STARCH IODINE POTENTIALS 



73 



In order to explain these facts, Gulland and Randall put forward a very inter- 

 esting suggestion which is not claimed to be fully proved but which is nevertheless 

 supported by much of the experimental evidence. It is suggested that the oxytocic 

 hormone contains two reversible oxidation-reduction groups, at least one of which 

 may be a disulphide group. On this view the hormone is fully active when both 

 groups are oxidised ; when one group is reduced the activity is decreased to one half ; 

 and, most surprising of all, when both groups are reduced full activity is restored. 

 It is to be hoped that further evidence will accumulate to test this interesting 

 hy])othesis. (See also Sealock and du Vigneaud, 1935.) 



CHORIONIC GONADOTROPIC HORMONE 



The activity of the hormone is lowered by oxidation and restored by reduction. 

 One electron is concerned in the oxidation-reduction reaction, which has the E^ 

 value at pH 5-9 of +0-354v. (Bowman, 1941). 



STARCH IODINE POTENTIALS 



Starch can be separated into two fractions by treatment of hot solutions with 

 alcohols (Schoch, 1945). Amylose, which crystallises from the solution, is the long- 

 chained fraction which gives a blue colour with iodine, whilst amylopectin, which 

 remains dissolved, has a branched chain structure and with iodine yields a reddish 

 purple colouration. It is possible to determine the amount of amylose in starch by a 



Water 



0-5 



I'O 1-5 

 ml.0-02 N. I2 



Fig. 15 

 Starch Iodine Potentials 



potentiometric method. (Bates, French and Rundle, 1943.) This depends upon the 

 effect of starches on the electrode potential of the iodine : iodide cell. Two effects are 

 shown by the results ; an absorption of iodine by amylopectin fractions and some 

 more firmer form of linking with amylose. When iodine solution is added stepwise to a 

 dilute iodide solution containing amylopectin there is a gradual rise in potential, the 

 curve differing little from that observed in the absence of starch ; but with amylose 



