Polyribonucleotides containing thiopurines. Synthesis and properties of poly(1-methyl-6-thioguanylic acid).

The synthesis of 1-methyl-6-thioguanosine 5'-diphosphate and its conversion to poly(1-methyl-6-thioguanylic acid) by means of polynucleotide phosphorylase are described. The polymer exhibited cooperative behavior (Tm = 294 K in the absence of added NaCl) characteristic of a highly stacked single-stranded helical array. In a high salt environment (0.5 M NaCl) the melting was much less cooperative and gave a higher Tm (313 K); this is suggestive of interstrand aggregation involving hydrogen bonding. The polynucleotide exhibited a remarkably high pKa (6.2) compared to that of the mononucleotide (2.6), and formed a very stable acid structure (Tm = 356 K in 50% ethylene glycol). Comparisons with poly(1-methyl-6-thioinosinic acid) and poly(6-thioguanylic acid) establish that both the 2-amino group and the 1-methyl group are required for the formation of the stable acid structure.