SEMIQUINONES 



161 



ammonia, ammonimn chloride and dilute (10""^ M) hexammino cobaltic chloride, and 

 a small amount of gelatine is added to suppress the " maximum " wave. Small 

 amounts of cytochrome (18 y per ml.) produce a sharp wave with the peak at 

 —1-5 V. (versus saturated calomel electrode). By subtracting the current due to 

 reduction of cobalt a quantitative measure of the cytochrome present can be obtained, 

 and the progress of purification through a refining process can be followed. The wave 

 is due to cystine, cysteine, etc., and no other amino acids give a catalytic wave. With 

 cysteine and cystine the peak is at — 1-75 volts and with glutathione at — 1-1 v. 

 The enzymes, ascorbic acid oxidase, tyrosinase, d-glyceraldehyde-3-phosphate 

 dehydrogenase and aldolase also give catalytic waves with a peak at about — 1-45 

 volts. On the other hand prolactin, papain, pepsinogen, desoxyribonuclease and 

 carboxypeptidase under the same conditions give two peaks at — 1-1 and — 1-7 volts 

 respectively. 



HEMOGLOBIN AND HEMATIN 



Hydrogen peroxide is reduced irreversibly at the dropping mercury electrode, 

 and the reduction wave is flat but in the presence of haemolysed erythrocytes, 

 haemoglobin or hsematin, the peroxide wave approaches the first wave of oxygen. 

 Curves are obtained in air-saturated solutions which make it possible to determine 

 quantitatively haemoglobin, haematin or coproporphyrin I, but cyanide inhibits 

 the effect. Bilirubin gives reduction curves at the dropping electrode which have 

 not yet been fully explained. 



SEMIQUINONES 



The formation of intermediate compounds in reductions involving two electrodes 

 has been discussed in Chapter III : — 



R + e->R- 

 R- + e->R-- 



The semi-reductant R~ may be recognised in the case of pigments by differences 

 in colour. Miiller (1942) has found two waves in the polarogram of a -- oxyphena- 



volts 



volts 



Fig. 44 

 Polarographic curves showing formation in acid solution of intermediate semiquinone in reduction ol 



a-oxyphenazine (after Miiller) 



zine in acid solution, the two points of inflection on the voltage-current curve corre- 

 sponding to the two stages in the reduction. The half-wave potentials of the two 



