MODES OF LINKAGE BY COVALENT BONDS 



59 



It is a product of the hydrolysis of cellulose, like lactose and maltose, it 

 exists in an a and ^ form. 



B. Nucleosides 



These substances are the result of the combination of a base (most 

 frequently a purine or pyrimidine base) with a sugar (D-ribose) or desoxy- 

 sugar (desoxyribose) by means of an "oside" bond. Since many of these 

 substances occur in nucleic acids, they are called "nucleosides". 



In these compounds the linkage is probably a /S-glucoside type of linkage. 



N=C— NH2 



HC C— N 



N— C— N 





CH 



CH 



H— C— Ori 



H— C— OH 



O 



H— C 



CH2OH 



Adenosine (adenine nucleoside) 



HN— CO 



HoN— C C— N 



\ 



CH 



N— C— N ^ CH 



H-C-OH 



O 



H— C— OH 



H— C 



CH2OH 



Guanosine (guanine nucleoside) 



Among the flavins, or isoalloxazine derivatives, one finds an important 

 natural derivative of this type, it is lactoflavin — more commonly known as 

 riboflavin. Chemically, it is 6,7-dimethyl-9-D-ribityl-isoalloxazine. It is 

 present in a combined state in a large number of animal and vegetable 

 tissues. Like the flavins in general, riboflavin is soluble in water, giving a 



