Supramolecular Structure and Conformation of a (1→3)(1→2)-β-D-Glucan from Lactobacillus suebicus CUPV221 as Observed by Tapping Mode Atomic Force Microscopy

Tapping mode atomic force microscopy (TM-AFM) has been used to analyze the supramolecular structure and conformation of the (1 -> 3)(1 -> 2)-beta-D-glucan produced by Lactobacillus suebicus CUPV221 isolated from cider. Solutions for TM-AFM observation were prepared by dispersing the solid glucan in distilled water and in alkaline aqueous solutions. It was found that from the distilled water at 10 mg/L or higher concentrations, the (1 -> 3)(1 -> 2)-beta-D-glucan forms networks. The heat resistance of the networks depends on the concentration. From the alkaline aqueous solutions, different supramolecular structures were observed depending on the pH. From the weakest alkaline solution, a fairly rough morphology with a high density of spikelike growth features was revealed. As the ionic force of the medium increased, the sizes of the spikelike growth features diminished, and even many disaggregated fibers could be found. At 0.4 M NaOH (pH 13.16), the aggregates had disappeared almost totally. NaOH aqueous solutions (0.1 and 0.4 M) were used to carry out the study of conformation. At 0.1 M NaOH, the aggregates were partially detached, and many free microfibers were found to which a helical conformation could be assigned due to their stiffness and rodlike character. At 0.4 M NaOH, the beginning of the dissociation of the helical structures was seen.