Induction of helicity in polyuridylic acid and polyinosinic acid by silver ions

Silver ions binding to poly(U) and poly(I) produce highly ordered multistranded helices under conditions which would otherwise lead to random coils. Evidence for helicity comes from the hypochromicity and high ellipticity generated in the polymers by Ag + binding, as well as from x‐ray studies and from the cooperativity of the Ag + complexing reaction. Continuous variation studies show that both polymers form 1:1 and 2:1 polymer–Ag + complexes. Low pH favors the 1:1 complex with poly(U) and the 2:1 complex with poly(I); the reverse is true at high pH. Ag + binding and proton‐release experiments make it clear that at low pH, unprotonated electron‐donor groups are complexed preferentially, but that at high pH, Ag + readily displaces H + from protonated groups. In poly(I) the unprotonated donor is N(7), leading at low pH to a 2:1 complex containing N(7)‐Ag‐N(7) bonds; at high pH, proton release from N(1) leads to a 1:1 complex containing N(1)‐Ag‐O bonds. In poly(U) there is no unprotonated donor; the low‐pH 1:1 complex involves deprotonation of only one N(3) per bound Ag + , leading to N 3 ‐Ag‐O bonding, while high pH causes deprotonation of two N(3) per Ag + and a 2:1 N(3)‐Ag‐N(3) complex. Thus silver ions react with the nucleotide bases in chemically predictable ways, and the formation of different Ag–nucleotide bonds leads to different multiple‐helix structures.