Bilayer , force measurement

In practice, van der Waals forces appear within a mix of forces. Measured between bilayers in free suspension, they are mixed with lamellar motions as well as with repulsive hydration forces. 

For measurements between crossed mica cylinders coated with phospholipid bilayers in water, see J. Marra andj. Israelachvili, "Direct measurements of forces between phosphatidylcholine and phosphatidylethanolamine bilayers in aqueous electrolyte solutions," Biochemistry, 24, 4608-18 (1985). Interpretation in terms of expressions for layered structures and the connection to direct measurements between bilayers in water is given in V. A. Parsegian, "Reconciliation of van der Waals force measurements between phosphatidylcholine bilayers in water and between bilayer-coated mica surfaces," Langmuir, 9, 3625-8 (1993). The bilayer-bilayer interactions are reported in E. A. Evans and M. Metcalfe, "Free energy potential for aggregation of giant, neutral lipid bilayer vesicles by van der Waals attraction," Biophys. J., 46, 423-6 (1984). 

V. A. Parsegian, "Reconciliation of van der Waals force measurements between phosphatidylcholine bilayer in water and between bilayer-coated mica surfaces," Langmuir, 9, 3625-8 (1993). 

The largest possible value of A, in Eq. (14) is obtained when s" = 1 for all the dipoles within the cavity (no screening), which provides A, =14.9 A [30], and Eq. (19) leads to Xh 1-67 A. This value is comparable with the decay length of the hydration force measured in neutral lipid bilayers [10]. 

Other techniques that can be used to measure distribution coefficients of a guest molecule between the bulk aqueous phase and a micellar or vesicular aggregate include gel filtration, electromotive force measurements, solnbility and vapor pressure measurements, muon spin rotation experiments and headspace gas chromatography. Theoretical models are, of course, also helpful, as, for example, in stndies of the enveloping of charged proteins by lipid bilayers. 

Figure 1.9 Short-range forces measured between two DPPC bilayers on crossed mica cylinders immersed in concentrated aqueous solutions [reproduced with permission from Ref. 26, copyright 1985, American Chemical Society], Abbreviation DPPC, dipalmitoylphosphatidylcholine.

Thus, it was questioned if there were any intrinsic hydration layers on a lipid membrane. The oscillatory profile observed in the A/ versus distance curve obtained by FM-AFM revealed the existence of the hydration layer on a DPPC bilayer (Fig. 18.8a). In FM-AFM, the force is measured with a tip having a nanometer-scale cross section, while the force measured by SFA and the osmotic pressure method is averaged over a micrometer-scale area. Such global averaging may smear out the local distance dependence, showing the oscillatory profile. The result clearly showed the importance of having local spatial resolution in the investigations on interfacial phenomena. 

In an extensive SFA study of protein receptor-ligand interactions, Leckband and co-workers [114] showed the importance of electrostatic, dispersion, steric, and hydrophobic forces in mediating the strong streptavidin-biotin interaction. Israelachvili and co-workers [66, 115] have measured the Hamaker constant for the dispersion interaction between phospholipid bilayers and find A = 7.5 1.5 X 10 erg in water. 

Pashley R M, McGuiggan P M, Ninham B W, Brady J and Evans D F 1986 Direct measurements of surface forces between bilayers of double-chained quaternary ammonium acetate and bromide surfactants J. Phys. Chem. 90 1637-42... 

Whereas the main challenge for the first bilayer simulations has been to obtain stable bilayers with properties (e.g., densities) which compare well with experiments, more and more complex problems can be tackled nowadays. For example, lipid bilayers were set up and compared in different phases (the fluid, the gel, the ripple phase) [67,68,76,81]. The formation of large pores and the structure of water in these water channels have been studied [80,81], and the forces acting on lipids which are pulled out of a membrane have been measured [82]. The bilayer systems themselves are also becoming more complex. Bilayers made of complicated amphiphiles such as unsaturated lipids have been considered [83,84]. The effect of adding cholesterol has been investigated [85,86]. An increasing number of studies are concerned with the important complex of hpid/protein interactions [87-89] and, in particular, with the structure of ion channels [90-92]. 

Rand, R. P. (1981). Interacting phospholipid bilayers Measured forces and induced structural changes, Ann. Rev. Biophys. Bioeng.. 10. 277-314. 

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