2G0 H. M. HABERMAiNN AND A. H. BUOW N 



(F) 2H2O ^2[H] +2[0H] 



(G) 2[H] +2[OH]-*2H20 



(H) quinone + 2[H] — >- hydroquinone 



(J) 2[0H] ^1/202 + H2O 



(K) 2[H] +02->H202 



(M) ascorbic acid + 2[0H] -♦ 2H2O + dehydroascorbic acid 



the quinone has all been reduced, the first step in the Mehler reaction 

 (K) may begin. We may think of the back reaction (G) as competing 

 in a sense with oxygen production (J), on the one hand, and with 

 either quinone (H) or oxygen reduction (K) on the other. 



We have generally found that the oxygen production rate via the 

 Mehler reaction is appreciably^ lower— often much lower — than 

 that of the quinone-Hill reaction. Therefore reaction (K) does not 

 compete with (G) as well as (H) does so that, during the Mehler re- 

 action, a back reaction of the photolysis products must occur at a 

 rate which is not negligible. When ascorbic acid is introduced into 

 the system, reaction (M) with the photolytic oxidation product may 

 be postulated, and this of course could compete with oxygen evolu- 

 tion (J), giving the decreased oxygen production which was observed. 

 If we suggest further that the ascorbic acid reaction (M) competes 

 also with the back reaction (G), then the ascorbic acid, serving as a 

 trap for [OH], would reduce the rate of the back reaction (G) as well 

 as that of the oxygen evolution (J). Slowing the back reaction must 

 inevitably promote oxygen uptake in reaction (K) ; thus ascorbic 

 acid has a dual effect of reducing oxygen evolution and enhancing 

 oxygen consumption, as the experimental results in Fig. 1 testify. 

 At the moment we favor this explanation of the ascorbic acid stim- 

 ulation of the Mehler reaction, and we are studying other aspects of 

 the system in an effort to apply additional tests of the proposed 

 mechanism. 



It is interesting to note that the Mehler reaction was discovered 

 when Alan Mehler attempted to identify as hydrogen peroxide the 

 oxidation product [OH] of the chloroplast reaction. Although this 

 attempt failed because he observed no reaction of his reagent (cata- 

 lase plus ethanol) with an oxygen precursor, he did perhaps dis- 

 cover that another substance, ascorbic acid, could play the role of 

 reactant with the [OH]. If our provisional interpretation is correct, 

 the ascorbic acid acts early enough in the reaction sequence leading 



