Unveiling the Interstitial Pressure between Growing Ice Crystals during Ice-Templating Using a Lipid Lamellar Probe

What is the pressure generated by ice crystals during ice-templating? This work addresses this crucial question by estimating the pressure exerted by oriented ice columns on a supramolecular probe composed of a lipid lamellar hydrogel during directional freezing. This process, also known as freeze-casting, has emerged as a unique processing technique for a broad class of organic, inorganic, soft, and biological materials. Nonetheless, the pressure exerted during and after crystallization between two ice columns is not known, despite its importance with respect to the fragility of the frozen material, especially for biological samples. By using the lamellar period of a glycolipid lamellar hydrogel as a common probe, we couple data obtained from ice-templated-resolved in situ synchrotron small-angle X-ray scattering (SAXS) with data obtained from controlled adiabatic desiccation experiments. We estimate the pressure to vary between 1 ± 10% kbar at -15 °C and 3.5 ± 20% kbar at -60 °C.

Freezing point depression of water in phospholipid membranes: a solid-state NMR study

Lipid-water interaction plays an important role in the properties of lipid bilayers, cryoprotectants, and membrane-associated peptides and proteins. The temperature at which water bound to lipid bilayers freezes is lower than that of free water. Here, we report a solid-state NMR investigation on the freezing point depression of water in phospholipid bilayers in the presence and absence of cholesterol. Deuterium NMR spectra at different temperatures ranging from -75 to + 10 degrees C were obtained from fully (2)H2O-hydrated POPC (1-palmitoyl-2-oleoylphosphatidylcholine) multilamellar vesicles (MLVs), prepared with and without cholesterol, to determine the freezing temperature of water and the effect of cholesterol on the freezing temperature of water in POPC bilayers. Our 2H NMR experiments reveal the motional behavior of unfrozen water molecules in POPC bilayers even at temperatures significantly below 0 degrees C and show that the presence of cholesterol further lowered the freezing temperature of water in POPC bilayers. These results suggest that in the presence of cholesterol the fluidity and dynamics of lipid bilayers can be retained even at very low temperatures as exist in the liquid crystalline phase of the lipid. Therefore, bilayer samples prepared with a cryoprotectant like cholesterol should enable the performance of multidimensional solid-state NMR experiments to investigate the structure, dynamics, and topology of membrane proteins at a very low temperature with enhanced sample stability and possibly a better sensitivity. Phosphorus-31 NMR data suggest that lipid bilayers can be aligned at low temperatures, while 15N NMR experiments demonstrate that such aligned samples can be used to enhance the signal-to-noise ratio of is 15N chemical shift spectra of a 37-residue human antimicrobial peptide, LL-37.

Dehydration of solute–lipid systems: hydration forces analysis

Sorption isotherms were obtained for a range of lipid/sugar/water mixtures. These were analysed using a simple hydration forces formalism. The results demonstrate that this simple analysis can be used to estimate dehydration parameters for these relatively complex systems. This in turn provides some insight into the location and role of sugars in the hydration behaviour of lipid systems. The relevance of these results to the phase behaviour of lipid/sugar mixtures during dehydration are discussed.

Freezing stresses and hydration of isolated cell walls

The hydration of the cell walls of the giant alga Chara australis was measured as a function of temperature using quantitative deuterium nuclear magnetic resonance (NMR) of samples hydrated with D2O. At temperatures 23-5K below freezing, the hydration ratio (the ratio of mass of unfrozen water in microscopic phases in the cell wall to the dry mass) increases slowly with increasing temperature from about 0.2 to 0.4. It then rises rapidly with temperature in the few Kelvin below the freezing temperature. The linewidth of the NMR signal varies approximately linearly with the reciprocal of the hydration ratio, and with the freezing point depression or water potential. These empirical relations may be useful in estimating cell wall water contents in heterogeneous samples.

Ice premelting during differential scanning calorimetry

Premelting at the surface of ice crystals is caused by factors such as temperature, radius of curvature, and solute composition. When polycrystalline ice samples are warmed from well below the equilibrium melting point, surface melting may begin at temperatures as low as -15 degrees C. However, it has been reported (Bronshteyn and Steponkus, 1993. Biophys. J. 65:1853-1865) that when polycrystalline ice was warmed in a differential scanning calorimetry (DSC) pan, melting began at about -50 degrees C, this extreme behavior being attributed to short-range forces. We show that there is no driving force for such premelting, and that for pure water samples in DSC pans curvature effects will cause premelting typically at just a few degrees below the equilibrium melting point. We also show that the rate of warming affects the slope of the DSC baseline and that this might be incorrectly interpreted as an endotherm. The work has consequences for DSC operators who use water as a standard in systems where subfreezing runs are important.

Freezing, drying, and/or vitrification of membrane- solute-water systems

Membranes are often damaged by freezing and/or dehydration, and this damage may be reduced by solutes. In many cases, these phenomena can be explained by the physical behavior of membrane-solute-water systems. Both solutes and membranes reduce the freezing temperature of water, although their effects are not simply additive. The dehydration of membranes induces large mechanical stresses in the membranes. These stresses produce a range of physical deformations and changes in the phase behavior. These membrane stresses and strains are in general reduced by osmotic effects and possibly other effects of solutes-provided of course that the solutes can approach the membrane in question. Membrane stresses may also be affected by vitrification where this occurs between membranes. Many of the differences among the effects of different solutes can be explained by the differences in the crystallization, vitrification, volumetric, partitioning, and permeability properties of the solutes.

Dehydration of biological membranes by cooling: an investigation on the purple membrane

The lamellar spacing dl of purple membrane (PM) multilayer systems was investigated with neutron diffraction as a function of temperature and of the level of hydration. The observed large T-dependent variations of dl indicate that PM is partially dehydrated when cooled below a "hydration water freezing point". This phenomenon is reversible, but a hysteresis is observed when PM is rehydrated upon reheating. The hydration water remaining bound to the membrane below about 240 K is non-freezing. Its amount was found to be hnf=0.24(+/-0.02) g 2H2O/g BR for all samples equilibrated at room temperature in the presence of 2H2O vapour at >/=84% r.h. It is evident, that the dehydration/rehydration behaviour of PM is strongly correlated with the temperature-dependent behaviour of the dynamical structure factor. Above the well-known "dynamical transition" announcing the onset of localized diffusive molecular motions between 190 K and 230 K, a second dynamical transition is caused by the temperature-induced rehydration of the PM starting near 255 K. This is also correlated with the deviation from a pure Arrhenius law of the rate-limiting process in the photocycle, known to occur upon cooling beyond the ice point into the same temperature region. Our results suggest that the phenomenon of dehydration and rehydration induced by cooling and reheating, respectively, is a general property of biological membranes.

The effects of solutes on the freezing properties of and hydration forces in lipid lamellar phases

Quantitative deuterium nuclear magnetic resonance is used to study the freezing behavior of the water in phosphatidylcholine lamellar phases, and the effect upon it of dimethylsulfoxide (DMSO), sorbitol, sucrose, and trehalose. When sufficient solute is present, an isotropic phase of concentrated aqueous solution may coexist with the lamellar phase at freezing temperatures. We determine the composition of both unfrozen phases as a function of temperature by using the intensity of the calibrated free induction decay signal (FID). The presence of DMSO or sorbitol increases the hydration of the lamellar phase at all freezing temperatures studied, and the size of the increase in hydration is comparable to that expected from their purely osmotic effect. Sucrose and trehalose increase the hydration of the lamellar phase, but, at concentrations of several molal, the increase is less than that which their purely osmotic effect would be expected to produce. A possible explanation is that very high volume fractions of sucrose and trehalose disrupt the water structure and thus reduce the repulsive hydration interaction between membranes. Because of their osmotic effect, all of the solutes studied reduced the intramembrane mechanical stresses produced in lamellar phases by freezing. Sucrose and trehalose at high concentrations produce a greater reduction than do the other solutes.

Hydration of DOPC bilayers by differential scanning calorimetry

The phase diagram of the unsaturated lipid dioleoylphosphatidylcholine (DOPC) in aqueous multibilayer dispersions has been constructed from a series of differential scanning calorimetry (DSC) thermograms over the temperature range from -40 to +10 degrees C, covering a range of hydration levels from the monohydrate to excess free water. Both the lipid chain melting transition and the ice melting point are found to be hydration dependent. From their respective variations it is found that the bilayer in the gel phase binds approximately 9 H2O per lipid, while the liquid-crystalline state has a saturation limit near 20 H2O. The water transition exhibits a hydration-dependent melting point depression, which can be explained in terms of newly incorporated water between the bilayer surfaces upon melting of the acyl chains, and which is reminiscent of the events that occur at the pre-transition for saturated lipids. From the melting point depression, the thermodynamic activity of the interbilayer water can be calculated and thus the repulsive hydration force characterized quantitatively. We evaluate a (non-isothermal) hydration force decay constant around 2.8 H20, which demonstrates that this DSC approach is well-suited for quantitatively characterizing the hydration properties of unsaturated lipid dispersions at low temperature.

Hydration forces and membrane stresses: cryobiological implications and a new technique for measurement

Very large, repulsive forces are measured between various surfaces in water at separations of about a nanometer or less. These forces are important in cryobiology because extracellular freezing usually causes extreme osmotic dehydration of cells. This brings membranes and macromolecules into close approach, and imposes large, anisotropic stresses on them. It is therefore important to study these forces at freezing temperatures. We have studied the freezing and thawing behaviour of lamellar phases of egg yolk lecithin and D2O. Force-hydration and force-separation relations are obtained from the deuterium nuclear magnetic resonance signal as a function of temperature. From these measurements we estimate the magnitude of freezing-induced membrane stresses and discuss their effect on the response of cells and organelles to freezing and thawing.