The role of cationic comb-type copolymers in chaperoning DNA annealing

G-rich oligonucleotides tend to fall into kinetically trapped unstable structures because of their conformational polymorphism. Nucleic acid chaperones accelerate association of nucleic acids assemblies into the thermodynamically most stable conformations by decreasing the energy barrier for breakage or re-assembly of base pairings. Here, we report that an artificial nucleic acid chaperone, a cationic comb-type copolymer, promotes tetramolecular quadruplex assembly from mixtures of two different G-rich sequences, 5'-TGGGGT-3' (TG(4)T) and 5'-TGGGGGT-3' (TG(5)T). A 1:1 mixture of TG(4)T and TG(5)T mainly gave [TG(4)T▪(TG(5)T)(3)], [(TG(4)T)(2)▪(TG(5)T)(2)] and [(TG(4)T)(3)▪TG(5)T] heteroquadruplexes when the mixture was annealed by cooling from 90 °C to 4 °C at 1.0 °C/min. At a cooling rate of 0.01 °C/min the mixture mostly assembled into [TG(4)T](4) and [TG(5)T](4) homoquadruplexes, indicating that homoquadruplexes were thermodynamically more stable than heteroquadruplexes. In the presence of the copolymer, mainly homoquadruplexes were obtained at cooling rate of 1 °C/min, suggesting that the copolymer promoted formation of the thermodynamically most stable structures. We also showed that the copolymer facilitated the recombination of heteroquadruplexes to homoquadruplexes even at 20-30 °C, implying that the copolymer can promote thermodynamically preferred quadruplex assembly from oligonucleotides trapped in metastable structures. These results show that the copolymer works as a DNA annealer that induces proper assembly of stable DNA structures from heterogeneous kinetically trapped mixtures of structures.