Elsevier

Carbohydrate Polymers

Volume 111, 13 October 2014, Pages 63-71
Carbohydrate Polymers

Synthesis, characterization and solution behaviour of oxidized pullulan

Paper dedicated to the 65th anniversary of “Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, Iasi, Romania.
https://doi.org/10.1016/j.carbpol.2014.04.060Get rights and content

Highlights

  • Four different samples of oxidized pullulan were synthesized using TEMPO protocol.

  • Longer reaction time leads to a total conversion of primary OH groups to carboxylic ones.

  • The viscometric behaviour of native and oxidized pullulans was investigated at 25 °C in water and NaCl solutions.

  • The intrinsic viscosity of oxidized pullulans, determined using Wolf approach is lower than that of unoxidized pullulan.

Abstract

Various amounts of carboxyl groups were introduced at C-6 of the non-ionic, water soluble polysaccharide, i.e. pullulan, by applying the well-established TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), sodium hypochlorite/sodium bromide oxidation protocol, varying the reaction time. The oxidized products, more water-soluble than pullulan, were further characterized by FT-IR, 1H NMR and 13C NMR techniques, in order to assess the degree of oxidation. The absolute molecular weight measurements performed using a multiangle laser light scattering molecular weight analyzer, reveals a sharp drop of the molecular weight of the samples oxidized for longer reaction times. The second virial coefficients (A2), increased from unoxidized pullulan to the oxidized samples. Dynamic light scattering (DLS) measurements provide zeta potentials and hydrodynamic radius for all studied samples. The viscosity of the initial and oxidized pullulan dilute aqueous solutions was studied in detail. All oxidized samples except the highest oxidized pullulan sample (OxPu8) showed strong polyelectrolyte behaviour, whereas this effect is less pronounced for OxPu8 due to the high degradation of the chains. The intrinsic viscosity and the interaction parameter were determined at 25 °C as a function of solvent ionic strength according to Wolf approach. The dependence of these parameters on the salt concentration follows Boltzmann sigmoid model.

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 single bondCOOH 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 C6single bondOH: OxPu0.1 (reaction time 10 min), OxPu1 (reaction time 1 h), OxPu4, and one sample (OxPu8) fully oxidized at C6single bond

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)

  • M. Bercea et al.

    In-situ gelation of aqueous solutions of entangled poly(vinyl alcohol)

    Soft Matter

    (2013)
  • M. Bishwambhar et al.

    The role of microbial pullulan, a biopolymer in pharmaceutical approaches: A review

    Journal of Applied Pharmaceutical Science

    (2011)
  • C.E. Brunchi et al.

    Hydrodynamic properties of polymer mixtures in solution

    Journal of Chemical & Engineering Data

    (2010)
  • D. Ciolacu et al.

    Amorphous cellulose–structure and characterization

    Cellulose Chemistry and Technology

    (2011)
  • S. Coseri

    N-hydroxyphthalimide (NHPI)/lead tetraacetate reactions with cyclic and acyclic alkenes

    Journal of Physical Organic Chemistry

    (2009)
  • S. Coseri

    Phthalimide-N-oxyl (PINO) radical, a powerful catalytic agent: Its generation and versatility towards various organic substrates

    Catalysis Reviews

    (2009)
  • S. Coseri et al.

    Mild and selective oxidation of cellulose fibers in the presence of N-hydroxyphthalimide

    Biomacromolecules

    (2009)
  • Cited by (0)

    View full text