91 



tron acceptor but at the same time also tunes the reactivity of this 

 central member of this chain enabling it to perform its function. 



This action of oxygen has rather remarkable features. The con- 

 centration of O2 in the atmosphere is low, 0.01 Af, and its concen- 

 tration in the water must be considerably lower, owing to the poor 

 solubility of O2. The experiment shows that while bringing the 

 riboflavine solution prior to freezing into equilibrium with pure 

 O2 does not increase the light emission, bringing the solution into 

 equilibrium with air, diluted with 10 volumes of N2, does not 

 decrease it. The action is maximal throughout this wide range of 

 concentrations and so the O2 can be expected to exert a maximal 

 influence even at the low tension present in the animal tissues. 



Most chemical reactions depend on the concentration of the re- 

 action partners. It is thus unusual to find the action of O2 inde- 

 pendent within such a wide range of its molarity. Since the concen- 

 tration of the O2 present in the solution, in balance with the 

 diluted air, had to be considerably lower than 10"^ Af, the concen- 

 tration of the riboflavine present, we can exclude a direct inter- 

 action between riboflavine and O2; we must look out for an un- 

 usual mechanism of action, based on some exceptional quality of 

 O2 which can be brought into a direct relation with the triplet 

 state. O2 has such a quality, being one of the very rare substances 

 which are paramagnetic in their ground state, and paramagnetic 

 molecules produce a magnetic perturbation of the electromagnetic 

 field which is known to be able to alter transition probabilities 

 between singlet and triplet excitations of electrons. 



This explains also the queer dependence on (or rather inde- 

 pendence of) concentrations. Until the concentration of the O2 is 

 high enough to ensure that statistically every riboflavine molecule 

 comes to lie within the action radius of an O2 molecule, we can 

 expect the influence to be maximal. The action radius of the O2 

 could be calculated from the concentration at which it ceases to 

 have a maximal effect. My experiments only allow me to state that 

 this radius must be a rather big one, of the order of 100 A. 



