High-Resolution 13C NMR Study of (1→3)-β-d-Glucans in the Solid State: DMSO-Induced Conformational Change and Conformational Characterization by Spin-Relaxation Measurements

Abstract

High-resolution 13C NMR spectra of a variety of (1→3)-β-d-glucans were recorded in the solid state after lyophilization from DMSO solution in order to study resulting conformations free from the triple-helix conformation. Lyophilization from DMSO solution caused conformational change of laminaran from Laminaria species (‾Dp=38) from the triple helix to the oligomer-type form (form I). On the contrary, single-chain conformation (form II) of high molecular-weight curdlan was unchanged before and after lyophilization from DMSO solution. Further, DMSO induced conformational change of both single-chain (form II) and triplehelical (form III) conformations of acid-degraded curdlan (‾Dp=14) to the form I. These findings support our view that the single-chain conformation is a major form in curdlan powder. We found that these three forms are distinguished by either the 13C spin-lattice relaxation times (T1C’s) of the laboratory frame or proton spin-lattice relaxation times in the rotating frame (T1ρH). In particular, the form I was distinguishable from others by its prolonged T1C values (3.5–12 s) of C-6 carbon. In addition, the T1ρH of the triple helix of annealed curdlan was one order of magnitude longer than that of the single chain, reflecting their difference in the manner of molecular packing.