Asymmetric Citric Acid 257 



Bentley: In the synthesis you reacted chloroacetoaeetie ester with 

 cyanide. How do you get the preferential condensation on the ketone 

 instead of reaction with the halogen? 



Heidelberger; It was an ordinary hydrocyanic acid addition which 

 was catalysed by cyanide ion. The experimental details are published 

 (1950, J. Amer. cheni. Soc, 72, 5019). 



Incidentally, the synthetic citric acid was characterized by a number 

 of methods, including a chromatographic one, in which we took an 

 equal amount of the [i*C]citric acid and of authentic non-labelled 

 citric acid and chromatographed them on silica gel. Only one peak 

 was found and again the titration paralleled the count. We feel that 

 there is no doubt about the citric acid being authentic. I wish some- 

 body would give us some better ideas about the peculiar result of finding 

 i*C in the wrong end of the citric acid in our biosynthetic experiments 

 using pyruvate and acetate. 



Krebs: Would it help in any way to assume that the reaction forming 

 citric acid from acetate and oxaloacetate was reversible and that the 

 reverse reaction did not involve a three-point contact? The reverse 

 reaction has not been shown in animal tissues but there are indications 

 that that type of reaction occurs in micro-organisms, citrate being split 

 into oxaloacetate and acetate. 



Heidelberger: I think that if such an explanation were true it would 

 help, but Professor Wood's laboratory has good evidence, I think, that 

 the split only takes place to a small extent. Furthermore, I would hate 

 to think of it taking place by a non-3-point contact. I think the same 

 enzyme that splits the citric acid would have to make it. 



•ISOTOPES 1 8 



