Water as a Link between Membrane and Colloidal Theories for Cells

Abstract

This review is an attempt to incorporate water as a structural and thermodynamic component of biomembranes. With this purpose, the consideration of the membrane interphase as a bidimensional hydrated polar head group solution, coupled to the hydrocarbon region allows for the reconciliation of two theories on cells in dispute today: one considering the membrane as an essential part in terms of compartmentalization, and another in which lipid membranes are not necessary and cells can be treated as a colloidal system. The criterium followed is to describe the membrane state as an open, non-autonomous and responsive system using the approach of Thermodynamic of Irreversible Processes. The concept of an open/non-autonomous membrane system allows for the visualization of the interrelationship between metabolic events and membrane polymorphic changes. Therefore, the Association Induction Hypothesis (AIH) and lipid properties interplay should consider hydration in terms of free energy modulated by water activity and surface (lateral) pressure. Water in restricted regions at the lipid interphase has thermodynamic properties that explain the role of H-bonding networks in the propagation of events between membrane and cytoplasm that appears to be relevant in the context of crowded systems.

Keywords: H bonding propagation; crowded systems; lipid hydration; restricted environments; water interphases.

Figure 3

Perturbation of a responsive membrane to the different bioffectors. (A) Typical changes of the monolayer surface pressure upon the addition of a bioeffector. (B) Difference of the final and initial surface pressure for different initial surface pressures induced by a soluble protease as a bioffector on DMPC (∆) and DMPE (∆) monolayers (taken from Ref. [55]). (C) Difference of the final and initial surface pressure for different initial surface pressures induced by chlorogenic acid on DPPC (●), Di 16:0 ether PC (●), DMPC (●) (taken from Ref. [56]). (D) Difference of the final and initial surface pressure as a function of the initial water activity in the monolayer calculated according to π = −C Γ_w RT ln (aⁱ_w) for DPPC (●), Di 16:0 ether PC (●), DMPC (●) (See text).

Figure 1

Different schematical descriptions of the interface: (A) The Gibbs model, (B) The Guggenheim model and (C) The membrane interphase model.

Figure 2

Schematic representation of the excluded volume in the head group region (red spheres) of a lipid membrane. (A). Blue region: hardcore first hydration shell. (B). Light blue region: water available as solvent (confined water) although of different properties than bulk water.

Figure 4

Molar fraction of water monomer (A) water bound with less than four H bonds (B) and tetracoordinated water molecules (C) as a function of temperature. Black symbols correspond to pure water, red symbols correspond to water in the presence of lipids.

Figure 5

Inversion of dipole direction by displacement of protons without molecular rotation. Protein conformation may change inducing a water polarization by proton displacement that propagates to the membrane interphase and vice versa.