1670 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



from enzyme to enzyme without undergoing full isotopic equilibration 

 with the interenzymatie reservoirs. No suggestion was made as to whether 

 the early tagging of malic acid could be ascribed to one of these two causes; 

 but it was implied that some such mechanism, rather than the previously 

 postulated more or less direct equilibration with the external tagged carlwn 

 dioxide, must be responsible for it. 



The mechanism of formation and the function of C4 bodies in the reac- 

 tion sequence of photosynthesis remains, at this writing, one of the uncer- 

 tain parts in the interpretation of the radiocarbon experiments. 



7. The C5 and C7 Sugars as Intermediates in Photosynthesis 



Benson, Bassham, Calvin, Hall, Hirsch, Kawaguchi, Lynch and Tolbert 

 (1952) described the identification (first announced by Benson, Bassham 

 and Calvin 1951, and Benson 1951) of two new photosynthetic intermedi- 

 ates — a pentose and a heptose sugar or, more precisely, their phosphate 

 esters. These compounds were found to appear in tracer quantities after a 

 few seconds of steady-state photosynthesis in unicellular algae, higher 

 plants, and purple bacteria (cf. figs. 36.17 and 36.21). Their identification 

 resulted from a further development of the area of the paper chromatograms 

 containing the phosphate esters — lower right corner in Figs. 36. 13 and 36. 15. 

 This area contained, in addition to spots attributable to phosphoenolpyru- 

 vic, phosphoglyceric, and phosphoglycolic acids, to triose (dihydroxyace- 

 tone) phosphate, and to several hexose phosphates, also two previously 

 unidentified major spots whose relative importance increased with de 

 creasing exposure to C'*02, thus indicating that they were due to early in- 

 termediates of photosynthesis. The sugar obtained by hydrolysis of one 

 of the unknown phosphate esters was found to form reversibly an anhydride 

 with the equilibrium constant characteristic of sedoheptulose. After the 

 oxidation of this sugar by periodic acid, 15% of its tracer was found in 

 formaldehyde, 55% in formic acid, and 28% in glycolic acid, in agreement 

 with expectations for a C7 ketose. Periodic acid treatment of the poly- 

 alcohol derived from the same sugar gave one molecule of tagged formalde- 

 hyde per three molecules of tagged formic acid. Assuming equal labelling 

 of all carbon atoms in the chain (a permissible assumption for carbohy- 

 drates found after 5 min. of steady photosynthesis in saturating light, cf. 

 table 36. V), the theoretical ratio is 1 : 2 for a hexitol, and 1 : 2.5 for a hepti- 

 tol. The empirical value of 3 thus confirmed that the chain was 

 longer than Ce. Co-chromatographing of the unknown sugar (or its an- 

 hydride) with chemically pure sedoheptulose (or its anhydride) led to a defi- 

 nite identification. 



The second unknown sugar was identified as a pentose by reduction. 



