202 ABIOGENIC ORGANIC-CHEMICAL EVOLUTION 



glycine is only possible in the presence of intact protoplasmic 

 structures. 



By decarboxylation a-amino-^-oxoadipic acid is converted 

 into 8-aminolaevulinic acid: 



HOOC.CH2.CH2.CO.CHNH2.COOH->HOOC.CH2.CH2.CO.CH2NH2 + CO2 



On condensation, two molecules of 8-aminolaevulinic acid 

 form a pyrrole, porphobilinogen. Four molecules of porpho- 

 bilinogen give a porphyrin structure which, by decarboxyla- 

 tion and dehydrogenation of the side chains forms proto- 

 porphyrin. 



It must be remarked that each link in this chain of chemi- 

 cal transformations requires a specific enzyme. However J. J. 

 Scott^^^ has recently succeeded in demonstrating the possi- 

 bility of converting 8-aminolaevulinic acid into porphobilino- 

 gen by purely chemical (not biological) means. In the course 

 of this work he established that this reaction is not peculiar 

 to 8-aminolaevulinic acid but can be undergone by a-amino- 

 ketones in general, with the formation of a-aminomethyl- 

 pyrroles. In addition to this A. Treibs"^ says that the trans- 

 formation of porphobilinogen into a mixture of porphyrins 

 can also be achieved abiogenically at high temperatures and 

 acidities. 



Certainly it is hard to tell at present to what extent 

 analogous processes could have taken place under natural 

 conditions independently of organisms. 



As we have seen above, a number of workers have done 

 many experiments in which pyrrole and pyrrolidine were 

 easily formed from ammonia, acetylene and other unsatur- 

 ated hydrocarbons by simple catalysis or under the influence 

 of ultraviolet radiations. The development of these hetero- 

 cyclic compounds in the primaeval atmosphere or hydro- 

 sphere can therefore scarcely be doubted. However, the 

 possibility of their combination there to form porphyrin 

 nuclei still needs to be substantiated. The porphyrins of 

 petroleum which have been found under natural conditions 

 are clearly of biogenic origin. They remained in the pet- 

 roleum after the decomposition of the organisms which had 

 synthesised them when alive. 



