Lateral Diffusion in Substrate-Supported Lipid Monolayers as a Function of Ambient Relative Humidity

Abstract

We analyzed the influence of water activity on the lateral self-diffusion of supported phospholipid monolayers. Lipid monolayer membranes were supported by polysaccharide cushions (chitosan and agarose), or glass. A simple diffusion model was derived, based on activated diffusion with an activation energy, E(a), which depends on the hydration state of the lipid headgroup. A crucial assumption of the derived model is that E(a) can be calculated assuming an exponential decay of the humidity-dependent disjoining pressure in the monolayer/substrate interface with respect to the equilibrium separation distance. A plot of ln(D) against ln(p(0)/p), where D is the measured diffusion coefficient and p(0) and p are the partial water pressures at saturation and at a particular relative humidity, respectively, was observed to be linear in all cases (i.e., for differing lipids, lateral monolayer pressures, temperatures, and substrates), in accordance with the above-mentioned diffusion model. No indications for humidity-induced first-order phase transitions in the supported phospholipid monolayers were found. Many biological processes such as vesicle fusion and recognition processes involve dehydration/hydration cycles, and it can be expected that the water activity significantly affects the kinetics of these processes in a manner similar to that examined in the present work.

Hydration Forces Between Lipid Bilayers: A Theoretical Overview and a Look on Methods Exploring Dehydration

“…Methods and models for determining/demonstrating hydration force Pressure gauge in solution under osmotic pressure (Leneveu et al 1976) Deflected spring within the surface force apparatus (SFA) (Marra and Israelachvili 1985) Modulated tip oscillations in atomic force microscopy (AFM) (Fukuma et al 2007) Adjusted chemical potential in osmotic stress method (OSM) (Parsegian et al 1979) Main phase transition pressure of lipids (PPM) (Pfeiffer et al 2003a) Main phase transition temperature of lipids (PTM) (Pfeiffer et al 2003b) Melting temperature of hydration water (Ulrich et al 1994) NMR Quadrupolar splitting in lipid (-CD) and/or water (D 2 O) (Volke et al 1994a) NMR relaxation times of lipids (Ulrich and Watts 1994b) Diffusion of lipids by fluorescence microscopy (Baumgart and Offenhäusser 2002) Selection of methods for water layer thickness a determination X-ray, using the Luzzati model (Luzzati and Chapman 1968) X-ray, analysing electron density profiles e.g. (Schmiedel et al 2001) Optical fringes observed in surface force apparatus (SFA) (Marra and Israelachvili 1985) Selected theories Langmuir VI isotherm (statistical mechanics) (Marsh 2011) Polarisation (electrodynamics, statistical mechanics and thermodynamics) (Cevc 1991) Protrusion (statistical mechanics) (Israelachvili and Wennerström 1990) Temperature dependence Only a few studies, obviously negligible (Simon et al 1995) Computer modelling Atomistic simulations (Schneck et al 2012), Molecular dynamics …”

“…One obtains functional pairs of chemical potential hydration which are also known as sorption isotherms. The water activity can be adjusted by the water vapour via saturated salt solutions, by inert co-solutes, such as dextran or polyethylene-glycols (PEG) or by devices that use a calibrated gas stream composed of water vapour and other gasses, such as nitrogen (Binder et al 1999a;Baumgart and Offenhäusser 2002).…”

“…In this sense, one can argue that all hydration-tuned exponentially decaying parameters have a more or less direct correlation with the hydration force, or at least a correlation with its common root cause. An interesting example is the logarithm of the hydration-dependent shift of the lateral diffusion in phospholipid monolayers (Baumgart and Offenhäusser 2002) that has a linear correlation with the water activity. It was accordingly shown that a measurement of the lateral diffusion would provide at least an estimate of hydration force parameters.…”

Hydration Effects on Membrane Structure Probed by Single Molecule Orientations

“…22,34 Baumgart and Offenh€ ausser used FRAP measurements to characterize the diffusion of the fluorescent analogue NBD-PC in LB monolayers of DMPC and DphyPC as a function of RH. 22 Generally, they found an increase in diffusion with RH for monolayers transferred onto supported polymer cushions of Figure 4. Comparison of the population of ordered emission dipoles (e10°) for LB/LS bilayers transferred at 23 mN/m and bilayers formed using vesicle fusion as a function of relative humidity.…”

“…2,3,[6][7][8][12][13][14][15][16][17][18][19][20][21] For supported lipid membranes, extensive studies have been devoted to understanding and characterizing the influence that the substrate can have on membrane properties. [22][23][24][25][26][27][28] These studies can take many forms but usually revolve around understanding the role that the substrate plays in influencing membrane structure, fluidity, and in modifying the activity of membrane-incorporated species. [22][23][24][25][26][27][28] These studies are important from both fundamental and practical viewpoints.…”