274 L. A. NIKOLAEV 



quickly oxidized by the oxygen of the air. The final product, sulphur, forms a 

 sol, which, obviously, also plays a part in the kinetics of this reaction. When this 

 reaction is carried out in alcoholic solution its rate is very markedly decreased 

 because, in this case, the sulphur which is produced forms a true solution and 

 causes a shifting of the equihbrium towards the formation of the leuco form.* 



To summarize the characteristic features of the models we have discussed, 

 we may tentatively state that complex compounds of copper containing four 

 atoms of nitrogen in the co-ordination sphere are models of the active group of 

 catalase. Some specific structures of the addenda leading to the formation of 

 rings including the ion-complex link show a high activity (compounds with 

 biuret or trimethylenediamine). 



Compounds of a similar type containing iron or copper may serve as models 

 of oxidases. The mechanism of action of models of catalases is obviously based 

 on the formation of intermediate products. In the course of this the high values 

 for the energy of activation taken with a high rate of reaction indicates a high 

 entropy of activation. This means that there is a transitional state in the course 

 of the reaction in which the structure of the catalyst is considerably loosened. 



Dyestuffs which can assume the leuco- form reversibly, when adsorbed on 

 carriers of high molecular weight, can act as models of dehydrogenases and, in 

 this case, activation by adsorption is achieved far more easily than in the case of 

 complexes containing metals. 



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* Catalysts of this type are somewhat reminiscent of the 'Faserkatalysatoren' of Bredig. 



