52 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



the rat and for certain bacteria), pyridoxal and pyridoxamine. These 

 substances are very widely distributed in nature. 



HOH,C 



CHoOH 



OH HOH.C 



\ /^CH3 



CHO 



OH HOHjC 



Pyridoxin (vitamin Bg) 



Pyridoxal 



CHjNHa 



OH 



Pyridoxamine 



{i) Pyrimidine Derivatives 

 In pyrimidine there is a heterocycHc ring containing two nitrogen atoms. 

 It is one of three isomeric diamines : 



HC N 



I I! 



HC CH 



^ / 

 C 



H 



Pyridazine 



HC 6 2 CH 



HC 5 



3 N 



C 



H 



Pyrimidine 



^ \ 



HC CH 



I II 



HC CH 



"^ / 



N 



Pyrazine 



To the organic chemist, pyrimidine derivatives are products of the 

 condensation of urea with certain acids. Barbituric acid or cyclomalonyl- 

 urea is a member of this group. It is prepared by reacting malonic acid 

 with urea in the presence of POCI3. 



COOH NH— CO 



o = c< 



Urea 



NH. 

 NH, 



+ 



CHo = OC 



CHj + 2H2O 



COOH 



Malonic acid 



NH— CO 



Barbituric acid 



1. Pyrimidines 



Among the oxypyrimidines (their formulae, like those of the oxypurines, 

 may be also written as if they are hydroxyderivatives) are a number of 

 important constituents of nucleic acids, cytosine, uracil and thymine. 



N = CH HN-C=0 



I 



0=C C— CH3 0=C C— H 



1 



HC 2 



3 



N 



6 I 

 5 CH 



4 



HN— C=0 



N=C— NH2 



0=C CH 



CH 



Pyrimidine 



HN— CH 



Uracil 

 (2, 6-dioxy- 

 pyrimidine) 



HN— CH 



Thymine 



(2, 6-dioxy-5-methyl- 



pyrimidine) 



HN— CH 



Cytosine 



(2-oxy-6-amino- 



pyrimidine) 



