This chapter concentrates on the characterization of Langmuir—Blodgett (LB) monolayer and multilayer films on solid surfaces. Experimental techniques to study the properties of monolayers floating on the surface of a subphase have been discussed by Gaines(1) and are, to some extent, covered in the earlier chapters of this book. A review of the properties of LB films could be arranged in a number of ways: subdivisions according to the type of material or to the nature of the experimental technique are probably the most straightforward approaches. In this chapter the latter plan is adopted. It is hoped that this will not only aid the researcher wishing to investigate monolayer and multilayer systems, but also keep the work relevant as more novel LB materials are synthesized. Examples used have generally been chosen from published data on the simpler types of LB assemblies (e.g., longchain fatty acids).

Characterization and Properties

The surface compartment model: a theory of ion transport focused on ionic processes in the electrical double layers at membrane protein surfaces.

In biological systems, many structures are too small to be considered bulk systems, but too thick to be called surface films. An important example is the natural membrane, an approximately 80-A-thick structure that contains several layers composed of lipids and proteins. To help elucidate the properties of membranes we have studied the thickness dependence of surface and bulk properties of films of the red cell membrane endofacial proteins, spectrin and actin (S+A). Five surface properties (yield, shear resistance, potential, viscosity, and elastic modulus) show extrema or saturation effects at 50- to 80-A thickness. Two bulk properties (viscosity and elastic modulus) become independent of thickness after about 80 A (four monolayers), while the bulk residual yield decreases with thickness after one monolayer. Above one monolayer, the diffusion coefficients also increase with the thickness of the S+A film, which along with the decreasing yield indicate that succeeding layers do not pack as well as the initial layer. Many of the properties studied show maxima, minima, or the development of thickness invariance in the range between the in vivo thickness of S+A layers and the total membrane thickness. The energetics of processes involving proteins appear to be determined by the surface layer only, as judged by calculations of the equilibrium constants of hemoglobin reactions and of membrane-ligand reactions.

The thickness dependence of properties of membrane protein multilayers

Preparations of intrinsic membrane proteins (IMP) from the red cell were characterized as primarily mixtures of the SDS-polyacrylamide gel bands 3, 4.1, 4.2, 6, and 7. The compression isotherms of spread films of different preparations showed variations in the slopes and extrapolated areas that correlated with the average molecular weights ( MW ) of the preparations. Binary mixtures of cytochrome c and bovine serum albumin (BSA) showed qualitatively similar variations with changes in the ratio of these two proteins, suggesting that the MW is a reasonable normalizing parameter that should be useful for characterizing protein mixtures. The slopes of the compression isotherms and the extrapolated areas of both the IMP monolayers and the binary mixtures were correlated with each other as well as with MW . The elasticities of all monolayers at π = 5 dyn/cm showed about the same values at MW > 40,000, and the surface potentials of all red cell protein films (IMP and earlier studies of extrinsic proteins) showed comparable values.

Surface isotherms of intrinsic red cell membrane proteins

Surface properties differ significantly from bulk properties. At charged membrane (or channel) surfaces the surface concentrations and surface potentials of ions differ from the bulk values, but the combined electrochemical potentials are the same. Any increase in surface concentration is exactly balanced by the decrease in electrical potential, so ions at the surface are in equilibrium with those in the bulk. Since ion transport is driven by electrochemical potentials, it is clear that the driving forces for the ions are the same at the surface as in the bulk solution.

R. E. Abbott

Papers

The Ionic Permeability of Adsorbed Membrane Protein Monolayers