Nano-Scale Molecular Shapes of Water-Soluble Chitin Derivatives Having Monodisperse Poly(2-alkyl-2-oxazoline) Side Chains

The molecular shapes and the sizes of structures formed by chitin derivatives with monodisperse poly(2-alkyl-2-oxazoline) side chains were investigated using atomic force microscopy (AFM), cryo-transmission electron microscopy (cryo-TEM), and small-angle neutron scattering (SANS) analyses. A ring structure with an outside diameter of 45-60 nm and a cross-sectional diameter of 10-18 nm was observed in the AFM image for chitin-graft-poly(2-methyl-2-oxazoline) 1d (DP of the side chain, 8.5; [side chain]/[glucosamine unit], 0.53). From the cryo-TEM observation of the graft copolymer 1d in 0.5 wt.-% D 2 O solution, an average diameter of 40 nm for the particles was determined, with a narrow size distribution. SANS measurements of the 0.5 wt.-% D 2 O solution of 1d revealed that the outside diameter of the particles and the cross-sectional diameter were 57 nm and 8 nm, respectively. The absolute weight average molecular weight of 1d was determined to be 5.4 × 10 5 by static light scattering. From these results it was concluded that 1d can form a unimolecular ring structure in aqueous solution. However, graft copolymers with fewer side chains (1c; [side chain]/[glucosamine unit], 0.30) and with more side chains (1e; [side chain]/[glucosamine unit], 0.96) did not form rings but instead formed monodisperse unimolecular spherical particles of diameters of 28-36 nm by AFM. A graft copolymer 1f with relatively long side chains (DP of side chain, 19.6; [side chain]/[glucosamine unit], 1.00) was also observed as a spherical particle by AFM (diameter: 30-40 nm by AFM; 40 nm by SANS). On the other hand, an intermolecular aggregate formation (diameter of the aggregate: 36-143 nm) was observed for graft copolymers 1a and 1b having short side chains (DP of side chains, 5.6; [side chain]/[glucosamine unit], 0.35 and 0.48, respectively), with a spherical molecular particle of diameter 36 nm by the AFM analysis. Chitin-graft-poly(2-ethyl-2-oxazoline) (2) (DP of side chains, 21.7; [side chain]/[glucosamine unit], 0.95) generated larger aggregates of diameter 100-400 nm by AFM. The complexation behavior of graft copolymer 1d with magnesium 8-anilino-1-naphthalenesulfonate (ANS) and with N-phenyl-1-naphthylamine (PNA) was also examined by fluorescence measurements in an aqueous solution. It was found that graft copolymer 1d complexed with both ANS and PNA, and the binding constants were calculated to be 7.5 × 10 4 M -1 and 5.3 × 10 4 M -1 respectively.