Competition between polysaccharide/polynucleotide complexation vs., polynucleotide hybridization; salt concentration dependence of the reaction direction

Schizophyllan is a natural β-(1→3)-D-glucan existing as a triple helix in water and as a single chain in dimethylsulfoxide (DMSO), respectively. As we already reported, when a homo-polynucleotide [ex. poly (A) or poly(dA)] was added to the schizophyllan/DMSO solution and subsequently DMSO was exchanged for water, the single chain of schizophyllan formed a complex with the polynucleotide. In this paper, using an adenine-thymine (or uracil) double strand as a model system, we explored whether s-SPG can unzip the polynucleotide duplexes, examining the competition between the schizophyllan/polynucleotide complexation and the polynucleotide hybridization with circular dichroism and fluorescence spectroscopy at various NaCl concentrations. The unzipping reaction happened for the hetero-duplexes (i.e., RNA-DNA duplexes) at low salt concentrations, on the other hand, it was not observed for the homo-duplexes at all the salt concentrations. When we compared dissociation temperatures, the unzipping reaction of the homo-duplexes was expected at low salt concentrations. This discrepancy between the experimental results and the expectation, as well as the different behavior between the hetero- and homo- duplexes, can be ascribed to the difference in the activation energy of intermediate states.