294 



LIBERATION OF OXYGEN 



CHAP. 11 



liberation in photosynthesis. To oxidize water to hydrogen peroxide, 

 the primary oxidation product must have an oxidation potential < — 1.35 

 volts, but if water (bound to a suitable catalyst) could be oxidized in 

 four approximately equal steps, the required potential would be < — 0.81 

 volt, a value approximated, for example, by organic radicals of the type 

 of porphyrexide (c/. page 232). 



The mechanism of oxidase action is unknown, but apparently most, 

 if not all, oxidases (e. g., the cytochrome oxidase) are hemin derivatives. 

 Since ferri-ferro systems are "univalent," it is probable that, in the 

 oxidase action, four hydrogen atoms (or electrons) are transferred one 

 at a time, and that the structure of the oxidases is such as to equalize 

 approximately the energies of these four steps. Ferro and ferri ions have 

 four or five unsaturated homopolar valences; even if two of them are 

 saturated in hemin by complex formation, two or three remain free, and 

 this may permit a chemical association with oxygen (as in oxyhemo- 

 globin), and may also help in the addition and internal transfer of 

 electrons. For instance, we may consider the following sequence of 

 transformations : 



(11.26) — Fe+++ + O2 

 I O 



■^ — Fe+++^ 



O 



+ e 



o 



->— Fe 



++y 



-Fe+++^ 



(resonance) 



o 

 o- 



(resonance) 



-Fe 



+++. 



o- 



OH- 



\ 



+ e 



o 



OH 



->— Fe 



++ 



6 



+ H+ 



(resonance) 



o 



-Fe+++ 



\ 



+ e I (resonance) | + e 



). — Fe++=0 , — Fe+++0- > 



-OH- 



+ H* 



o 



-OH- 



-^— Fe 



+++ 



I (resonance) | 



— Fe++OH . — Fe+++OH- 



+ 4e 



Net result: Oo > 2 0H~ 



+ 2H+ 



The essential point in this (otherwise arbitrary) scheme is that the 

 intermediates of peroxidic character are supposed to be stabilized by 

 resonance between the forms containing ferrous and ferric iron, respec- 

 tively; this may equalize the potentials of the four reduction steps. 



In photosynthesis, the evolution of oxygen may occur by a mechanism 

 of type (11.26) running in reverse, with four primary oxidation products, 

 Z, serving as acceptors for the four electrons taken away from water in 

 its oxidation to oxygen. (For the first suggestion that water is oxidized 

 in photosynthesis by ferric iron, cf. Weiss 1937.) The primary oxidation 

 product, Z, may itself be a hemin derivative; it seems more probable, 



