Polysaccharide–polynucleotide complexes (15): thermal stability of schizophyllan (SPG)/poly(C) triple strands is controllable by α-amino acid modification

Schizophyllan (SPG), a β-1,3-glucan polysaccharide which is known to form macromolecular complexes with certain polynucleotides, was modified by a reductive amination method with α-amino acids (Arg, Lys, and Ser). The thermal stability of the complexes as estimated by Tm was enhanced in SPG–Arg and SPG–Lys conjugates which have pI values higher than the pH of the medium (8.0). The Tm shift increased with the increase in the percentage of α-amino acid introduced and the highest Tm values attained were 64 °C for SPG–Arg conjugate and 62 °C for SPG–Lys conjugate, which are higher by 13 and 11 °C, respectively, than those of the unmodified SPG + poly(C) complex. In the SPG–Ser conjugate with a pI lower than the medium pH (8.0), the Tm values decreased with an increase in the percentage of Ser. Formation of the macromolecular complex was no longer detected above 13.2% Ser. The findings indicate that the Tm values are easily controllable by the type and percentage of the introduced α-amino acids. We believe, therefore, that the present conjugates, consisting of naturally originated SPG and α-amino acids, provide an important lead for developing nontoxic artificial vectors and to control the affinity with polynucleotides in response to medium pH and temperature.