Thermal analysis of semi-dilute hyaluronan solutions

The freezing and melting of water in semi-dilute (0.5-3.0%) solutions of the polysaccharide hyaluronan have been investigated by modulated differential scanning calorimetry. High molecular weight hyaluronan inhibited nucleation of ice and significantly depressed the freezing temperature in a dynamic scan conducted at -3.0 °C min^-1. Low molecular weight hyaluronan had a weaker and more variable effect on nucleation. The effects on nucleation, especially by the high molecular weight hyaluronan, are attributed to the influence of a hyaluronan network on the formation of critical ice nuclei. Both high and low molecular weight hyaluronan reduced the melting temperature of ice by 0.4-1.1 °C, depending on concentration. The enthalpy change associated with this transition was significantly reduced. If all of the enthalpy difference is attributed to the presence of non-freezing water, approximately 3.65 g water/g hyaluronan would be non-freezing. This result appears incompatible with published studies on hyaluronan samples of low water content. An alternative hypothesis and quantitative approach to analysis of the data are suggested. The data are interpreted in terms of a small amount of non-freezing water, and a much larger boundary layer of water surrounding hyaluronan chains, which has slightly altered thermodynamic properties relative to those of bulk water. The boundary layer water behaves similarly to water trapped in small pores in solid materials and hydrogels.