Microscopic structure and thermoresponsiveness of a hydrogel nanoparticle by self-assembly of a hydrophobized polysaccharide

Various cholesterol-bearing pullulans (CHPs) with different molecular weights of the parent pullulan and degrees of substitution (DS) of the cholesteryl moiety were synthesized. The structural characteristics of CHPs in water were studied by static (SLS) and dynamic light scattering (DLS) and the fluorescence probe method. Irrespective of the molecular weight of the parent pullulan and the DS, all of CHPs provided unimodal and monodisperse self-aggregates in water. The size of the self-aggregate decreased with an increase in the DS of the cholesteryl moiety (hydrodynamic radius, 8.4-13.7 nm). However, the aggregation number of CHP in one nanoparticle was almost independent of the DS. The polysaccharide density within the self-aggregate (0.13-0.50 g/mL) was affected by both the molecular weight and the DS of CHPs. The mean aggregation number of the cholesteryl moiety (3.5-5.7), which was estimated by the fluorescence quenching method using pyrene and cetylpyridinium chloride, was almost same for all the CHP self-aggregates. The CHP self-aggregate is regarded as a hydrogel nanoparticle, in which pullulan chains are cross-linked noncovalently by associating cholesteryl moieties. The microenvironment inside or the structural characteristic of the self-aggregate was spectrometrically studied using a fluorescence probe, ANS. The characteristic temperature to cause a structural change of the nanoparticle (T*) decreased with an increase in the DS of CHP and the ionic strength of the medium. The thermoresponsiveness of the nanoparticle hydrogel is related to the partial dehydration of the hydrophobized pullulan upon heating.