Polysaccharide–polynucleotide complexes. Part 7. Hydrogen-ion and salt concentration dependence of complexation between schizophyllan and single-stranded homo RNAs

Schizophyllan belongs to a β-1,3-d-glucan family, which exists as a random coil in dimethyl sulfoxide (DMSO) and as a triple helix in water, respectively. The schizophyllan single chain forms a complex with single-stranded homo RNAs in water/DMSO mixed solvents. Using circular dichroism, we studied the complexation and its stability as a function of apparent pH (pH*) in a mixed solvent system and as a function of the salt concentration. The complex is formed in the pH* range 6.5–10, and dissociated in the pH* range 4–6. Both poly(A) and poly(C) adopt a double strand in the pH* range 4–6 and a single strand in the pH* range 6.5–10. Therefore, the conformational change of each polynucleotide is responsible for dissociation/association of the complex, i.e., the single strand of the polynucleotides can form complexes, whereas the double one cannot. This result indicates that hydrogen bonding and similarity of the helix parameters are essential for the complex formation. The melting temperature of the complex reaches the maximum around 0.05 M of NaCl and KCl, and the value of the maximum temperature depends on the cation species.