The purpose of this study was to investigate the stabilizing properties of osmolytes, specifically sugars on biomolecule such as protein. The strategy used in this study involved the utilisation of surfactant-rich micelles; where by the impact sugars have on the free energy of exposure of hydrocarbon groups present within the surfactant micelles was examined. The observation made for sugar-surfactant study was then applied to explain the stabilisation of the native structure and thus the physiologically active form of the protein by sugars. The sugars that have been studied include sucrose, trehalose, maltose, raffinose and mannitol. The surfactants studied were sodium decyl sulphate (SDeS), sodium dodecyl sulphate (SDS) and sodium tetradecyl sulphate (STS).
Tensiometry was used to examine the impact of sugars on the critical micelle concentrations (CMC), Gibbs free energy change of micellization ( Gmic), surface pressure, surface excess concentration and area occupied per surfactant molecule. The free energy penalty of hydrocarbon chain exposure was obtained from the Gibbs free energy change of demicellization( Gdemic) which is equal but opposite in sign to the Gmic. Measurements were carried out to elucidate the influence of sugar on the aforementioned surfactant properties as a function of increasing sugar concentration. Isothermal titration calorimetry (ITC) was then used to study -sugar surfactant interactions to give enthalpy ( Hmic) and entropy ( Smic) of micellization in addition to CMC and Gmic, thus obtaining a full thermodynamic characterisation, complementing the results obtained by tensiometry.
Tensiometric results revealed that at increasing concentration of sugar, the CMC of the surfactants was decreased and a more negative Gmic was obtained. ITC results revealed a similar trend for the effect of sugar on CMC and Gmic while the Hmic and Smic was increased in the presence of the sugars. The results from surfactant studies suggest an increase in the free energy penalty of hydrocarbon group exposure to the aqueous environment, due to an unfavourable interaction between the hydrophobic groups and the aqueous sugar solution. Consequently, the aggregation process is thermodynamically favoured and more spontaneous in sugar solutions. For instance in SDeS the Gmic in water and in sugar solution showed that micellization was more favourable in sugar solution ( Gmic = -19.14 kJ mol−1 at 1.0M Trehalose) than in water ( Gmic = -18.44 kJ mol−1). In addition, significant increases in surface pressure of the surfactants in the presence of sugars suggest an enhancement of the surface activity of the surfactants.
Increases in area occupied per surfactant molecules in the presence of sugars suggest increase in the size of the head group area thus, possible interactions between surfactant head group – sugar or sugar-water mediated interactions. Also increases Hmic in comparison to lower values of Smic obtained by calorimetry suggest possible hydrogen bonding. In conclusion, surfactant studies suggest that sugars would stabilize biological structures by a combination of both an exclusion from the hydrophobic group due to unfavourable interactions between the hydrophobic groups and possible polar interactions between polar groups.
Differential scanning calorimetry (DSC) was used to study and characterise the effect of the sugars on the thermal stability of RNase A. The results revealed an increase the thermal stability of RNase A as shown by higher Tm values in the presence of sugars. Results obtained from surfactant studies were then related to DSC results, a linear relationship between the Tm and CMC values suggests a similar mechanism. Hence, though proteins are large complex molecules, their interaction with sugars or other small solutes could be related to simple model systems such as micelles
