Synthetic trehalose glycolipids confer desiccation resistance to supported lipid monolayers

Abstract

Lipid-derived desiccation resistance in membranes is a rare, unique ability previously observed only with trehalose dimycolate (TDM), an abundant mycobacterial glycolipid. Here we present the first synthetic trehalose glycolipids capable of providing desiccation protection to membranes of which they are constituents. The synthetic glycolipids consist of a simple trehalose disaccharide headgroup, similar to TDM, with hydrophobic tail groups of two 15- or 18-carbon chains. The synthetic trehalose glycolipids protected supported monolayers of phospholipids against dehydration even as minority components of the overall membrane, down to as little as 20 mol % trehalose glycolipid as assessed by assays of membrane fluidity. The dependence of the desiccation protection on the synthetic trehalose glycolipid fraction is nearly identical to that of TDM. The striking similarity of the desiccation resistance observed with TDM and the synthetic trehalose glycolipids, despite the variety of hydrophobic tail structures employed, suggests that interactions between the trehalose headgroup and surrounding molecules are the determining factor in dehydration protection.

Figure 1

Schematic of trehalose glycolipid and lipid structures. (1,2,12) Synthetic trehalose glycolipids designed to provide desiccation protection. (1) Ester linked trehalose-dipentadecanoyl. (2) Trehalose-dioleoyl. (12) Ether linked trehalose-dipentadecanoyl. (13) DOPC, a common phospholipid with similar hydrophobic tail structure to synthetic compounds 1, 2, and 12. (14) Trehalose dimycolate, desiccation resistant lipid found in all mycobacteria and investigated previously in Ref. (12).

Figure 2

Illustration of a synthetic, supported lipid and glycolipid monolayer. The system consists of a silicon/silicon oxide substrate with covalently bound octadecyltrichlorosilane on top of which sits a fluid, lipid and glycolipid monolayer. The upper monolayer contains a small fraction (1%) of fluorescently labeled lipids that permit measurements of monolayer integrity and fluidity.

Figure 3

Membrane fluidity and dehydration resistance quantified by fluorescence recovery after photobleaching (FRAP). (a-e) Fluorescence images of supported monolayers containing 1 mol % Texas Red DHPE. Initially, images of 99 mol % synthetic trehalose glycolipids (a and c) and 99 mol % DOPC (e) display intact monolayers with characteristically bright and uniform fields of fluorescence. (a2,c2,e2) When the monolayers are photobleached in a defined circular region, they recover a uniform field of intensity, indicating fluidity. (b and d) After dehydration and rehydration, FRAP images of 99 mol % trehalose glycolipid display a similarly intact, bright, and fluid monolayers. (e) Monolayers of 99 mol % DOPC show no measurable intensity above background noise after dehydration and rehydration and are destroyed by the desiccation process. The intensity of f has been increase by a factor of 5 relative to e.

Figure 4

The ratio, D_r, of the membrane diffusion coefficient after rehydration to its initial value (circles) shows recovery of the membrane above a critical fraction. The curves represent simulated percolation on a triangular lattice with the saturation, p_sat, and critical fraction, p_c, as fit parameters. In the fit, the mapping from site occupation probability to mole fraction includes the ratio of the area per trehalose glycolipid to the area per DOPC. Synthetic trehalose glycolipids (a,b) show the same recovery behavior as the previously investigated TDM (c) despite their structural differences.

Scheme 1

Synthesis of C-15 trehalose glycolipid.

Scheme 2

Synthesis of C-18 unsaturated trehalose glycolipid.