Structural Examination of the Impact of Glycerol on Water Structure

Abstract

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.

On the structure of an aqueous propylene glycol solution

“…82 This change in the bulk water structure has been postulated to contribute to the mechanism by which cryoprotectants, such as PG, prevent ice formation. 81 With two hydroxyl groups, the results here show that PG is readily hydrated by surrounding water molecules, through both hydrogen bond donation and acceptance, with the SDM plots in Fig. 5 showing the nearest neighbor water molecules in similar highly localized positions about both -OH groups.…”

Balance of enthalpy and entropy in depletion forces

“…A complimentary perspective addresses the explicit cosolute-induced changes occurring in the solution (cosolute and solvent mixture) itself. The existence of water that is structured differently at the macromolecular interface than in the bulk has been frequently found experimentally and in simulations for many different solutions of macromolecules, including, for example, osmolytes [63,64], lipids [65,66], proteins [67,68] and nucleic acids [69]. The "hydration force" [70] typically associated with this different water structuring decays exponentially with a typical decay length of roughly the dimensions of a 1-2 water molecules (2-4 Å) [71][72][73], and is insensitive to macromolecule identity.…”

Slow rheological mode in glycerol and glycerol–water mixtures

“…al. used neutron scattering in combination with Reverse Monte Carlo (RMC) simulations to study the hydrogen-bonded structure in glycerol [59] as well as in glycerol-water mixtures [60][61][62][63]. They find that in neat glycerol each molecule forms on average 5.68 ± 1.51 hydrogen bonds, while for neat water the average number of hydrogen bonds per molecule is about 3.56±1.10.…”