868 MILLER [chap. 32 



was reducing. Thermodynamic calculations show that all lower oxidation states 

 of phosphorus are inistable in the presence of the pressures of hydrogen assumed 

 in this paper. For example, the equilibrium constant for the reactioii 



H2PO4- (aq) + 2H2 = H2P02-(aq) + 2H20(i) 



is 2.5 X 10~27 at 25°C. It is possible that stronger reducing agents than hydrogen 

 (such as formaldehyde or hydrogen cyanide) reduced the phosphate, or that 

 some process other than reduction solubihzed the calcium phosphate. This 

 problem deserves careful attention. 



The synthesis of porphyrins is considered by many authors to be a necessary 

 step for the origin of life. Porphyrins are not necessary for living processes if 

 the organism obtains its energy requirements from fermentation of sugars or 

 other energy -yielding organic reactions. According to the heterotrophic theory 

 of the origin of life, the first organisms would derive their energy requirements 

 from fermentations. The metabolism of sulfate, iron, N2, hydrogen and oxygen 

 appears to require porphyrins as well as photosynthesis. Therefore, porphyrins 

 would probably have to be synthesized before free energy could be derived 

 from these compounds. While porphyrins may have been present in the 

 environment before life arose, this is apparently not a necessity. Porphyrins 

 may have arisen as a result of biochemical experimentation of very primitive 

 organisms. 



F. Intermediate Stages in Chemical Evolution 



The major problems remaining for an understanding of the origin of life are: 

 (1) the synthesis of peptides, (2) the synthesis of purines and pyrimidines, 

 (Jif (3) a mechanism by which "high energy" phosphate or other types of bonds 

 '' could be synthesized continuously, (4) the synthesis of nucleotides and poly- 

 nucleotides, (5) the synthesis of polypeptides with catalytic activity (enzymes), 

 and (6) the development of polynucleotides and the associated enzymes which 

 are capable of self-duplication. 



The free energy change for the reaction 



amino acid + amino acid — dipeptide-t-H20 



is about -(- 3 to -f 5 kcal {K — 10"^). Since the concentrations of amino acids in 

 the oceans would be low, the percentage of amino acid in the form of dipeptide 

 would be extremely small. The equilibrium constant for a peptide of n amino 

 acids is about K". The K^ must be corrected for the fact that the addition of 

 an amino acid to a long peptide requires less energy than to a small peptide, 

 but this difference still leaves the corrected K^ extremely small. This reaction 

 can be pushed to the right by removing water or by heating as Fox has done, 

 but, as pointed out earlier, these methods would probably not be effective on the 

 primitive Earth. Adsorption on mineral surfaces might aid in the synthesis of 

 peptides, but this would require the peptides to be firmly bound to the mineral. 



