1566 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



that when the Hill reaction is carried out with other oxidants— quinone, 

 Hill's mixture, or cytochrome c — no hydrogen peroxide is formed as an 

 intermediate oxidation product of water. He found, in fact, that no acet- 

 aldehyde is produced upon addition of ethanol and catalase to these com- 

 mon Hill reaction systems. This is the most convincing confirmation to 

 date of the surmise — made in chapter 11, p. 286 — that hydrogen peroxide 

 is not an intermediate in the photochemical oxygen formation by plants. 



-90 - 



20 40 60 80 



MINUTES 



100 



Fig. 35.17A. Evolution and absorption of O2 by chloroplast material (300 

 /ig. chlorophyll in 2 ml. phosphate buffer pH 6.8) (Mehler 1951). O2 evolved in 

 vessel 1 (5 X 10~« mole quinone); O2 consumed in vessel 2 (4 mg. catalase, lO"' 

 mole ethanol; O2 first evolved, then consumed at double speed in vessel 3 (5 ^mole 

 quinone, 4 mg. catalase, 10 ~^ mole ethanol). Vessel 4; catalase, ethanol, and 

 5 jumole hydro quinone. 



Mehler then could proceed with the demonstration that acetaldehyde does 

 arise— as a reduction intermediate — if chloroplast suspensions containing 

 ethanol and catalase are illuminated in the absence of added Hill oxidants, 

 but in the presence of oxygen. Under these conditions, oxygen is con- 

 sumed instead of being liberated, as in the usual versions of the Hill reaction 

 (figure 35.17A). Two moles of acetaldehyde were found to be formed per 

 mole of oxygen consumed, which is consistent with the reaction scheme 

 (35.31A). 



