Multiple Conformation Transitions of Triple Helical Lentinan in DMSO/Water by Microcalorimetry

The hydrogen bonding interactions of the triple helical lentinan, beta-(1-->3)-D-glucan from Lentinus edodes, in the mixtures of dimethyl sulfoxide (DMSO) and water were investigated by light scattering, viscometry, and ultrasensitive differential scanning calorimetry (US-DSC). The results revealed that two conformation transitions occurred in the lentinan solution with an increase of temperature. A reversible transition from triple helix I to triple helix II, namely, from a high degree of immobilization of the backbone to a more freely rotating one, occurred in the temperature range from 8 to 45 degrees C. The other was an irreversible conformation transition from triple helix to single strand flexible chain at 90-140 degrees C, depending on the DMSO concentration. The side chain of the triple helix I lentinan combined with water clusters through hydrogen bonds to form an associating water layer in the first region, leading to a high degree of immobilization of the backbone. However, the side chain could rotate in triple helix II at slightly elevated temperature, as a result of the breaking of the associating structure with relatively lower energy. The second transition resulted from the destruction of the intra- and intermolecular hydrogen bonds in lentinan, which sustain the triple helical structure. Furthermore, the transition in the high temperature region showed high cooperation, suggesting that the intra- and intermolecular hydrogen bonds with the relatively high energy were destructed simultaneously. Therefore, the diversity of the hydrogen bonds created the multiple conformational transitions of the triple helical polysaccharide in the aqueous solution.