Sugars are widely known for their cryoprotective effects. Their presence in lipid membranes systems is known to raise lipid phase transition temperatures, thereby avoiding deleterious transitions to non-fluid bilayer phases or non-bilayer inverse phases. A debate has centred over the characteristic of the interaction between the sugar and lipid bilayer membrane: i.e. does the sugar hydrogen bond with the lipid headgroup, becoming partially incorporated into the bilayer and thereby preventing a reduction in area per lipid during compressive stress experienced at low hydration, or are non-specific properties of the sugars between the bilayers prevent close approach of the bilayers, or is the mechanism dependent on the sugar concentration.
The location of the sugar in a stacked lipid bilayer system at reduced hydration – resemblant of a biological system undergoing water stress – is central to elucidating the mechanism of protection. Recently we demonstrated the use of the neutron membrane diffraction technique on partially hydrated aligned stacks of dioleoylphosphatidylcholine to obtain information about the distribution of trehalose between bilayers . In this study we compare the location of glucose, sucrose and trehalose, between bilayer stacks. Using partial deuteration the sugars, we found that the sugar molecules followed a distribution profiles centred in the middle of the water layer, regardless of the sugar concentration. This result demonstrates that the protective effects of the sugars must be due to mechanisms that do not rely on direct interactions between the sugars and lipid headgroups, and instead are due to the non-specific osmotic and volumetric effects of the sugars.