ORGANIC PEROXIDE HYPOTHESIS 291 



the enzymes active in photosynthesis has a certain similarity with 

 ordinary catalase, and the suggestion that this similarity may be ex- 

 plained by assuming that this enzyme is a kind of "catalase," adapted 

 to the dismutation of an organic peroxide. However, sensitivity to 

 hydroxylamine may be caused also by noncatalatic enzymes containing 

 a heavy metal, e. g. a "deoxidase" which brings about the liberation of 

 oxygen without the intermediate formation of free 'peroxide. 



One may ask whether the substitution of organic peroxides for 

 hydrogen peroxide could be advantageous from the point of view of the 

 energy balance of photosynthesis. The answer is that organic peroxides 

 of the type R'O— OR", as well as hydroperoxides of the type RO— OH, 

 share fully the instability of hydrogen peroxide. D'Ans and Frey (1914) 

 measured the equilibrium of the reactions: 



O O 



II II 



(11.17) R— OH + H2O2 . R— OOH + H2O 



(acid) (peracid) 



for R = methyl, ethyl, propyl, etc., and obtained, for the equilibrium 

 constant: 



(11.18) K = l^P^^"^^^^ 



[hydroperoxide] [acid] 



values ranging from K = 2 (for performic acid) to K = 5 (for the higher 

 members of the series) . This shows that the formation of peracids from 

 hydrogen peroxide and acid has a free energy of less than 1 kcal. The 

 peracids are thus only insignificantly more stable than hydrogen peroxide. 

 Probably, all peroxides of the type RO— OH and R'OOR" decompose 

 bimolecularly (by dismutation), into alcohols, ethers, acids or aldehydes, 

 and oxygen, with the liberation of an amount of energy similar to that 

 liberated in the decomposition of hydrogen peroxide into water and 

 oxygen. 



An important difference between organic peroxides and hydrogen 

 peroxide appears when one considers the possibility of a monomolecular 

 decomposition (into RH and O2, or R'R" and O2). The monomolecular 

 decomposition of hydrogen peroxide: 



(11.19) H2O2 > Ho + O2 



consumes 35 kcal. A similar decomposition of an organic hydroperoxide: 



(11.20) ROOH > RH + O2 



requires, with the standard bond energies, only 7 kcal, while the de- 

 composition of a purely organic peroxide: 



(11.21) R'OOR" > R'R" + O2 



should liberate 6 kcal ; reactions of these two types could thus be reversible. 



