THE CHEMISTRY OF PHOTOSYNTHESIS 179 



this back reaction in which quinone would act as a catalyzer. If the pH of 

 the grana suspension is increased from 6.5 to 8 quinone reaction changes from 

 the stoichiometric state to the catalytic state and we find an over-all reaction 

 of zero : 



2 quinone + 2H2O > 2 hydroquinonc + O2 



2 hydroquinone + O2 > 2 quinone + 2H2O 







The back reaction of hydroquinone may be either enzymatic by means of the 

 copper of the phenoloxidase or directly chemical by autoxidation. The en- 

 zymatic reoxidation takes place rapidly at low O2 pressures; it is inhibited by 

 HCN. By contrast, the autoxidation proceeds only at higher O2 pressures 

 and — in contrast to the enzymatic reoxidation — results in formation of H2O2: 



2 hydroquinone + 2O2 > 2 quinone + 2H2O2 



The ratio H2O2/ O2 equals 1 . The less quinone added in these catalytic 

 quinone reactions, the closer conditions are to the physiological. According 

 to Warburg, the optimal amount of quinone to be added is equal to the 

 chlorophyll content of the grana, e.g., 1 /xmole benzoquinone or 1 /^mole 

 /3-naphthaquinone sulfonic acid for a quantity of grana containing 1 ^mole 

 chlorophyll. In stoichiometric experiments 20 /^moles quinone are added to 

 1 ^mole chlorophyll. 



As already mentioned, the enzymatic back reaction is inhibited by HCN. 

 However, the strange thing is that the forward reaction of O2 cannot be in- 

 hibited by HCN. This can be shown in the stoichiometric quinone reaction 

 — i.e. under the proviso that no back reaction of O2 can take place — when 

 HCN is added. The O2 evolution is then not inhibited by 0.01 A^HCN and 

 only slightly inhibited by 0.1 A^ HCN. Thus, as the O2 production is not 

 inhibited by HCN and the enzymatic back reaction of O2 is inhibited by 

 HCN, quinone catalysis at low O2 pressures must always be inhibited by 

 HCN. If the Oo pressure is increased to such an extent that autoxidative 

 back reaction proceeds rapidly enough, neither the forward reaction nor the 

 back reaction of O2 will be inhibited by HCN. Thus, in 0.01 A^ HCN we 

 have complete quinone catalysis; however, H2O2 is produced instead of 

 water, HCN inhibiting the catalase of the grana. In 0.01 A^ HCN we there- 

 fore have the following reactions : 



2 quinone + 2H2O > 2 hydroquinone + O2 



2 hydroquinone + 2O2 > 2 quinone + 2H2O2 



2H2O + o~ > 2H2O2 



The over-all reaction is no longer zero. O2 is consumed and H2O2 produced. 

 The addition of HCN thus gives, manometrically, an alteration in pressure 

 changes from zero to markedly negative values. In contrast to simple autoxi- 

 dation, the ratio H2O2/O2 now equals 2 (see Table 23). 



