Helix-with-Loops Structure of Polynucleotide. I. Poly(C+IC) and Poly(C+GU)

A copolymer of 57% riboinosinic acid plus 43% ribocytidylic acid has been prepared. The ultraviolet absorbances of aqueous solutions of mixtures of this copolymer and polyribocytidylic acid of various mole ratios were measured. The results indicate that, in these aqueous solutions, a “helix-with-loops” structure is formed; this structure has the hypoxantine-cytosine base pair and the unpaired cytidylic acid residues in the loops. A copolymer of 58% riboguanylic acid plus 42% ribouridylic acid has also been prepared. A similar ultraviolet examination was made of a mixture of this copolymer and polyribocytidylic acid; again the formation of a “helix-with-loops” structure was indicated. This structure is considered to have the guanine-cytosine base pair and the uridylic acid residues in the loops.