THE C2-FRAGMENT 1675 



8. The C2 Fragment 



The appearance of a C3 compound (PGA) as the first carboxylation 

 product in photosynthesis naturally pointed to a C2 compound as carbon 

 dioxide acceptor. If the carboxylation were not coupled with reduction, 

 or phosphorylation (or both), the acceptor would have to be phosphogly- 

 col: 

 (36.9) n.POaCHOHCHaOH + CO, > H2PO3CHOHCHOHCOOH 



In the case of reductive carboxylation, the acceptor could be phospho- 

 glycol aldehyde. 



Although C2 compounds have been found among the early tagged photo- 

 synthesis products, none of them could be identified as the CO2 acceptor, 

 and the hypothesis arose that this acceptor does not occur in the free state 

 at all, its formation by splitting of a long-chain compound being coupled 

 with carboxylation. The findings described in section 7 support this 

 hypothesis. 



Nevertheless, the study of the early tagged C2 products remains of 

 interest, because even in a reaction of the type (36.7), fleeting appearance 

 of free C2 fragments remains possible. The actually found labelled C2 

 compounds may be, if not identical, at least related to an evasive C2 inter- 

 mediate in the main reaction chain. 



Labelled phosphoglycolic acid, glycohc acid, and glycine are the main 

 C2 compounds appearing on the chromatograms (cf. for example, fig. 

 36.14c). Glycolic acid has a reduction level (L = 0.75) too low to produce 

 GA by carboxylation (which would require, according to equation 36.9, 

 glycol with L = 1.50); however, glycohc acid could be a by-product of 

 oxidation of the acceptor. 



Calvin and co-workers (1951) observed that if plants are illuminated in 

 0*02 until labelled C3 and C4 compounds appear, and illumination then 

 continued in absence of CO2, the C*3 and C*4 compounds disappear, and 

 labelled glycolic acid (CHoOH-COOH) and glycine (NH2CH2-COOH) 

 increase on the radiograms. The two compounds may be related to the C2 

 acceptor, and accumulate whenever this acceptor is prevented from being 

 converted to PGA by the absence of carbon dioxide. It is further of im- 

 portance that glycolic acid is found to be labelled symmetrically in both 

 carbon positions. This shows that it is derived from a symmetric precur- 

 sor, such as glycol, OHCH2CH2OH. 



The appearance of glycolic acid was studied in more detail by Shou, 

 Benson, Bassham and Calvin (1950). They confirmed that even after 

 very short exposure the two carbons in glycolic acid are uniformly labelled 

 (similarly to the a and j8 carbons in PGA). Shou et at. fed Scenedesmus 

 C(14)-labelled glycolic acid at pH 2.8 (to suppress ionization) and found an 



