Studies on interaction between poly(L‐lysine) and DNA of varied G + C contents

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Abstract
Interaction between polylysine and DNA's of varied G + C contents was studied using thermal denaturation and circular dichroism (CD). For each complex there is one melting band at a lower temperature tm, corresponding to the helix–coil transition of free base pairs, and another band at a higher temperature t′m, corresponding to the transition of polylysine‐bound base pairs. For free base pairs, with natural DNA's and poly(dA‐dT) a linear relation is observed between the tm and the G + C content of the particular DNA used. This is not true with poly(dG)·poly(dC), which has a tm about 20°C lower than the extrapolated value for DNA of 100% G + C. For polylysine‐bound base pairs, a linear relation is also observed between the t′m and the G + C content of natural DNA's but neither poly(dA‐dT) nor poly(dG)·poly(dC) complexes follow this relationship. The dependence of melting temperature on composition, expressed as dtm/dXG·C, where XG·C is the fraction of G·C pairs, is 60°C for free base pairs and only 21°C for polylysine‐bound base pairs. This reduction in compositional dependence of Tm is similar to that observed for pure DNA in high ionic strength. Although the t′m of polylysine‐poly(dA‐dT) is 9°C lower than the extrapolated value for 0% G + C in EDTA buffer, it is independent of ionic strength in the medium and is equal to the tm0 extrapolated from the linear plot of tm against log Na+. There is also a noticeable similarity in the CD spectra of polylysine· and polyarginine·DNA complexes, except for complexes with poly(dA‐dT). The calculated CD spectrum of polylysine‐bound poly(dA‐dT) is substantially different from that of polyarginine‐bound poly(dA‐dT).