On the decrease in lateral mobility of phospholipids by sugars

Abstract

Upon cold and drought stress, sucrose and trehalose protect membrane structures from fusion and leakage. Similarly, these sugars protect membrane proteins from inactivation during dehydration. We studied the interactions between sugars and phospholipid membranes in giant unilamellar vesicles with the fluorescent lipid analog 3,3'-dioctadecyloxacarbocyanine perchlorate incorporated. Using fluorescence correlation spectroscopy, it was found that sucrose decreased the lateral mobility of phospholipids in the fully rehydrated, liquid crystalline membrane more than other sugars did, including trehalose. To describe the nature of the difference in the interaction of phospholipids with sucrose and trehalose, atomistic molecular dynamics studies were performed. Simulations up to 100 ns showed that sucrose interacted with more phospholipid headgroups simultaneously than trehalose, resulting in a larger decrease of the lateral mobility. Using coarse-grained molecular dynamics, we show that this increase in interactions can lead to a relatively large decrease in lateral phospholipid mobility.

FIGURE 1

Trehalose (α-D-glucopyranosyl-α-D-glucropyranoside) (a) and sucrose (β-D-fructofuranosyl-α-D-glucopyranoside) (b).

FIGURE 2

Sugars decrease the lipid mobility. The diffusion of the fluorescent lipid analog DiO in membranes consisting of a 3:1 molar mixture of DOPC/DOPS is shown. On the x axis, the bulk concentrations (a) and viscosities (b) of sucrose (▪), trehalose (▾), and maltose (▴), and the bulk concentrations of (c) sucrose (▪), glucose (□), and fructose (♦), and (d) sucrose (▪), glucose (□), maltose (▴), maltotriose (∇), and maltotetrose (Δ) are plotted.

FIGURE 3

The MD simulations. Snapshot taken from an MD simulation of a DOPC bilayer with 0.8-M sucrose system after 100 ns. Only one of the leaflets of the bilayer is shown. For clarity, the sucrose molecules are presented in green on the left side of the figure, whereas the lipids are presented as ball and stick, with the carbon atoms shown in blue, the oxygen atoms shown in red, and the hydrogens in white. Only hydrogens capable of forming hydrogen bonds are shown. On the right side of the figure, the lipids are presented in yellow and the sucrose in ball and stick representation.

FIGURE 4

Density profiles along the axis perpendicular to the membrane for the 0.8-M sugar simulations. (a) The mass densities are shown of trehalose (▾), water (▴), and the headgroups (•) and tails (▪) of DOPC. (b) The same as for (a), only with the densities for the glucose (○) and fructose (□) moieties of sucrose plotted separately. For clarity of the figure, symbols spaced 0.4 nm apart are plotted.

FIGURE 5

Distribution of the diffusion constants of the lipids obtained from the MD simulations with no sugar (▪), 0.4 M (•), 0.8 M (▴), or 1.5 M (▾) sucrose, and 0.4 M (♦), 0.8 M (◂), or 1.5 M (▸) trehalose.

FIGURE 6

Hydrogen-bond analysis. (a) The average number of hydrogen bonds between the sugar molecules and the lipids (solid symbols) or between sugar molecules (open symbols) for sucrose (▪, □) and trehalose (•, ○). (b) The average number of sucrose (▪) and trehalose (•) molecules that formed hydrogen bonds with a lipid. (c) The average number of lipids that formed hydrogen bonds with a sucrose (▪) or trehalose (•) molecule. (d) Distribution of the number of lipids that bound to a sucrose (▪) or trehalose (•) molecule (for 0.8 M of sugar).

FIGURE 7

Increased cross-linking decreases the lipid mobility. (a) Snapshot of a coarse-grained MD simulation. The lipids are represented by small rods consisting of two beads (dark gray), whereas the sugars are represented by rectangles consisting of four beads (black). The position of the molecules is restrained to planes parallel to the light gray square. The affinity of each of the sugar beads to the lipids can be varied. A total of 1000 lipids and 500 sugar molecules were simulated in the unit cell, but only a fraction is shown. (b) Diffusion constant obtained from the coarse-grained simulation as a function of the number of high affinity beads of the sugar, relative to the lipid with weakly interacting sugar.