Structure in an Aqueous Solution

Nuclear magnetic resonance (NMR) spectroscopy has been used to study the structure of inulin in aqueous solutions. In addition, the use of low-angle laser light scattering, dynamic light scattering, and small-angle x-ray scattering following size exclusion chromatography has yielded information on the molecular weight distribution, hydrodynamic radii, and geometry of Jerusalem artichoke inulin (Eigner et al., 1988). Inulin was found to have a rod-like shape with maximum dimensions of 5.1 x 1.6 nm (length x mean diameter). 13C relaxation rate measurements indicate that the fructofuranoside units are not part of the polysaccharide backbone; therefore, the structure is like a polyethylene glycol polymer with furanosides attached (Figure 5.1). This greatly increases the flexibility of the chains, which is reflected by a segmental motion that is two to three times faster than amylose (Tylianakis et al., 1995).

13C NMR assessment of oligomers from GF3 to GF6 and inulins with an average degree of polymerization of 17 and 31 indicated that simple helical structures are not the predominant conformation in solution (Liu et al., 1994), but rather inulin consists of randomly ordered saccharide chains. With crystallization, the molecules form helices that are stabilized by intermolecular hydrogen bonds. During gel formation, there is an increasing number of hydrogen bonds, the formation of helix domains, and an increasing crystallinity (Haverkamp, 1996). The helix domains do not contain a core in which linear molecules can be included (Dvonch et al., 1950).

13C NMR spectra of the inulooligosaccharides 1-kestose (GF2) and nystose (GF3) have also been assessed (Jarrell et al., 1979), as have the :H and 13C chemical shifts of 1-kestose (Calub et al., 1990), nystose (Liu et al., 1993; Timmermans et al., 1993a), and 1,1,1-kestopentaose (GF4)

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