Interactions of the Graft Polymers

Interactions of the paft copolymers of adenine and thymine on linear and branched polyethylenimine with polynucleotides, poly A and poly U, were studied.57 From the Job plots58 of the A-PEI/T-PEI system at pH 7.4, formation of the complex was observed, the overall stoichiometry of the latter being 1 1 hypochromicity of the 1 1 complex was about 16%. The melting profiles are reversible upon heating and cooling. 

At pH 1.5, however, no complex formation between A-PEI and T-PEI was detected. At the pH in question, adenine units in the graft polymer are protonated and cannot form the complex with the thymine units. 

The formation and stoichiometry of the polymer complex between nucleic acid analogs are affected by two important factors. The first is the base-base distance in the polymer chain, represented as xA and yA in Fig. 17. This distance is one factor in intramolecular base-base interactions in the polymer chain, which also influences what can be called the penetration ability of the polymer. 

A second factor is the nature of the polymer backbone its flexibility, steric regularity, electric charge, and branching. This factor also reflects the compatibility and penetration ability of the polymer and the stability of the complex. A polynucleotide has a flexible, sterically regular and negatively charged polymer backbone, whereas that of a vinyl polymer obtained by free-radical polymerization is probably less flexible, sterically inhomogeneous and neutral. 

These assumptions can provide explanations for some of the results observed. The base-base distance in the N-vinyl type polymer is shorter than that in the graft polymer, such as A-PEI. Thus, in A-PEI, the intramolecular interactions between bases in the polymer chain may be small. This could explain why the formation of the complex between A-PEI and T-PEI at a neutral pH was observed whereas interactions between the N-vinyl type models were negligible59.  

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The enthalpy of interaction of the grafted polymer with the filler surface is given by... 

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