Chemical Modification to Enhance Affinity

We prepared a 2-aminoethanol-appended schizophyllan with a modification level of 2.4% with the method presented in Fig. 4.35, and hereinafter designated as N-s-SPG. When N-s-SPG is mixed with poly(C), it forms a complex that exhibits the same spectroscopic behavior as the poly(C)/s-SPG complex.This indicates that the presence of the anion group in the side chain does not obstruct the complexation at all. Furthermore, the coincidence of the CD spectra implies that the complex structure is almost identical between the poly(C)/s-SPG and poly(C)/N-s-SPG complexes. In fact, the Job plot for the poly(C)/N-s-SPG system shows that the stoichiometric number n is close to 0.40, similar to that of poly(C)/s-SPG. 

SO that the ion-pair formation between the polymer chains tends to suppress the cooperative dissociation of the multivalent hydrogen bonding. In other words, the ion pair has the ability to pin the polymer chains together to prevent them from dissociating. There is no doubt that this pinning effect contributes to enhance the melting temperature. 

Thermal stabilization was observed not only for poly(C) but also for poly(A), poly(dT) and poly(dA). Surprisingly, it induced the complexation with poly(U), which does not form any complexes with s-SPG under a nonsalt condition. Furthermore, the affinity for short-chain polynucleotides was also enhanced by the amino modification. Fig. 4.37 compares the base length dependence for dA and dT oligomers between s-SPG and N-s-SPG. As can be seen, the critical base pair length is shifted through the small modification. 

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