Inulin

Soluble in water with low viscousity [3]. Low solubility in water at room temperature (<0.5 %) [4]. In solution, inulin form at helical structure with a semilenght of 25 Å and a radius of 10 Å [8].

Inulin can be crystallized by lowering the temperature of a solution slowly (1 °C/min) from 96 to 20 °C (dynamic crystallization). A 12 h stop at the isothermal crystallization temperature (65 or 77 °C) can be done (isothermal crystallization). The two methods give two different crystal forms. The crystals are white 8-like shaped [1].

Water: 70 g/l (20 °C) [16]
Ethanol: almost insoluble [16]

Exposure of inulin to digestion with a Bacillus circulans-derived fructosyltransferase leads to both linear and cyclic oligosaccharides. The circular oligosaccharides contain 6-8 fructofuranose units [14].

♦ Hypohalides: Inulin can be oxidized by hypohalides. The most common hypohalide for oxidation of polysaccharides is hypochlorite. An improvement in oxidation can be achieved by using hypobromite and even more by hypoiodite. In the present oxidation, hypobromite is the preferred oxidation agent. The hypobromite is made in situ by adding NaBr to the aqueous inulin solution and slowly adding NaClO. The addition of catalytic amounts of salts of Co, Mn, Cu, Ni or Fe (0.1-1 %) increases the reaction rate. The reaction is carried out at 10-25 °C at pH 7-11. The reaction time is 3-20, depending on the required quality of the product and the yield ranging from 50-90 %. A more extensive oxidation will oxidize C(6), it is however not intented in the present synthesis [13].


Oxidation of inulin by hypohalides [13].


♦ Hypohalides and TEMPO: The oxidation of inulin can selectively be done at C(6) using hypohalides and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). The reaction is carried out as described above for hypohalides. The reaction is much faster (reaction time = 20 min) with a high yield (>87 %) and selectivity (>90 %). Degree of oxidation = 1.0 [12].

Oxidation of inulin by hypohalides and TEMPO [12].

♦ O-aminopropyl-inulin can be synthesized by hydrogenation of O-cyanoethyl-inulin with a Raney-cobalt catalyst in methanol/water 90:10 [10].

Synthesis of O-aminopropyl-inulin from cyanoethyl inulin [10].

Conversion of cyanoethyl to propylamine decreases with increasing DP and DS as the amine group has a higher affinity for the catalyst, thus inhibiting the cyanogroup. This effect can be countered by lowering pH [10]. Unreacted cyanogroups can be removed by 1 M NaOH at 70 °C for 1h, which will dealkylate the O-cyanoethyl groups but not the O-aminopropyl. The reaction give a 90 % conversion of cyanoethyl to aminopropyl [10].

Aminopropyl-inulin can also be synthesized by cyanoethyl-inulin by reduction with NaBH₄ and CoCl₂. A conversion of 90 % has been achieved. The reaction requires an excess of CoCl₂, as Co(II) is reduced to Co by NaBH₄ [10].

A third methid is reduction of cyanoethyl-inulin with metals in liquid NH₃. The method is not very good, causing severe dealkylation. The addition of an alcohol as a secondary proton donor will reduce the problem. Of the metals tested (Na, Li, Ca, La), Li gave the best results. Using only an alcohol as proton donor does not hydrogenate the cyano group, and using propylamine instead of NH₃, causes complete dealkylation [10].


♦ O-carboxymethyl-inulin
Synthesis in [11].

NMR experiments show that C(4) is the most reactive site. The reactivity is explained by the secondary alcohol being easier to deprotonate than the primary. The reason for C(3) not being equally reactive is steric hindrance. The preference for C(4) is reduced with decreasing DP and increasing DS. At DS = 0.68, 41 % of the monosaccharides are monosubstituted, 10 % is disubstituted and only traces (<1 %) of trisubstituted monosaccharides are found. Increasing DS from 0.68 to 1.04 does non significantly change the amount of monosubstituted monomers or the position of the substitutions. The increase is in 3,4 and 4,6 disubstituted monomers. At DS=1.76, C(4), disubstitution and trisubstitution is almost equally favoured [11].


♦ O-cyanoethyl-inulin
Synthesis in [11]

NMR experiments show that C(4) is the most reactive site. The reactivity is explained by the secondary alcohol being easier to deprotonate than the primary. The reason for C(3) not being equally reactive is steric hindrance. The preference for C(4) is reduced with decreasing DP and increasing DS. At increasing DS, there is a preference for 3,4 and 4,6 disubstitution and at DS=2.0, trisubstituted monomers are preferred. 3,6 disubstitution is not observed [11].


♦ Triton X-100-inulin Inulin has been modified, grafing the detergent Triton X-100 on beads. The grafting was done by converting the Triton X-100 to an epoxide derivative. Epoxidized Triton X-100 was added to an inulin solution (20 g inulin in 100 ml 10 % (w/w) NaOH) at 50 °C under gentle stirring. After 24 h the soluton was neutralized to pH 7 with 1 M HCl. The water was then removed by evaporation and the modified inulin dissolved in acetone and recrystallized after filtration by adding toluene. The substitution was 18 % Triton X-100 (w/w) [15].