Effects of monovalent cation and anion species on the conformation of gellan chains in aqueous systems
Introduction
Gellan is an anionic polysaccharide that contains a carboxyl group in each repeating unit (Fig. 1). Aqueous solutions of gellan form transparent, heat-resistant, and acid-resistant gels with a decrease in temperature (T) (Brownsey et al., 1984, Jay et al., 1998). Gelation of gellan solutions is regarded to occur through a mechanism that involves a change in the conformation of gellan chains from random-coil to helix with a decrease in T. Further decrease in T leads to the aggregation of the helices to form the gel (Chandrasekaran et al., 1988, Crescenzi et al., 1986, Grasdalen and Smidsrod, 1987, Matsukawa et al., 1999, Matsukawa et al., 1999, Robinson et al., 1991). It has been reported that the physical properties of gellan gels are strongly affected by the type and concentration of added salt species (Chandrasekaran and Thailambal, 1990, Crescenzi et al., 1987, Milas et al., 1990, Ogawa, 1999, Ogawa et al., 2001, Zasypkin et al., 1997). The effects of the type of added salt species on the macroscopic properties of the gellan gels have been examined by thermal and mechanical measurements (Miyoshi et al., 1994, Miyoshi et al., 1995, Miyoshi et al., 1996, Moritaka et al., 1995, Miyoshi and Nishinari, 1999, Nishinari, 1997). However, these effects have been insufficiently examined from the molecular point of view.
Optical rotation (OR) measurement is a powerful method for examining the helical conformation of polymer chains. Using this technique, we have examined the influence of salt concentration on the conformation of gellan chains in aqueous solutions and revealed that the salt concentration only affects the conformation of gellan chains in the helix state under the condition studied (Horinaka, Kani, Itokawa, Ogawa, & Shindo, 2004). Fluorescence anisotropy measurement is useful for evaluating polymer chain mobility, which is affected by the conformation of the polymer chain when it is isolated in solution, by the degree of interaction between the chains, and the conformation when it is aggregated. The mobility of gellan chains below the coil–helix transition temperature (Ttr) decreases with an increase in the salt concentration; however, the mobility of gellan chains in the random-coil state isolated in aqueous solution is not influenced by the salt concentration. These results indicate that the salt concentration does not influence the conformation of gellan chains in the random-coil state; however, the Ttr is clearly affected by the salt concentration.
In this study, the effects of the type of cation and anion species, derived from the added salt, on the conformation of gellan chains are examined by OR and fluorescence anisotropy measurements. The effects of the type of monovalent species on the Ttr and on the aggregation of gellan chains are separately discussed. Finally, we compare the effects of the type of cation species with those of the cation concentration.
Section snippets
Sample preparation
Gellan (ELF7892, Wako, Japan) was purified three times by reprecipitation from water to 2-propanol. Table 1 shows the weight-average molecular weight (Mw) and the number-average molecular weight (Mn) of gellan obtained by GPC and the metal content of gellan estimated by the inductively coupled plasma method. Gellan labeled with fluorescein isothiocyanate (FITC) was used for the fluorescence anisotropy measurement. The preparation of labeled gellan is described in a previous paper (Horinaka,
Effect of cation species
Fig. 3 shows the T-dependence of the OR values for LiCl15, NaCl15, KCl15, RbCl15, and CsCl15. None of these samples formed a gel within the T range examined. The T-dependence is qualitatively similar among the cases studied. The [α] value decreases with a decrease in T; however, it starts increasing at approximately 30 °C. This inflection point probably corresponds to the Ttr of gellan chains from random-coil to helix, as previously reported (Horinaka et al., 2004, Horinaka et al., 2004, Rees et
Conclusion
The effects of the cation and anion species on the conformation and mobility of gellan chains were examined by OR and fluorescence anisotropy measurements. Cation species that have a larger ionic radius have a higher ‘effective’ shielding ability for electrostatic repulsions between intramolecular segments and intermolecular chains. The fact that the [α] value decreased due to the formation of aggregates suggests that the aggregation of gellan chains results in the formation of supramolecular
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