometrically (Conant and Fieser, 1925). 

 Difficulties are encoimtered in that high concentrations of the components have to be 

 employed and the potentials are obtained only sluggishly. Between pH 6-8 and 8*5, 

 the system is independent of alterations in hydrogen ion concentration and the 

 electrode potentials obtained may be defined approximately by the equation 



^ , RT , [MetHb] 

 E,= +0.1 + -^lnLj^J 



