SOME ASPECTS OF NITRATE REDUCTION 



designate as nitroxyl is extremely unstable and liable to various con- 

 versions. 



So it is quite acceptable that a molecule HNO arising from a re- 

 duction of nitrite will be at once subject to a further hydrogenation 

 which will almost inevitably lead to the formation of hydroxylamine : 



(3) HNO+2H -> H 2 NOH 



In the assimilatory nitrate reduction hydroxylamine has frequently 

 been postulated as a precursor of the ammonia or of the amino acid 

 nitrogen in which this element is present in the same reduction stage. 

 I will not enter here into a discussion of the problem whether in these 

 cases hydroxylamine is reduced either before or after the nitrogen has 

 entered into the organic state. It may suffice that the investigations of 

 Virtanen and Csaky [1948] and of Virtanen, Csaky and Rautanen 

 [1949] leave no doubt that in the nitrate assimilation by the yeast 

 Torulopsis utilis oxime nitrogen, and also small quantities of free hydro- 

 xylamine regularly occur. 



For the dissimilatory nitrate reduction by Denitrobacillus licheniformis 

 the occurrence of free hydroxylamine has recently been demonstrated 

 by Verhoeven [1952]. 



The acceptance of hydroxylamine as a precursor of ammonia in those 

 dissimilatory nitrate reduction processes in which this compound is for- 

 med seems, therefore, fully justified which means that the last step in 

 the ideal course of nitrate reduction proceeds according to the equation : 



(4) H0NOH+2H -> NH 3 +H 2 



In the preceding discussion the undeniable fact that in dissimilatory 

 nitrate reduction nitrous oxide and nitrogen gas are the main reduction 

 products of the nitrate has been left out of consideration. The most 

 striking aspect of these products certainly is that, in contrast to the 

 nitrate and to the hydrogenation products discussed until now, they 

 contain two nitrogen atoms per molecule. This implies that in some 

 stage of their formation a reaction between two molecules of a hydro- 

 genation product must have occurred. 



In this connexion it is clearly indicated to think of nitroxyl for 

 which very unstable compound a spontaneous conversion into a dimer 

 has been well established: 



(5) 2HNO -> H 2 N 2 2 



487 



