60 



OXIDATION-REDUCTION POTENTIALS 



being the active compound ; and in the synthesis and breakdown of tryptophan. 

 In some cases these coenzymes apparently function as carriers of amino groups and 

 in others as hydrogen acceptors. 



ASCORBIC ACID (VITAMIN C) 



It is established that vitamin C is ascorbic acid (3-keto-l-gulofuranolactone), 

 a substance with reducing properties. The vitamin-C content of foodstuffs may be 

 determined by titration with oxidation-reduction potential indicators, such as 

 2 : 6-dichlorophenol-indophenol (Georgescu, 1932 ; Tillmans, Hirsch and Hirsch, 

 1932 ; Tillmans, Hirsch and Dick, 1932) or prune (Melville and Richardson, 1934). 

 The reducing properties of the vitamin suggested that potentiometric study could 

 be made of the reversible oxidation-reduction system concerned. Unfortunately, 

 the oxidant is unstable and is irreversibly oxidised so that at an electrode the potential 

 difference set up is due only to the reduced form — a result superficially similar to 

 that observed in the case of sulphydryl compounds. The instability of the oxidised 

 form in any but very acid solutions makes it difficult to understand how the vitamin 

 remains undestroyed in tissues which have an oxidising level of potential higher 

 than that of ascorbic acid. It is possible that some stabilising substance is present. 

 Laki (1933) found difficulties with electrode potential determinations as did Green 

 (1933), who derives the following equation for ascorbic acid (in volts) : — 



Eh = + 0-375 — 0-06 pH 



Borsook and Keighley (1933) found difficulties in solutions less acid than pH 5-75, 

 whilst Wurmser and de Loureiro (1933) suggest that two reducing groups in ascorbic 

 -acid behave normally whilst a third alcohol grouping is responsible for the irregular 

 potential behaviour. Other papers dealing with this subject include Szent-Gvorgi 

 (1928), Birch, Harris and Ray (1933), Herbert, et alios (1933) and Zilva (1934). ' 



Borsook and his co-workers (1937) find that dehydroscorbic acid is rapidly 

 reduced by sulphydryl compounds and observe the oxidation -reduction potentials 

 of three separate steps in the oxidation of ascorbic acid. These conclusions are 

 disputed by Ball (1937) who finds that in the presence of " mediators " (generally 

 oxidation-reduction indicator dyes) the electrode potentials of the ascorbic acid 

 system could be measured and has obtained the following figures foi Eq^ values at 

 different pH levels (30 °C.). 



1-05 

 2-16 

 3-04 

 4-00 

 5-19 

 6-32 

 7-24 

 8-57 



E, 



