Synthesis, characterization and solution behaviour of oxidized pullulan
Introduction
Pullulan is a bacterial polysaccharide, extracted from the fermentation medium of the Aureobasidium pullulans (Bishwambhar, Suneetha, & Ramalingam, 2011), consisting of maltotriose units connected through α-(1 → 4) glycosidic bonds, whereas consecutive maltotriose units are linked to each other by α-(1 → 6) glycosidic bonds. Pullulan is extensively used as food ingredient and as a pharmaceutical bulking agent. The daily intake of pullulan is estimated around 10 g per day for a person based on food categories (Rekha & Sharma, 2009). The use and application of this polysaccharide is increasing rapidly as it is an important and industrially available alternative material for replacing natural gums produced from marine algae and other plants.
For some applications, there is often a need to increase the hydrophilicity of the native pullulan; this can be done by the introduction of various functional groups, such as carboxylic ones, by using chemical functionalization. The presence of ionic groups allows obtaining polymer solutions with various properties and viscosity due to polyelectrolyte swelling of macromolecules. The viscosity can be controlled by polymer concentration and solvent ionic strength. However, to the best of our knowledge, the rheological properties of oxidized pullulan have never been reported. De Nooy et al. (De Nooy et al., 1994, De Nooy et al., 1995) described a method for the oxidation of polysaccharides employing sodium hypochlorite, sodium bromide and catalytic amounts of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) in homogeneous conditions, using water as solvent, at pH around 10.5. It was assumed that primary alcohol groups are selectively converted into carboxylic ones. Recently an alternative protocol for the mild and efficient oxidation of polysaccharides, particularly cellulose, mediated by N-hydroxyphthalimide (NHPI) and various co-catalysts has been proposed (Coseri, 2009a, Coseri, 2009b, Coseri et al., 2009, Coseri et al., 2013). Both protocols, i.e., TEMPO and NHPI, claim to be selective, namely the oxidation reaction converts only primary OH groups to carboxylic ones.
The goal of this work was to study and understand the influence of oxidation on the properties of pullulan solutions, oxidized vs. native. Pullulan was oxidized using TEMPO, sodium hypochlorite and sodium bromide. Using four different reaction times, samples of oxidized pullulan having various degrees of oxidation were prepared. The oxidation was confirmed by FT-IR, 1H NMR and 13C NMR analysis. Moreover, the absolute molar weight of pullulan and oxidized samples was determined by using a multiangle laser light scattering molecular weight analyzer. The properties of the pristine and oxidized pullulan dilute solutions were studied using viscometry. The presence of ionic groups on pullulan macromolecules induces new length scales; thus to obtain the intrinsic viscosity we used a model developed for polyelectrolyte solutions (Wolf, 2007). In order to screen the repulsive forces between the charged groups and decrease the osmotic pressure of free counter-ions, NaCl of various concentrations was added. The discussion will be focused mainly on the changes in the viscometric behaviour induced by introduction of COOH groups in the pullulan structure and on the effect of salt addition to aqueous solutions.
Section snippets
Materials
Pullulan sample purchased from TCI Europe was dried under vacuum at 100 °C overnight prior uses.
TEMPO (99% Aldrich), sodium hypochlorite (NaOCl, 3% chlorine, Chemical Company Romania) and sodium bromide (99% Alfa Aesar) were used as received. Distilled water of HPLC grade was used for dissolution of water soluble compounds and the viscometric analyses.
Pullulan oxidation
Oxidation of pullulan was carried out as follows: 1 g of pullulan was dissolved in distilled water (120 mL). TEMPO (0.02 g) and NaBr (0.1 g) were
Characterization of oxidized pullulan
Pullulan has been oxidized by using sodium hypochlorite and sodium bromide, a protocol adapted from De Nooy et al. (De Nooy et al., 1994, De Nooy et al., 1995), at room temperature, and TEMPO serving as mediator for the oxidation reaction. Four samples were prepared, differing on the amount of negatively charged groups introduced after oxidation: three partially oxidized samples at C6OH: OxPu0.1 (reaction time 10 min), OxPu1 (reaction time 1 h), OxPu4, and one sample (OxPu8) fully oxidized at C6
Conclusions
Four oxidized pullulan samples, having different degrees of oxidation, which depend on the reaction time, were synthesized and characterized by FTIR and 13C NMR techniques. The oxidation reaction was carried out for 10 min, 1, 4 and 8 h respectively, at room temperature, pH = 10, using TEMPO as mediator, sodium bromide, and sodium hypochlorite (10 mmol/g pullulan) as actual oxidant.
The viscometric behaviour of native and oxidized pullulans was investigated at 25 °C in water and NaCl solutions. Longer
Acknowledgment
The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007–2013) under grant agreement no. 264115 – STREAM.
References (34)
- et al.
Temperature and molecular weight dependence of the unperturbed dimensions of aqueous pullulan
International Journal of Biological Macromolecules
(1987) - et al.
Oxidized cellulose – Survey of the most recent achievements
Carbohydrate Polymers
(2013) - et al.
Highly selective nitroxyl radical-mediated oxidation of primary alcohol groups in water-soluble glucans
Carbohydrate Research
(1995) - et al.
Intrinsic viscosities of polyelectrolytes in the absence and in the presence of extra salt: Consequences of the stepwise conversion of dextran into a polycation
Carbohydrate Polymers
(2012) - et al.
Synergistic behavior of poly(aspartic acid) and Pluronic F127 in aqueous solution as studied by viscometry and dynamic light scattering
Colloids and Surface B: Biointerfaces
(2013) - et al.
On the chain flexibility of arabinoxylans and other β-(1 → 4) polysaccharides
Carbohydrate Research
(2002) - et al.
Blood compatibility and in vitro transfection studies on cationically modified pullulan for liver cell targeted gene delivery
Biomaterials
(2009) - et al.
Nuclear magnetic resonance spectroscopy of pullulan and isomaltose: Complete assignment of chemical shifts
Starch/Stärke
(1995) - et al.
Dextran-based polycations: Thermodynamic interaction with water as compared with unsubsituted dextran. 2. Flory–Huggins interaction parameter
Macromolecular Chemistry and Physics
(2011) - et al.
Polyelectrolyte complexes: Phase diagram and intrinsic viscosities of the system water/poly(2-vinylpyridinium-br)/poly(styrene sulfonate-Na)
Macromolecular Chemistry and Physics
(2012)