1570 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



Purity of the quinone used is important. Commercial pure benzoqui- 

 none was purified by distillation with water vapor and drying in vacuum. 

 Dissolving the yellow crystals in pure water (even if carried out in the ab- 

 sence of air) gives a strongly colored solution; it contains an impurity 

 which inhibits the photochemical activity. To avoid this, quinone was 

 dissolved in 0.01 N sulfuric acid. Illumination of quinone solution in the 

 absence of chloroplasts with the blue-violet light absorbed by quinone 

 causes darkening; no reaction can be observed afterward upon the addition 



T3 

 0) 



E 

 E 



50 



40 



30 



20 



10- 



10 



20 



30 



TIME, minutes 

 Fig. 35.18. Effect of light on oxygen consumption by chloroplast preparations in 

 the presence of pyrocatechol (after Warburg and Liittgens 1948). 



of chloroplasts (Clendenning and Ehrmantraut, 1950). A chloroplast 

 suspension to which a pure, acid quinone solution was added at pH 6.5 

 in the absence of oxygen showed in the experiments of Warburg and Lutt- 

 gens no gas exchange in the dark (except for occasional liberation of traces 

 of CO2) ; upon illumination, it produces oxygen until the volume of the 

 latter reached 80-90% of that calculated for the oxidation of water by the 

 available quinone, according to equation (35.32), thus confirming the valid- 

 ity of this equation (see figure 35.19). 



That the liberated gas was oxygen was confirmed by absorption in 

 phosphorus. lodometric assay showed that when the photochemical gas 



