Micellar Structure of Hydrophobically Modified Pullulan in Aqueous Solution

概要

Amphiphilic polysaccharides are more suitable for biomedical applications, e.g., drug delivery systems for novel nanoparticle therapeutics, than synthetic polymers, structural studies of their self-assemblies reported so far are still limited. Akiyoshi et al. investigated the self-assembly of cholesterol-bearing pullulan in water by static and dynamic light scattering, fluorescence probe method as well as small-angle neutron scattering, and concluded that the self-assembly is a (aggregating) hydrogel nanoparticle, in which pullulan chains are cross-linked non-covalently by associating cholesteryl moieties. However, the inner structure of the hydrogel nanoparticle could not be revealed only by light scattering and fluorescence.

While amphiphilic vinyl copolymers can be synthesized by random and alternating copolymerizations of hydrophilic and hydrophobic monomers, amphiphilic polysaccharides must be synthesized by the polymer reaction with reagents bearing a hydrophobic group.

In this thesis, I have chosen the hydrophobically modified pullulan of which chemical structure is illustrated in Chart I-3 to study the micellar structure of the amphiphilic polysaccharide in aqueous solution. This octenyl succinic anhydride (OSA)-modified pullulan (PUL-OSA) has substituents bearing both octenyl hydrophobe and hydrophilic carboxy group, so that its primary chemical structure is characterized by the single degree of substitution DS as well as the degree of polymerization (number of glucose residues) n of the pullulan chain. The OSA substitution reaction was performed under the mild alkaline conditions at room temperature, using the method of Eenschooten et al. where the pullulan chain little degraded during the substitution reaction. I prepared PUL-OSA samples by reacting six narrow distribution pullulan samples with n ranging from 142‐4540 with OSA. Thus, PUL-OSA samples used in this study possess narrow distributions in n.

Prior to the investigation of the micellar structure of PUL-OSA in aqueous solution, the local and global chain conformation of pullulan (PUL) with DS = 0 in aqueous solution was studied by small-angle X-ray scattering (SAXS) in Chapter II. It was found that SAXS results could be fitted by the wormlike chain model perturbed by the excluded volume effect, and the pullulan chain took a local conformation being different from the amylose chain.

In Chapter Ⅲ, the micellar structure of the PUL-OSA with relatively high DS and six PUL-OSA samples with different n in aqueous solution was studied by SAXS and fluorescence probe methods. Both SAXS and fluorescence data were analyzed consistently by using the flower necklace model originally proposed for amphiphilic vinyl copolymers (cf. Section I-3). Parameters charactering the flower necklace of PUL-OSA were compared with those of amphiphilic vinyl copolymers. It was found that the chemical structure of the amphiphilic copolymer backbone chain strongly affected the micellar structure formed in aqueous solution.

In Chapter Ⅳ, the micellar structural analysis was extended to PUL-OSA samples with lower DS to investigate the transformation from the flexible wormlike chain conformation to the flower necklace by increasing DS from zero. Both SAXS profile and fluorescence from solubilized pyrene changed smoothly with DS, and they were consistently analyzed in terms of the loose flower necklace model, interpolating the flexible wormlike chain conformation and the full flower necklace.

Chapter Ⅴ summarized the results and conclusions obtained in this thesis.