1530 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



for comparison oxygen produced from the same water by electrolysis. 

 The results are shown in Table 35. Ill; they confirm that in all studied 

 cases of the Hill reaction — including that with chromate — the oxygen came 

 from water. (In the experiment with chromate, pH 8.3 borate buffer was 

 used to retard the isotopic exchange of oxygen between water and chro- 

 mate.) 



Table 35.III 



Isotopic Composition of Oxygen Evolved bt Illuminated Chloroplasts 



(after Holt and French, 1948^) 



100[O(18)O(16)]/[O(16)O(16)l 

 oxygen produced by 



Water used 

 Normal 



Enriched 



1. Can Carbon Dioxide Serve as Oxidant in Hill Reaction? 



Claims of having obtained oxygen evolution from water by isolated 

 chloroplasts, with simultaneous reduction of carbon dioxide, if not to a 

 carbohydrate, at least to formic acid, have been made by Boichenko (1943, 

 1944). Since these mvestigations have only been published in Russian, 

 we will describe them in some detail, despite the fact that techniques used 

 were rather primitive and the results not too convincing. 



Boichenko further claimed that chlorophyll preparations can reduce 

 carbon dioxide not only in light, "photosynthetically," at the expense of 

 water, but also in the dark, "chemosynthetically" (with hydrogen as re- 

 ductant) . 



Boichenko thought (quoting Lubimenko) that under natural conditions the medium 

 surrounding the chloroplasts has a very low value of rH- — in other words, is strongly 

 reducing. Since photosynthesis produces both oxidation products (O2) and reduction 

 products (carbohydrates), the reducing properties of the medium can be understood if 

 the oxidation product (oxygen) is removed much more efficiently than the reduction 

 products (sugai's) ("removal" may mean translocation or a chemical change, such as 

 polymerization of reducing sugars to sucrose or starch). The accumulation of reducing 

 compounds around the chloroplasts is thus a consequence of photosjoithesis, and, as 

 such, more likely to inhibit than to stimulate it. Boichenko considered, to the contrary, 

 that a low value of rH is a -prerequisite for photosjTithesis, and attempted to imitate 



