XANTHAN PROPERTIES FOR SPECIFIC APPLICATIONS
3.3 STRATEGIES FOR CHANGING XANTHAN PROPERTIES THROUGH PHYSICOCHEMICAL PROCESSES
3.3.2 CONFORMATIONAL TRANSITIONS
3.3.2.1 CONFORMATIONAL TRANSITIONS INDUCED BY SALT ADDITION AND pH CHANGE
In aqueous solutions at 25°C, the backbone of xanthan is disordered or partially ordered in the form of a randomly broken helix but extended, due to the electrostatic repulsions interactions between COO− groups on the side chains, allowing them to align and associate by hydrogen bonding and to form a weakly structured material. By salt addition, counter ions shield the intramolecular charge–charge interactions and the side chains fold down on the backbone forming a fivefold ordered helical structure (Rochefort and Middleman, 1987). The conformational changes occurring in the presence of salts diminish the chain rigidity reflected in the decrease of the persistence length and intrinsic viscosity. The intrinsic viscosity decreases by salt addition due to the enhancement of the non-Coulombic interactions (Brunchi et al., 2014). The increase of environmental pH determines a conformational change of xanthan chains from double helix
to coils in disordered state (Brunchi et al., 2016a). At low (acidic) pH value the protonation of the anionic carboxylic groups occurs and the chains become uncharged and partially flexible. In weak acid environ- ment, the carboxylic groups become partially protonated (around pH = 4) or completely deprotonated (around pH = 6) (Bueno et al., 2013; Bueno and Petri, 2014; Shiroodi and Lo, 2015).
3.3.2.2 CONFORMATIONAL TRANSITIONS INDUCED BY TEMPERATURE
Besides salt and pH solution, conformational transition can be induced by changing temperature and the content of acetyl and pyruvyl groups (Rochefort and Middleman, 1987; Smith et al., 1981).
Temperature at which the transition occurs is known as melting temperature (Tm). The value of Tm depends on salt concentration (Milas and Rinaudo, 1979; Liu et al., 1987), and also pyruvate substitution of the outer mannose (Holzwarth and Ogletree, 1979; Smith et al., 1981) increases of pyruvate substitution lead to a decrease of transition tempera- ture. For a xanthan sample with the ratio of pyruvyl/acetyl substitution of 0.9, Tm was found to be 66°C and for a ratio of 0.4, Tm was 74°C (Kierulf, 1988). Considering the xanthan existence in two ordered structures (i.e., native and renatured) and one disordered (denatured) (Milas et al., 1996;
Capron et al., 1998; Matsuda et al., 2009), the heating at a temperature above Tm (80–90°C) can destroy reversibly or irreversibly the ordered conformation of xanthan. By heating the solution of ordered xanthan for few minutes, the side chains become free to rotate and macromolecules adopt a disordered coiled structure. Fast cooling (at room temperature or below) of hot solution avoid the chains degradation and so, the renatured ordered structure is obtained (Rinaudo et al., 1999; Milas et al., 1996).
The ordered renatured structure of the xanthan is similar to the native one only at the local level and it is more flexible (Capron et al., 1997; Oviatt and Bran, 1994). The investigation related to thermal behavior of double- helical xanthan in aqueous solution reports the effect of polymer and salt solution concentration on the denaturation, renaturation, and aggregation process (Matsuda et al., 2009). The denaturation of xanthan in dilute solu- tion (cp ≤ 1 mg/cm3) occurs by dissociation of double-helical structure in two single chains. At sufficiently low ionic strength each single chain
reconstructed the double-helical structure with the hairpin loops at one end by renaturation. In the case of concentrated xanthan solution (cp = 10 mg/
cm3), the denaturation leads to a double-helical structure unwounded at both ends stabilized by the ionic strength of xanthan itself. Renaturation process occurs by multiple and mismatched molecule aggregations that led to apparent molecular weights higher than in native state. In the most cases, the commercial samples of xanthan available on market are found in the renatured state (Milas et al., 1996).
Heating xanthan solution (T > Tm) more than 2 h the structural degra- dation takes place (Capron et al., 1997; Nishinari et al., 1996; Fitzpat- rick et al., 2013). Taking into account that a high ionic strength stabilizes the ordered double-stranded xanthan conformation, it is expected that the degradation rate of its chains (measured as loss viscosity) to be lower in salt solutions. In addition, it was found that highly pyruvated samples show a greater thermal stability than lower pyruvated ones (Sandford et al., 1997).
Annealing is, also, a thermal treatment which consists of heating xanthan solution at a temperature higher than that of the gel–sol transition (temperature of gel–sol transition is lower than Tm) followed by cooling.
Thus, xanthan chains undergo conformational transitions through intra- molecular rearrangements which lead to a decrease of the shear viscosity (Milas and Rinaudo, 1984). Temperature of the gel–sol transition is influ- enced by the concentration of xanthan solution, its molar mass and the nature of the existing cations. By annealing in the sol state, as other non- gelling polysaccharides, xanthan may form hydrogels (Fujiwara et al., 2000a, 2000b; Iseki et al., 2001; Quinn and Hatakeyama, 1994). Some reports show that solutions with cp > 0.5 wt% form gels through the junc- tion zone composed of oriented bundles of xanthan double helix. On the other hand, the gelling is favored by temperature and the time of annealing process, as well as by the polymer concentration.
3.3.2.3 CONFORMATIONAL TRANSITIONS INDUCED BY THE PRESENCE OF OTHERS POLYMERS
Mixtures of natural and synthetic polymers can give polymeric mate- rials with improved mechanical properties and biological performances.
The viscometric and Fourier transform infrared spectroscopy (FTIR)
investigations of xanthan/poly(vinyl alcohol) (PVA) mixtures revealed the formation of intermolecular associates through hydrogen bonds interac- tions (Brunchi et al., 2016b). In solid state, the carboxylate groups from xanthan chains interact preferentially with OH groups of PVA through hydrogen bonding interactions. In aqueous solutions, associations with less or more compact structures between the carboxylate groups from the pyruvic moieties and the hydrophilic units from PVA as well as inter- molecular associations of glucuronate units of the side chain with water molecules were evidenced.
A synergistic combination of xanthan with other polysaccharides changes considerably the rheological characteristics and this aspect was extensively investigated in order to obtain physical gels. The feature was attributed to different chain topologies and stiffness of two polysaccha- rides in the mixture especially if one component is xanthan. Thus, the addition of a small amount of xanthan to a guar gum homogeneous solu- tion induced gel-like properties. The increase of the molecular weight of xanthan or the presence of an electrolyte determines a stronger synergistic effect (Schorsch et al., 1997).
Mixtures of xanthan and galactomannans or glucomannan give ther- mally reversible gelation attributed by many researchers to chain asso- ciations. In the case of galactomannans, the gel strength depends on the mannose to galactose ratio (the interaction decreases as galactose content decreases), increasing from guar to tara and locust bean gum.
Associations were attributed to interactions between ordered helix of xanthan and unsubstituted regions of galactomannan chains (Dea et al., 1977). Other explanations discussed by Annable et al. (1994) are briefly given here:
- Mixing above transition temperature, thus xanthan interacts in disordered state.
- Deacetylated xanthan increases the interactions and gives stronger gels due to enhanced side chain mobility.
- The ionic strength controls the gelation for xanthan/Konjac mannan mixtures, the synergistic interaction appears in pure water above the transition temperature (which is located at approx.
82°C), whereas in 0.04 M NaCl around 42°C.
- Xanthan and guar gum interact favorably in both disordered and ordered states.
The charge density of polysaccharide chains and the nature of counter ions play an important role in synergistic interaction and gel forma- tion (Annable et al., 1994). The interaction between the xanthan side chain backbone and glucomannan (Konjac mannan) in aqueous solu- tions occurs nearly instantaneously bellow the conformational transition temperature of xanthan, determining the associated formation which is thermodynamically favored, the driving force being the decrease of the number of xanthan/water contacts. The presence of electrolyte shifts the conformational change of xanthan to higher temperatures. Divalent cations present a more pronounced effect than monovalent ones because they are more effective for promoting the aggregation or ordering of xanthan chains. The polymer–polymer heterocontacts involve both ordered and disordered xanthan sequences. The gelation temperature decreases in the presence of electrolytes because the self-association of xanthan macromolecules is favored in a greater extent than xanthan/
Konjac mannan associations.