K. Akiyoshi et al., MICROSCOPIC STRUCTURE AND THERMORESPONSIVENESS OF A HYDROGEL NANOPARTICLE BY SELF-ASSEMBLY OF A HYDROPHOBIZED POLYSACCHARIDE, Macromolecules, 30(4), 1997, pp. 857-861
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 o
f CHPs in water were studied by static (SLS) and dynamic light scatter
ing (DLS) and the fluorescence probe method. Irrespective of the molec
ular weight of the parent pullulan and the DS, all of CHPs provided un
imodal and monodisperse self-aggregates in water. The size of the self
-aggregate decreased with an increase in the DS of the cholesteryl moi
ety (hydrodynamic radius, 8.4-13.7 nm). However, the aggregation numbe
r of CHP in one nanoparticle was almost independent of the DS. The pol
ysaccharide density within the self-aggregate (0.13-0.50 g/mL) was aff
ected by both the molecular weight and the DS of CHPs. The mean aggreg
ation 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 sel
f-aggregate is regarded as a hydrogel nanoparticle, in which pullulan
chains are cross-linked noncovalently by associating cholesteryl moiet
ies. The microenvironment inside or the structural characteristic of t
he self-aggregate was spectrometrically studied using a fluorescence p
robe, 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 hyd
rophobized pullulan upon heating.