Location of sugars in multilamellar membranes at low hydration

Abstract

Severe dehydration is lethal for most biological species. However, there are a number of organisms which have evolved mechanisms to avoid damage during dehydration. One of these mechanisms is the accumulation of small solutes (e.g. sugars), which have been shown to preserve membranes by inhibiting deleterious phase changes at low hydration. Specifically, sugars reduce the gel to fluid phase transition temperatures of model lipid/water mixtures. However, there is debate about the precise mechanism, the resolution of which hinges on the location of the sugars. In excess water, it has been observed using contrast variation SANS that the sugar concentration in the excess phase is higher than in the interlamellar region (Demé and Zemb, J. App. Cryst., 2000, 33:569). This raises two questions regarding the location of the sugars at low hydrations: first, does the system phase separate to give a sugar/water phase in equilibrium with a lipid/water/sugar lamellar region (with different sugar concentrations); and second, is the sugar in the interlamellar region uniformly distributed, or does it concentrate preferentially either in close proximity to the lipids, or towards the centre of the interbilayer region. In this paper we present the preliminary results of measurements using contrast variation SANS to determine the location of sugars in lipid/water mixtures.

Reconciliation of opposing views on membrane–sugar interactions

“…For the disaccharides the amount of bound sugar at the maximum is 0.07 mol∕mol lipid; i.e., there is 1 sugar for each 14 lipids in the membrane. At higher sugar concentrations, Γ 3 decreases linearly to negative values thus signifying preferential exclusion of sugar from the membrane interface in accord with earlier reports (38,39). These data are compiled and compared with the current results in Table S2.…”

“…This ubiquitous correlation is explained by a preferential expulsion or exclusion that increases the interfacial free energy and thus promotes the stability of lipid phases with low water accessible areas. The exclusion hypothesis has been supported by some spectroscopic evidence (28), and recently both small-angle scattering studies (36,38) and vapor pressure measurements (39) have provided direct evidence for a partial depletion of sugar in the hydration zone of lipid bilayers. Several aspects of the exclusion hypothesis were recently discussed by Lenné et al (40) who concluded that "sugars partition away from the phospholipid headgroups, rather than inserting between the headgroups.…”

Direct Comparison of Disaccharide Interaction with Lipid Membranes at Reduced Hydrations

“…An alternative mechanism for the effect has been suggested, ascribing a key role to non-specific volumetric and osmotic effects of the sugars which mediate the compressive stresses induced in membranes brought into close contact by dehydration 3,[20][21] . This explanation is supported by a model which quantitatively predicts the hydration dependence of the fluid-gel transition temperature 22 , as well as (indirect) experimental evidence that sugars tend to be excluded from the regions close to the membranes [23][24][25][26] . Andersen et.…”

Kinetics of the lamellar gel–fluid transition in phosphatidylcholine membranes in the presence of sugars

“…In order to shed some light on this problem we have recently reported results using small angle neutron scattering (SANS)-contrast variation (Lenné et al, 2006), small and wide angle X-ray scattering SAXS/WAXS (Lenné et al, 2009) and differential scanning calorimetry (DSC) (Lenné et al, 2007). This work has made significant progress in clarifying and quantifying the effects of sugars on the gel-fluid transition.…”