Phospholipid bilayers are viscoelastic
Entry by Sandeep Koshy, AP 225, Fall 2010
Title: Phospholipid bilayers are viscoelastic
Authors: Christopher W. Harland, Miranda J. Bradley, and Raghuveer Parthasarathy
Journal: Proceedings of the National Academy of Sciences
Whether bilayers were completely viscous or viscoelastic has long been debated. Lipid bilayers are not amenable to bulk rheology. Harland et. al used an optical microrheological approach to probe the viscoelastic properties of lipid membranes. They found that membranes do indeed have an elastic component to their behavior. This work will have impact on studying protein behavior in biological membranes as well as molecular signal transduction and mechanotranduction in cells.
Soft Matter Keywords: rheology, membrane, viscoelastic, fluctuation-dissipation
The fluidity of biological membranes is an important and long-debated topic. Membrane fluidity influences properties such as signal transduction, protein assembly on cell surfaces and presentation of macromolecules. In the past, most of these processes have been modeled assuming the lipid bilayer is a simple Newtonian fluid containing only a viscous component without an elastic response. However, problems have arisen when trying to fit molecular diffusion data using this assumption, suggesting the true behavior of such membranes should be investigated in detail. The authors use a microrheological method to show that lipid membranes demonstrate viscoelastic behavior.
The authors used a Langmuir-Schaefer deposition technique to form a bilayer. Briefly, a hydrophobic substrate containing a circular aperture is lowered into a lipid monolayer (Figure 1 – A) and a bilayer forms at the aperture (Figure 1 – B). The authors incorporated biotin into the lipid bilayer which complexed with neutravidin coated fluorescent nanoparticles. Nanoparticle trajectories were captured at 200 frames/second and analyzed. Using fluctuation-dissipation theorem, the diffusion of particles along the membrane was used to determine the viscoelastic response function of the material.
Elastic response of DMPC bilayers
The authors first explored the viscoelasticity of 1,2-dimyrisoyl-sn-glycero-3-phophocholine (DMPC) bilayers. They showed that the membrane elastic modulus increases with frequency and becomes larger than the viscous modulus at some cross-over frequency ω (Figure 2) at all temperatures. The data is well fit by the Maxwell model of viscoelasticity (solid lines Fig. 2).
Crossover frequency as a function of temperature
The crossover frequency was determined at various temperatures above and below the phase transition temperature for a collection of lipids: DMPC (Fig. 3A); DMPC with 10% 1,2-dioleoyl-sn-glycero-3- phosphocholine (DOPC), a phospholipid with the same phospha- tidylcholine headgroup but with unsaturated acyl chains (Fig. 3B); 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS), a phos- pholipid with a different, anionic, headgroup than zwitterionic DMPC but the same 14-carbon acyl chains, with 7.5% DOPC. The form of viscoelasticity for all of these compositions was found to be the same even though they differed in melting temperature.
Possible shortcomings of the optical microrheological method
The authors discuss many potential issues with their methodology. First, they state that the tracers used must be small enough that their behavior is not influenced by the water surrounding the membrane. They show by a simple calculation that their 100 nm particles meet this criterion. Next they state that the number of linkages between the particles and the membrane may differ for each particle. They say that this might influence the actual magnitudes of the moduli determined but will not change the fact that the membrane shows viscoelastic behavior.
The frame rate of data collection limits the frequencies that can be measured with this method. This may lead to artifacts near the extremes of the frequencies used. They explored this problem by measuring the viscoelastic properties of glycerol (purely viscous) and gellan gum (viscoelastic), both of which have well known behavior. They found that artifacts were found above 100 Hz and established a cutoff frequency for their experiment of 33 Hz to safely avoid such artifacts.
The probe itself may alter the local environment of the membrane enough to change its behavior. They showed that the results of their method is insensitive to the size of particle that is used, effectively disproving the presence of this potential problem. They also showed that they could use two particle measurements to calculate the moduli. This situation would lead to aberrant results if one particle were modifying the local environment of another. They found however that their method was not influenced by using a two particle method (Figure 4).
This work shows that at sufficiently short timescales, membrane behavior is viscoelastic. This may explain many unusual results previously obtained in protein biophysics when the assumption that the membrane is purely viscous was used. This discovery is thus crucial to the understanding of the interaction between transmembrane proteins such as ion channels and pumps with the membrane. These interactions may hold fundamental knowledge about these proteins which will be important to understanding their function and in the design of strategies to overcome diseases in which their function fails. In addition, the viscoelastic properties of the membrane may be vital in understanding mechanical signal transduction and intermolecular communication at the cell membrane.