Complex Compounds and Models of Enzymes 265 



sulphate were added to the solution. It can be seen that the presence of the 

 crystals did not affect the rate of the reaction, that is to say that in this case the 

 ions in the crystal of the salt are in a state which almost completely suppresses 

 their activity. 



Thus, the transition from ions which are present in a crystalline lattice, 

 CuS04-5H20 to ions in a lattice of a semiconductor (cupric and cuprous oxides) 

 brings about a smaller increase in activity than the transition from ions sur- 

 rounded by a hydrating envelope in solution to ions surroimded by a complex 

 envelope. In the latter case we find a sharp rise in activity which gives rise to 

 the idea of the structural and thermodynamic peculiarity of the mechanism of 

 catalysis by complex compounds. 



The next step consisted in the elucidation of the effect of the addenda and of 

 the complex as a whole on the magnitude of the activity. Cuprammoniimi was 

 one of the most active; only the compoimd of copper with biuret seemed to act 

 more efficiently as a model of catalase. Alteration in the structure of the addenda, 

 however, has a very marked effect on the catalase-like activity. 



The most important factor enhancing the catalase-like activity of 'copper' 

 models is the presence of four nitrogen atoms in the sphere of co-ordination; if 

 atoms of oxygen are substituted for them the activity is lessened. The part 

 played by oxygen in the sphere of co-ordination is demonstrated in its simplest 

 form by a comparison between the activities of methylamine (i6o) and a-methyl- 

 hydroxylamine (80). If one decreases the number of nitrogen atoms in the sphere 

 of co-ordination, by changing over, for example, to the internally complex 

 compounds formed with amino acids, then there is a sharp decline in activity 

 (Table 2). 



Table 2 



The role of oxygen in the sphere of co-ordination 



On the other hand, an increase in the concentration of the addendum in the 

 solution (a slight excess of it is necessary to prevent destruction of the complex 

 by the hydrogen peroxide) lowers the activity owing to the formation of com- 

 pounds having six as their co-ordination number. Correspondingly, cobalt 

 complexes, of which this is the characteristic co-ordination number, have 

 almost no catalase-like activity. 



In ascending the methane series one may observe the competition of two 

 factors. Lengthening of the chain brings about an increase in activity but 

 lengthening, like branching of the chain, also increases steric hindrance. The 

 activity of the complex formed with methylamine is, therefore, almost equal to 



