Comb-type cationic copolymer expedites DNA strand exchange while stabilizing DNA duplex.

The accelerating effect of cationic substances on the DNA strand exchange reaction between a 20 bp DNA duplex and its complementary single strand was studied. A polycationic comb-type copolymer, that consists of a poly(L-lysine) backbone and a dextran graft chain (PLL-g-Dex) and known to stabilize triplex DNA, expedites the strand exchange reaction under physiological relevant conditions. Electrostatically a small excess of the copolymer let to a 300-1500-fold increase in the DNA strand exchange while large excess of spermine or cetyltrimethylammonium bromide, a cationic detergent known to promote markedly hybridization of complementary DNA strands, shows only a slight effect. The efficacy of the copolymer was not affected by a 10 mM Mg2+ concentration. Notably the copolymer promotes the strand exchange reaction while it stabilizes double-stranded DNA. The stabilization of strand exchange intermediates consisting of the parent duplex and the single strand by the copolymer is believed to be responsible for the observed acceleration behavior.