Unfrozen water in amylosic molecules is dependent on the molecular structures—A differential scanning calorimetric study

The amount of unfrozen water (UFW) associated with linear and cyclic (1→4)-α-d-glucans and oligomers thereof, including maltopentaose, two enzymatically synthesized amyloses; A-55 (Mw=54,500) and A-820 (Mw=820,000), three cyclodextrins; α-CD, β-CD and γ-CD and two cycloamyloses; CA-5 (Mw=4700) and CA-13 (Mw=13,100) were determined by differential scanning calorimetry (DSC). UFW, estimated from a plot of the melting enthalpy of ice against the weight fraction of water, showed that the glucans examined contain surprisingly different amounts of UFW. The molar ratio of UFW per glucose residue was increased in the order of α-CD (1.24), β-CD (1.78), γ-CD (1.96), A-55 (2.77), A-820 (2.98), CA-5 (2.93) and CA-13 (3.52). These values can be interpreted in terms of their hydrated conformational structures.