Preparation of Cationic Comb-Type Copolymer Having Tetra-Alkylammonium Groups and its Interaction with DNA

The interactions between DNAs and cationic comb-type copolymers like those with quaternary ammonium with N-methylated groups have applications in nanotechnology. N-methylated poly(L-lysine)-graft-dextran copolymers (MPLL-g-Dex) were obtained by methylation of poly(L-lysine)-graft-dextran copolymers (PLL-g-Dex) using dimethyl sulfate under weak basic conditions. The resulting copolymers were characterized using NMR spectroscopy and gel permeation chromatography-multiangle light scattering (GPC-MALS). Both primary and secondary amino groups of the PLL backbones were thoroughly methylated to form quaternary ammonium groups without fragmentation of PLL-g-Dex chains or other detectable side reactions. The interactions of the copolymers with DNAs were assessed by UV melting curves and fluorescence correlation spectroscopic measurements. PLL-g-Dex interacts with double-stranded DNA (dsDNA) in a sequence-independent manner. In contrast the MPLL-g-Dex affinity was lower for dsDNA with higher GC content than for DNA with low GC content. The results suggest that MPLL-g-Dex interacts with DNAs not only through ionic interactions, but also by hydrophobic interactions.