An alternative DSC approach to study hydration of hyaluronan

Differential scanning calorimetry (DSC) was used to determine the number of water molecules in the hydration shell of hyaluronan of different molecular weights and counterions. First, traditional experiments including freezing/thawing of free water in semi-diluted solutions were carried out leading to the determination of melting enthalpy of freezable water. Non-freezing water was determined using extrapolation to zero enthalpy. For sodium hyaluronan within the molecular weight range between 100 and 740 kDa the hydration shell was determined as 0.74 g g(-1) HYA. A larger hydration shell containing 0.84 and 0.82 g g(-1) HYA was determined for hyaluronan of 1390 kDa in its sodium and protonized form, respectively. Second, melting enthalpy of freezing water was further studied applying water evaporation experiments. Resulted plot of enthalpy vs concentration indicated an additional heat evolution process which occurs at specific concentration and decreases the measured evaporation enthalpy. The heat evolution was attributed to the mutual approaching of hyaluronan molecular chains, their mutual interactions and formation of the ordered hyaluronan structure which starts immediately when the hydration water is desorbed from the hyaluronan surface. The concentration at which the process occurred was related to non-evaporable water which was determined as 0.31-0.38 g g(-1) for sodium hyaluronan and 0.84 g g(-1) for its protonized form. The second approach provides additional information enabling a deeper insight into the problem of hyaluronan hydration. (C) 2010 Elsevier Ltd. All rights reserved.