Characterization of the secondary structure and order-disorder transition of a β-(1 → 3, 1 → 6)-glucan from...

This study revealed the secondary structures of the water-soluble Aureobasidium pullulans β-(1 → 3, 1 → 6)-d-glucan (APG) whose primary structural unit is a β-(1 → 3)-d-glucan backbone with four β-(1 → 6)-d-glucosyl branching units every six residues. Solid-state NMR spectroscopy, X-ray diffractometry (XRD), and small-angle X-ray scattering (SAXS) experiments involving samples prepared from lyophilized APG showed that APG forms a triple helix in H₂O and a random structure in DMSO. In addition, it was revealed that the transformation from the triple helix of APG to the random structure occurs reversibly, and that the triple helix is recovered from the random structure in DMSO/H₂O mixtures containing more than 30% H₂O. Solid-state NMR and diffraction studies revealed that the triple helix of APG is more stable than that of schizophyllan (SPG) whose structure comprises a β-(1 → 3)-d-glucan backbone with one β-(1 → 6)-d-branching unit every three residues. The APG helical pitch is 1.82 nm, which is about 10% longer than that of the triple helix of SPG. These findings show that the β-(1 → 6) side-chain frequency strongly affects the stability and helical pitch of a β-(1 → 3, 1 → 6)-d-glucan.