NUCLEOPROTEINS, NUCLEIC ACIDS, RELATED SUBSTANCES 1^7 



Adenine and ribose are joined by a j8-glycosidic linkage from the 

 nitrogen of position-9 of the adenine to carbon- 1 of the ribose. The 

 adenosine and phosphoric acid are united by an ester hnkage. Adenosine 

 and adenylic acid have now been synthesized by Todd and co-workers, 

 so that there is no doubt remaining as to their structure. These workers 

 have also synthesized a number of other nucleosides and nucleotides. 



A second type of adenylic acid has been obtained from yeast nucleic 

 acid. In this type the phosphoric acid is thought to be linked to carbon-2' 

 instead of 3'. If this proves to be correct, it provides strong support for 

 the view that nucleotides are linked together through phosphoric acid, 

 which is joined to one nucleotide at carbon-2' and to the other nucleotide 

 at carbon-3'. These structures are complicated and difficult to deter- 

 mine, but distinct progress is being made toward their final solution. 



In the pyrimidines the y8-glycosidic linkage is between the nitrogen 

 at number 3 position and carbon-l' of the ribose. The phosphoric acid 

 is located at carbon-3', as in the purine nucleotides. These structures 

 have been established beyond doubt by synthesis of cytidine and uridylic 

 acid. 



The nucleosides and nucleotides of desoxyribose are believed to have 

 the same linkages between base, sugar, and phosphoric acid as those of 

 ribose, but the data are not so conclusive as for the ribose compounds. 



Polynucleotides 



Nucleic acids found in nature are usually polynucleotides, consisting 

 of many purine and pyrimidine nucleotides joined together to form a 

 single structure. Estimates ranging from 60 nucleotides for yeast nucleic 

 acid to 4000 for thymus nucleic acid have been given. Such estimates 

 are in accord with the large molecular weights obtained for these nucleic 

 acids. Formerly, it was believed that these large molecules were made up 

 of many tetranucleotide units, but this view is now generally abandoned. 



The molar ratios of the different purines and pyrimidines to one another 

 do not bear out the idea of a regularly occurring tetranucleotide unit. 

 For example, Chargaff and co-workers found that the desoxynucleic acid 

 of salmon sperm gave molar ratios of the constituents as follows: Adenine 

 to guanine, 1.43; thymine to cytosine, 1.43; adenine to thymine, 1.02; 

 guanine to cj^tosine, 1.02; purines to pyrimidines, 1.02. Adenine oc- 

 curred in excess of guanine, and thymine was more abundant than 

 cytosine. Oddly enough the ratios are the same in both cases, and the 

 total purine is equal to the total pyrimidine content. In other desoxy- 

 nucleic acids Chargaff found adenine exceeded guanine, and thymine out- 

 weighed cytosine, but the ratios were different than in the salmon nucleic 

 acid. 



The nucleotides are joined together through phosphoric acid groups, 

 but just how is not known. One possibility is a linkage from carbon-2' 



