264 



HEINZ SCHUSTER 



possible to determine the molecular weight or chain length of ribonucleic 

 acids by purely chemical techniques analogous to those successfully em- 

 ployed with proteins and peptides. This is easily understood considering 

 that a nucleic acid of high molecular weight is much larger than protein 

 molecules are and significantly more labile. Furthermore, no homogeneous 

 high molecular RNA has been prepared to date on a preparative scale. 

 Homogeneity or uniformity in molecular size is a prerequisite for structural 

 studies. 



If it is assumed that the molecular ratio of base : ribose : phosphoric acid = 

 1:1:1 is preserved in a high molecular weight polynucleotide and that the 

 nucleotides in the chain are held together by phosphoric acid diester bridges 

 between carbons 3' and 5' of adjacent sugar residues, then two types of 

 polynucleotides are possible which are differentiated merely by the posi- 

 tion of the singly esterified phosphoric acid on the end of the chain (Fig. 4). 

 According to the findings of Brown and Todd, 70 compound (II), on hy- 

 drolysis with alkali, splits as indicated by the dotted lines to yield a nucleo- 

 side diphosphate from one end, a nucleoside from the other end, and n 

 nucleoside monophosphates. Compound (I) yields n + 2 nucleoside mono- 

 phosphates and no nucleoside or nucleoside diphosphate (see Chapter 

 12). After alkaline degradation of TMV-RNA, which had been prepared by 

 heat denaturation or phenol treatment, only nucleotides but no nucleosides 

 or nucleoside-diphosphates could be demonstrated by paper electro- 

 phoresis. 71 The limit of sensitivity for the determination of nucleoside di- 



BASE 



BASE 



BASE 



1' 2' 3' V 5' 



V 



1' 2' 3' V 5' 

 4' 2' 3' V 5' 



BASE 



BASE 



BASE 



\ 



1' 2' 3' V 5' 

 1' 2' 3' V 5' 



X, 



H' 2' 3' V 5' 



n 



(I) 



(ID 



Fig. 4. Schematic representation of RNA structures. A further RNA type may be 

 derived from (I) in which the terminally located phosphoric acid is cyclically linked 

 to both the C-2' and C-3' of the terminal ribose. 



70 D. M. Brown and A. R. Todd, J. Chem. Soc. p. 52 (1952). 



71 K. K. Reddi and C. A. Knight, Nature 180, 374 (1957). 



