Melting of Complexes of Polyribocytidylic Acid Plus Copolymers of Inosinic and Guanylic Acids in Solution

Three copolymers of riboinosinic acid and riboguanylic acid (poly IG), with guanylic acid contents of 21.6%, 33%, and 45%, have been prepared, as well as three homopolymers of ribocytidylic acid (poly C), riboinosinic acid (poly I), and riboguanylic acid (poly G). On mixing both the poly IG and the poly G with poly C in various mole ratios in an aqueous solution, a marked hypochromicity was observed in the 250 mμ region, and it was shown that there are double helical complexes similar to the well-known poly(I+C) formed in the solutions. The optical melting profile of each complex into random coils was examined, and it was found that the guanine-cytosine pair is 20% (in enthalpy) more stable than the hypoxanthine-cytosine pair. It was also found that actinomycin D, which has no effect on the melting of poly (I+C), stabilizes the helical structure of poly(IG+C).