94 S. L. MILLER 



The kK (RCHO) means the sum of this term over the different aldehydes. The 

 hydrolysis of hydrogen cyanide to formic acid is a competing reaction with the 

 rate 



- d (HCN)/dr = r (HCN) 



where r is the rate constant for the hydrolysis of hydrogen cyanide. Thus if the 

 concentrations of aldehydes are so low that ^i<r(NH3)(RCH0)/r<^ij then cyan- 

 ide wiU not be available for the Strecker synthesis because of hydrolysis to formic 

 acid. It is necessary to know the values of K, k, H, h, and r, their pH and tem- 

 perature dependence for a quantitative treatment of this problem. 



From a qualitative standpoint it can be seen that the Strecker synthesis will 

 operate in very dilute solutions. The H for acetaldehyde at 25° is 1-4 x 10^ [23] 

 and the K is probably greater. The experiments reported here indicate that 

 h, k, and r are of the same order of magnitude. Thus the hydrolysis of the 

 nitriles in the hydrosphere is by the same mechanism as in these experiments 

 (probably OH" attack on the carbon of the nitrile) then (RCHO) can be as low 

 as I0"4 or io~5 m and the Strecker synthesis will still operate. If the value of k 

 relative to r (and h) is increased by catalytic hydrolysis (e.g., SH", HP04~) then 

 the concentration of aldehydes could be much lower. 



The ratio of hydroxy acid to amino acid is given by equation (i). If the con- 

 centration of ammonia is very low and (RCHO) and (HCN) are high enough, 

 then hydroxy acid will be synthesized rather than amino acid or formic acid. 



There are competing reactions which the aldehyde can undergo instead of a 

 Strecker synthesis. The aldehydes can be reduced or oxidized, the latter being 

 important if any oxygen were present. The most important competing reaction 

 would be aldol condensations. These condensations would give products that 

 are of biological importance such as trioses, tetroses, pentoses and hexoses. The 

 rate of these condensations relative to the Strecker synthesis would not depend 

 markedly on the concentrations of aldehydes, since the aldol condensations 

 would be second-order reactions. Therefore, the competing reactions of the 

 aldehydes would not predominate at low concentrations. 



The composition of the primitive Earth atmosphere has been assumed to be 

 reducing in the above discussion. The general geochemical argument for the 

 reducing atmosphere, advanced by Oparin and Urey, is that the ratio of hydrogen 

 to oxygen in the Universe is about a thousand to one, the Earth being rather 

 anomalous. No one has shown any mechanism which, before the planets were 

 formed, would produce oxygen in the region of the Earth but not in the region 

 beyond Mars. The formation of oxidizing conditions on Mercury, Venus, Earth, 

 and Mars after their formation is explained by the escape of hydrogen from these 

 planets. Their atmospheres are hot enough and their gravitational fields weak 

 enough so that hydrogen can escape into outer space from the atmosphere. The 

 escape of the strong reducing agent H2 results in oxidizing atmosphere. In the 

 region beyond Mars, the planets are at low temperature and have a high gravi- 

 tational field. These conditions prohibit the escape of hydrogen from their 

 atmospheres, as a result of which they are still reducing. 



A second argument for the existence of a reducing atmosphere on the primi- 

 tive Earth is based on the assumption that for life to arise there must be present 



