31. SYNTHESIS OF POLYNUCLEOTIDES 121 



3' — » 5' phosphodiester linkage, a 5'-phosphomonoester group at one end, 

 and a 3 '-hydroxy 1 group at the other end. The second series of compounds 

 are the cyclic oligonucleotides (XXXV) which arise by end to end intra- 

 molecular cyclization. The extent of the cyclization process decreases with 

 increase in chain length. In addition to these major series of compounds, 

 minor byproducts are also produced and there is invariably present, in small 

 amount, the monomelic thymidine-3' , 5 '-cyclic phosphate (XXXVI). 



Chemical polymerization of thymidine-5 '-phosphate has so far furnished 

 only small sized polymers; the longest polymer contains about twelve units 

 in a chain and the amounts fall off after tri- and tetranucleotides. It is clear 

 that in order to investigate formation of higher linear polymers at least two 

 problems must be solved. These are (1) the inhibition of the intramolecular 

 cyclization process which competes with linear polymerization and (2) the 

 elimination of side product formation so as to induce the polymerization to 

 go further and to facilitate the isolation of pure linear polymers. With re- 

 spect to the first problem, the concentration used in the reaction will greatly 

 influence the proportion of the linear and cyclic products. Furthermore, a 

 procedure which does, in fact, greatly reduce the cyclization reaction has 

 been devised. This consists of adding some (25-50%) 3'-0-acetylthymi- 

 dine-5 '-phosphate to thymidine-5'-phosphate before polymerization. 25, 26 

 The protected nucleotide forms the terminating unit of the greater portion 

 of polymeric mixture and the compound (XXXVII) and higher homologs 

 cannot undergo intramolecular cyclization. 



OH 



o 



I 



CH 3 C=0 



(XXXVII) 



The minor side products in the polymerization reactions have been in- 

 completely investigated 11 • 26 as yet but they appear to arise from the direct 



25 H. G. Khorana, J. Cellular Comp. Physiol. 54, Suppl. 1, 5 (1959). 



26 H. G. Khorana, and J. P. Vizsolyi, J. Am. Chem. Soe. in press. 



