68 



OXIDATION-REDUCTION POTENTIALS 



and the pigment may be of value as an oxidation -reduction potential indicator (Ball, 

 1934). 



Friedheim (1934) has investigated the pigments obtained from walnut husks 

 and henna leaves ; they are juglon and lawson and the Eq^ values at pH 7-0 are 

 -f 0-033 V. and —0-139 v. respectively. 



Phoenicin, a pigment isolated from Penicillnmi plweniceum by Friedheim (1933) 

 has an Eq^ value at pH 7 of -j- 0-047 v. An intermediate product in melanin forma- 

 tion, 5 : 6-diketohydroindole-2-carboxylic acid, has also been studied by Friedheim 

 (1935). This compound, red in the oxidised form, gains two electrons on reduction 

 and at 17°C. the Eq^ value at pH 4-62 is + 0-171 v. 



Posternak (1938) identifies phoenecin as 2 : 2-dihydroxy-4 : 4'-ditoluquinone. 



Other 2-alkyl-3 hydroxynaphthaquinones have also been studied by Ball 

 (1936). Lapachol is a yellow pigment present in the wood of many trees, and 

 lomatiol is obtained from the seeds of Lomatio ilicifolia. In alkaline solution the 

 yellow pigments become red and when reduced they are colourless. The normal 

 potentials at 30° are closely similar as shown in the table. 



TABLE 13 

 Normal Potentials of 1!-alkyl-3 hydroxy-1 : 4-Naphthaquinones 



Ashley and Raistrick (1938) have isolated from the mycelium of a mould a 

 quinone or semiquinone, Luteoleersin, and the corresponding phenol, Alboleersin, 

 which are readily interconvertible by oxidation and reduction. It is suggested 

 that these substances may form an oxidation-reduction system of biological signi- 

 ficance in the life processes of HelmintJiosporium leersii (Atkinson). 



VITAMIN K 



Vitamin K is necessary for the formation of prothrombin in the body and in 

 Vitamin K deficiency the clotting time of the blood is prolonged. Normally there is 

 sufiicient Vitamin K in the diet, and intestinal bacteria can effect its synthesis, but 

 deficiency occurs if intestinal absorption is disorganised in diseases such as jaundice. 

 There is a deficiency of the vitamin in the new-born child and many neo-natal deaths 

 have been ascribed to this deficiency. 



Vitamin K is 3-phytyl-2-methyl-l : 4-Naphthoquinone and a number of related 

 compounds have similar effects, the synthetic substitute being menadione which has 

 the same structure without the phytyl group. In sweet clover disease animals fed 

 on spoiled clover were found to suffer from haemorrhages and the blood did not clot 

 after minor injuries. This was traced to the presence in the spoiled clover of 

 dicoumarol. Inspection of the formulae of Vitamin K (I) and dicoumarol (II) shows 



