Interaction of Poly-5-bromocytidylic Acid with Polyinosinic Acid

We have shown that a 1:1 complex is formed between polyinosinic acid (I)n and poly-5-bromocytidylic acid (BC)n and have characterized the complex by several spectroscopic methods. We attribute the resolved, longest wave length band in the ultraviolet spectrum to a 5-bromocytidine transition and suggest that the first positive and negative bands in the circular dichroism curve may arise primarily from splitting interaction in the (BC)n chain. The infrared spectrum indicates interaction at the carbonyl groups and shows specificity of base pairing. We have determined the dependence of Tm of (I)n·(BC)n upon BC content of mixed (C, BC)n copolymers and have carried out a comparative study of the effect of organic solvent upon the Tm of (I)n·(C)n and (I)n·(BC)n. The dependence of Tm upon BC content is linear, indicating that the stabilizing effect of bromine residues does not depend significantly upon sequence or Br-Br proximity in the chain. The effect of ethylene glycol upon Tm is similar for both helices, suggesting that differential solvation of C and BC is probably not the source of the stabilizing effect of bromine. We present further experiments and arguments in support of our conclusion that the stabilizing effect of bromine is probably not due to solvent effects or to in-plane I-C interactions within the helix but that it may result instead from the rather high polarizability of Br acting upon cytosine residues in the same chain.