Structural examination of the impact of glycerol on water structure.

The sugar alcohol glycerol is essential for cryopreservation, an important process used for the storage of biological molecules, cells, or tissues at low temperatures. A key hypothesis for the cryoprotective action of glycerol is that the glycerol molecule acts to modify the hydrogen bonding ability of water molecules, thus inhibiting ice formation. In this study, high-resolution neutron diffraction has been used in conjunction with hydrogen/deuterium isotopic labeling to determine with unprecedented detail the structure of a dilute aqueous glycerol solution. Contrary to some expectations, at the first neighbor level no modification in the position of the coordination shell of water is observed. However, at the second neighbor level the presence of only small quantities of glycerol in the solution has the same impact on water structure as increasing the pressure. Evidence is also found of more glycerol monomers than would be expected in the solution. This prevalence of isolated glycerol molecules results in a very well mixed solution with glycerol-water hydrogen bond interactions being very favorable. Our results indicate that while the local structure of water is relatively unperturbed by the presence of glycerol, the hydrogen bonded network is highly mixed between glycerol and water. These results indicate that efforts to explain the action of glycerol should focus on not just local water structure, but the extended hydrogen bonded network in the system.