THE MECHANISM OF ORGANIC SYNTHESIS 129 



propionic acid or alanine. The formation of the higher amino-acids 

 involves a process of reduction of the sugar first formed in the chloro- 

 phyll granules. It is possible, however, that the starting-point for the 

 amino-acid synthesis may be, not a hexose itself, but some other sub- 

 stances, formed, so to speak, as by-products in the assimilation of sugar 

 from carbon dioxide. We have seen reason to believe that the first 

 result of the action of the sun's rays within the chlorophyll corpuscle 

 is formaldehyde. This substance in the presence of calcium carbonate 

 when exposed to the light gives a mixture of glyceryl aldehyde and 

 dihydroxyacetone. If we can assume that acetone is formed from the 

 latter by a process of reduction, we might possibly derive leucine from 

 an interaction of this substance with lactic acid and ammonia. Thus ; 



CH 3 CH 3 

 CH, CH 3 N}H/ 



CO + CH.OH + NH 3 + H 2 CH 2 + 2H 2 



CH 3 COOH CH.NH 2 



COOH 



As an intermediate product in the synthesis of starch, glyoxylic acid 



CHO 



has been described as occurring in the green parts of plants. 



COOH 



This substance with ammonia gives f ormyl glycine, and by the splitting 

 oil of formic acid, glycine or amino-acetic acid. Why nitrates are 

 necessary for certain forms of plants is not at present understood. In 

 the proteins nitrogen always occurs in an unoxidised form as NH or 

 NH 2 , and the nitrates taken up from the soil must therefore undergo 

 reduction before they can be built into the protein molecule. It is 

 supposed that they may pass through a series of reductions, namely : 



HN0 3 HN0 2 HNO H 2 N OH 



nitric acid nitrous acid hyponitrous acid hydroxylamine 



and that the latter substance then reacts with formaldehyde or other 

 substance derived from the carbon dioxide assimilation to form amino- 

 compounds. In general we may say that the probable mechanism 

 of formation of amino-acids is the production of a-oxyacids, which then 

 react with ammonia to form the amino-acids of the protein molecule ; 

 but of the exact steps in this process we are at present ignorant. 

 Knoop's work would point to the ketonic acids as forming one step, 

 and as interacting with ammonia, with simultaneous reduction, to 

 form amino-acids. 



9 



