66 



OXIDATION-REDUCTION POTENTIALS 



Friedheim (1933) has described a similar pigment (hallochrome) obtained 

 from Halla parthenopea. 



TABLE 11 



Semiqutnone pigments 

 (After Stern, 1934) 



The green pigment of B. chloraphis, Guignard and Sauvageaii, is a semiquinone 

 stable only in very acid solution. On oxidation it gives chloraphine, a yellow holo- 

 quinone with the constitution phenazine-a-carbonamide, and is therefore related 

 to pyocyanine (a-hydroxy-N-methyl-phenazine). The electrode potential of the 

 half-reduced pigment at pH 7-0 (Eq^) is — 0-115 v (Elema, 1933). 



Friedheim (1933) has extracted from Arion rufus (red slugs), a pigment with 

 E^atpHT'Oof -0-027 V. 



The pigment obtained from Urechis eggs is subject to an oxidation-reduction 

 reaction, involving one electron and has an Eq^ value of -f 0-186 v. at pH 7-0. It 

 is suggested that the pigment is related to the hasmins since reduction in alkaline 

 pyridine gives rise to the spectrum of a pyridine hsemochromogen (Horowitz and 

 Baumberger, 1941). 



Triphenyltetrazolium chloride has been used as a test for viable tissues which 

 are stained red by reduction to a red dye by some enzyme system (Mattson, Jensen 

 and Dutcher, 1937). The apparent oxidation-reduction potential of the dye is — 0-08 

 volts. 



NAPHTHAQUINONES, MOULD PRODUCTS, PIGMENTS 



Very many natural products from bacteria, moulds and plants have been 

 identified as 1 : 4-naphthaquinones. A few other natural products of quinonoid 

 structure will also be dealt with in this section. Some of the products have pronounced 



