Polycation comb-type copolymer reduces counterion condensation effect to stabilize DNA duplex and triplex formation

We have previously demonstrated that the polycation comb-type copolymer having abundant grafts of hydrophilic polymer chains significantly stabilizes DNA duplexes and triplexes [Maruyama et al., Bioconjugate Chem., 8 (1997) 3, Ferdous et al., Nucleic Acids Res., 26 (1998) 39]. This study was designed to estimate the mechanisms involved in the copolymer-mediated stabilization of DNA duplexes and triplexes. The melting temperatures, T(m), of DNA duplex and triplex increased with increasing salt concentration, as well documented by the Poisson-Boltzmann and counterion condensation theories that were originally proposed by Manning [J. Chem. Phys., 51 (1969) 924] and further elaborated by Manning [Biopolymers 11 (1972) 937, Biopolymers. 15 (1976) 2385] and Record [Biopolymers, 14 (1975) 2137-2158, Biopolymers, 15 (1976) 893]. In the presence of the copolymer, however, the T(m) values of DNA duplexes and triplexes did not show significant change with salt concentration. It was concluded that the copolymer is capable of reducing the counterion condensation effects to stabilize DNA duplexes and triplexes. Strong but exchangeable interaction between the copolymer and DNA is seemingly involved in the stabilization behavior.