A High-Resolution Solid-State 13C NMR Study of the Secondary Structure of Branched (1→3)-β-d-Glucans from Fungi: Evidence of Two Kinds of Conformers, Curdlan-Type Single-Helix and Laminaran-Type Triple-Helix Forms, as Manifested from the Conformation-Dependent 13C Chemical Shifts

The high-resolution 13C NMR spectra of a variety of fungal branched (1→3)-β-d-glucans were recorded in a DMSO solution and a lyophilized solid in order to gain an insight into the primary and secondary structures in relation to their gel-forming property. We found that all the five lyophilized branched (1→3)-β-d-glucans examined so far, except lentinan, exhibit the laminaran-type triple-helix form, as established by the close resemblance of the 13C NMR spectra among them. On the contrary, lentinan taking the curdlan-type singlehelix conformation was readily converted to the triple-helix form by lyophilization after having been dissolved in a 8M urea solution (1M=1 mol dm−3) and dialysis against distilled water. This finding should be compared with the similar conformational change of curdlan: the conversion by this procedure was at most 50%; annealing at 150°C, followed by slow cooling, was essential for a complete conversion. This treatment did not induce the reverse conformational change from the triple helix to the single helix. The differential behavior of gelformation between the linear and branched (1→3)-β-d-glucans was explained in terms of the molecular conformations in the solid and gel states.