DIFFERENT OXIDANTS 



1571 



liberation was ended, practically all quinone had disappeared from the solu- 

 tion. 



Similar experiments with o-benzoqiiinone were unsuccessful because of 

 the instability of this compound; but another orthoquinonoid compound, 

 a-naphthaquinone sulfonic acid, could be reduced in the same way as p- 

 benzoquinone. This, and the stimulation of the oxygen consumption in 

 green extracts by the addition of pyrocatechol (section 5(c) below) makes it 

 probable that o-quinones react similarly to p-quinones. 



200 



TIME IN LIGHT, minutes 

 Fig. 35.19. Evolution of o.xygen from chloroplast suspension in quinone solu- 

 tion in light (after Warburg and Liittgens 1946). 2 ml. suspension in 0.05 M 

 phosphate buffer, pU 6.5, 0.05% KCI, 1 nM chlorophyll in vessel, 20° C, argon 

 atmosphere. 2 mg. quinone in 0.2 ml. 0.01 A'' H2SO4 added at time 0. 



Since 18.5 X 10"" mole quinone could be reduced by a suspension con- 

 taining only 0.2 X 10 ~" mole chlorophyll, the latter obviously could not 

 have served as a reactant, but only as a catalyst; the same applies to all 

 other components of the chloroplasts, none of which is available in such 

 large quantities. 



As shown in equation (35.32) the oxidation of water by quinone is endo- 

 thermal to the extent of 52 kcal./mole oxygen — somewhat less than one 

 half as much as the oxidation of water by carbon dioxide. This energy 

 must be supplied by light. 



