Water replacement hypothesis in atomic detail--factors determining the structure of dehydrated bilayer stacks

Abstract

According to the water replacement hypothesis, trehalose stabilizes dry membranes by preventing the decrease of spacing between membrane lipids under dehydration. In this study, we use molecular-dynamics simulations to investigate the influence of trehalose on the area per lipid (APL) and related structural properties of dehydrated bilayers in atomic detail. The starting conformation of a palmitoyloleolylphosphatidylcholine lipid bilayer in excess water was been obtained by self-assembly. A series of molecular-dynamics simulations of palmitoyloleolylphosphatidylcholine with different degrees of dehydration (28.5, 11.7, and 5.4 waters per lipid) and different molar trehalose/lipid ratios (<1:1, 1:1, and >1:1) were carried out in the NPT ensemble. Water removal causes the formation of multilamellar "stacks" through periodic boundary conditions. The headgroups reorient from pointing outward to inward with dehydration. This causes changes in the electrostatic interactions between interfaces, resulting in interface interpenetration. Interpenetration creates self-spacing of the bilayers and prevents gel-phase formation. At lower concentrations, trehalose does not separate the interfaces, and acting together with self-spacing, it causes a considerable increase of APL. APL decreases at higher trehalose concentrations when the layer of sugar physically separates the interfaces. When interfaces are separated, the model confirms the water replacement hypothesis.

Figure 1

(A) Time evolution of the APL with different water contents; (inset) effect of dehydration. (B) Evolution of the APL with different trehalose contents at low and intermediate levels of dehydration; (inset) effect of trehalose on APLs. Gray thick lines are the fits y = A₁^∗exp(-x/t₁) + y_0. The SD is indicated; when error bars are not visible, they are smaller than the symbols.

Figure 2

Mass density profiles of POPC, water, trehalose, and trehalose+water for different systems as designated in Table 1. Leaflets of adjacent bilayers through periodic boundary conditions from both sides are shown for clarity. All profiles are centered in the interbilayer space.

Figure 3

RDFs between N-n (top) and N-p atoms (right) from headgroups of adjacent bilayers, and 2D maps correlating RDF (N-n) and RDF (N-p) for models with different water and trehalose contents. All distances are in nanometers. Blue indicates the minimal level (close to zero) and red indicates the maximal level of RDFs.

Figure 4

Orientation of PN vectors in the bilayers (cosine of the angle of the headgroup with the bilayer normal) with different water contents (A–C), at intermediate water content and different amounts of trehalose (D–F), and at low water content and different amounts of trehalose (G and H). Designation of the models is described in Table 1.

Figure 5

Lipid potentials and mass density profiles of N and P atoms in the bilayers with different water contents (A–C), at intermediate water content and different amounts of trehalose (D–F), and at low water content and different amounts of trehalose (G and H). Designation of the models is described in Table 1. (Insets) Total potential profiles.

Figure 6

(A) Water potential profiles in the bilayers with different water contents; (inset) correlation between water potential and density at z = 0. (B) Changes in water and trehalose potentials in models with different water and trehalose contents; (inset) potential profiles.