74 OXIDATION-REDUCTION POTENTIALS 



the first additions of iodine have little effect on the potential, but there is a point of 

 inflection when further iodine produces an abrupt rise, the curve afterwards smoothing 

 out asymptotically with the amylopectin curve. The difference in behaviour of the 

 two fractions is great enough to enable the potentiometric titration to be used for the 

 determination of the amount of the fractions in starch samples. Measurement of the 

 colours produced with iodine by the two fractions by photometric methods is less 

 sensitive. 



BIO-LUMINESCENCE 



In the presence of the enzyme, lucif erase, luciferin is slowly oxidised ^dth develop- 

 ment of luminescence, but since the reaction appears not to be thermodynamically 

 reversible electrode potential measurements cannot be applied to its investigation in 

 a direct manner (Harvey, 1926). Korr (1936) concludes that the reaction is rever- 

 sible but that oxyluciferin is unstable, and that the potential reached in the system. 

 is close to that of quinhydrone. 



Anderson (1936) has studied the reversible reaction of luciferin with oxidising 

 agents, and Harvey (1940) the oxidation-reduction reactions of luciferin in the 

 presence of dehydrogenases and lucif erase. Cyanide was found to inhibit the 

 luminescence of symbiotic bacteria but not the self luminosity of animals. 



McElroy and Ballentine (1944) find that purified luciferin contains acid-labile 

 phosphate which is converted to inorganic phosphate during luminescence. Bio- 

 luminescence is suggested to be almost a reversed photosynthesis whereby the 

 group E, • CO • CHgOH is oxidised in four steps to K. • COOH. The luminescence of 

 PJiotohacterium phospJioreum is inhibited by blue light (Kluyver, van der Kirk and 

 van der Burg, 1942) probably due to photochemical inactivation of dehydroluciferin. 

 It is suggested that this may be a vitamin K derivative and not of the coenzyme- 

 flavin type as suggested by McElroy and Ballentine (1944) and Johnson and Eyring, 

 (1944). Quinine inhibits bacterial luminescence. (Johnson and Schneyer, 1944.) 

 For reviews on bioluminescence, see Harvey (1941) and Johnson and others (1945). 



POTENTIALS OF CELLULAR CONTENTS 



A number of attempts have been made to determine the oxidation-reduction 

 condition in the interior of cells by micro-injection of dyes and by diffusion of dyes 

 into the interior of the cell. These experiments, naturally, are subject to the errors 

 involved in the use of oxidation-reduction indicators, amongst which the following 

 seem pertinent : — 



(i) Damage to the cell by the dye or by injection. Although the cell may 

 survive, the injury may have altered the electrode potential. Either the 

 oxidised or reduced form of the dye, or both, may be toxic. 



(ii) The dye may catalyse biological oxidations (or act as oxygen carrier) 

 and hence alter equilibrium. 



(iii) The capacity of the dye system may be so large, compared with the 

 system, that equilibrium may be disturbed, and the time involved for the 

 reattainment of equilibrium may be very long. 



(iv) The properties of the dye may be altered by combination with cellular 

 contents, etc. 



