Poly(9-vinylhypoxanthine) was prepared by three different methods and its properties studied. The mutual interactions of four neutral analogs of nucleic acids, poly(1-vinyluracil), poly(1-vinylcytosine), poly(9-vinyladenine), and poly-(9-vinylhypoxanthine), and their interactions with polyribonucleotides, were investigated in dilute aqueous solutions. Poly(9-vinylhypoxanthine) shows considerably less tendency for any complexing than the other vinyl polymers, resembling thus the behavior of polyinosinic acid at high ionic strength. In other respects the formation of complexes follows the specificity pattern of polynucleotide interactions—except that the nature of the interactions is modified by two additional factors. First, because the bases of a vinyl polymer cannot completely pair with a single strand of any other polymer, some of the bases of the complexed vinyl polymer are potentially free to bind to additional strands. Secondly, because of the incomplete base pairing, the strand-strand bonding is weaker and could be offset by coulombic repulsion forces. As a result, the nature of these interactions is determined by the electric charges on the polymers. If neither component carries any charge, which is the case when both are vinyl polymers, no coulombic repulsion acts in the system and the growth of the complex continues until a high aggregate is formed. When one component is neutral (vinyl polymer) and the other negative (polynucleotide), a soluble, nonaggregated complex is formed, the repulsive forces acting against progressive joining of further negative strands. These complexes have the following characteristics: (a) broad and incompletely reversible melting and (b) filamentous appearance upon electron microscopic examination (observed for the poly(l-vinylura-cil) · polyadenylic acid complex). Similarly, the complex between poly(9-vinyladenine) with intercalated negative detergent, and poly(1-vinyluracil), is nonaggregated. When both components carry an electric charge of the same sign, as in the case of two protonated vinyl polymers, or of a negative polynucleotide and a vinyl polymer with intercalated anionic detergent, then no complex is formed; apparently the incomplete pairing cannot overcome the coulombic repulsion between two strands.
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