H. Morgan

Models for interpreting surface potential measurements and their application to phospholipid monolayers

Three models for estimating group dipole moments from surface potential measurements on monolayers at the air/water interface are critically reviewed. The Helmholtz model favored by most workers seriously underestimates the group moments since no account is taken of the polarizability of the monolayer. The partial dipole compensation technique recently introduced by Vogel and Mobius, while in principle very sound, is shown to be flawed. A basic assumption of their model, that the conformation of the two carboxyl groups in octadecylmalonic acid is identical to that in stearic acid, is unlikely to be correct. Consequently, their estimation of the contribution of the distal methyl group to the surface potential of long chain alkanoic compounds is overestimated. On the other hand, despite its limitations, the Demchak and Fort model provides good agreement between the group dipole moments of several aliphatic compounds estimated from monolayer surface potential and the values determined for the same groups from measurements on bulk material. In view of this success, the Demchak and Fort model is applied to the surface potential obtained from four phospholipids, phosphatidylcholine, phosphatidylethanolamine, phosphatidic acid, and phosphatidylserine, and their headgroup dipole moments are estimated. It is concluded that the hydrophilic headgroups contribute significantly to the surface potential, contrary to the findings of Vogel and Mobius who suggested that the terminal CH3 groups of the lipids were mainly responsible for the observed surface potential.

Modelling the surface potential-area dependence of a stearic acid monolayer

Abstract Based on a 3-layer capacitor approach used previously to describe the surface potential of condensed phase monolayers we have developed a model which appears to explain the main features of the surface potential-area (ΔVA) isotherm of stearic acid. The model suggests that a large reduction in the local permittivity in the neighbourhood of the headgroup dipoles from 78.4 to 6.5 is the cause of the rapid increase in ΔV when the monolayer is compressed below a critical area per molecule ( A c ≌ 30 A 2 molecule ). We associate this with the growth of domains in the monolayer.

Surface plasmon resonance studies of chemisorbed biotin-streptavidin multilayers

Abstract Surface plasmon resonance spectroscopy has been used to confirm that a monolayer of biotin may be immobilized by chemisorption onto evaporated gold films and subsequently used to bind a monolayer of streptavidin. As expected, the strong affinity of the protein for the biotin ligand ensures that the binding of the protein layer is highly specific. No non-specific binding occurs to the biotinylated gold surface; the protein layer is resistant to washing in 1 M NaCl. It is suggested that multilayer structures may be assembled by chemisorption of successive protein monolayers using a bifunctional bisbiotin ligand for connecting individual layers.

Two-dimensional proton conduction at a membrane surface: Influence of molecular packing and hydrogen bonding

Abstract Evidence for two-dimensional protonic charge transport along a monolayer/water interface is presented, supporting the concept of localised charge transport along membrane surface. Charge transport proceeds only at and above a critical molecular packing density, and probably occurs via a network of hydrogen-bonded water molecules and lipid headgroups.

The surface potential of monolayers formed on weak acidic electrolytes: Implications for lateral conduction

Abstract Surface potential measurements on stearic acid monolayers formed on dilute NaCl solutions and ultrapure water show that the double-layer potential, ψ 0 , obeys the simple Gouy-Chapman theory only if account is taken of the effect of subphase ion concentration on monolayer dissociation. It is concluded that the enhanced lateral conductance observed during compression of stearic acid monolayers cannot arise from the higher proton concentration in the diffuse double-layer but rather from a H-bonded network at the monolayer/water interface as described previously for phospholipid monolayers.

Molecular-scale neural nets: an approach to the self-assembly of molecular networks

Abstract It is suggested that molecular self-assembly techniques may be utilised in molecular scale electronic systems. Taking a neural network as an example, it is shown how the affinity of proteins for their complementary ligand could be used to form the molecular scale network essential to such a system. The approach is based on the polymerisation of the tetrameric proteins avidin and streptavidin using biotinylated ligands. By synthesising a range of aromatic bisbiotin ligands, some simple ground rules are established for ensuring complexation of protein and ligand. Based on these findings, a tetrabiotinylated tetrapyridylporphyrin salt (TBPP) was synthesised with a view to forming a two-dimensional protein polymer. Gel chromatography experiments and a UV-visible investigation of solutions containing the protein/TBPP mixture provide strong evidence for complex formation. The specific immobilisation of streptavidin to a monolayer of TBPP formed at the air-water interface is also demonstrated using fluorescence microscopy. Based on the promising results of these initial studies, a number of suggestions are made for the further exploitation of the techniques described herein.

Self-assembly of avidin and streptavidin with multifunctional biotin molecules

Abstract We report the synthesis of a water-soluble tetrafunctional biotin ligand based on the porphyrin moiety and its behaviour at the air-water interface. The addition of streptavidin or avidin to the subphase is shown to cause a significant expansion of the isotherm of the tetrabiotinylated ligand, indicating a strong interaction between the protein and the ligand. The addition of inactive protein to the subphase caused no such change from which it is deduced that non-specific interactions could not have been responsible for the effects observed with the active proteins. Supplementary experiments using column chromatography provide evidence for the formation of a high molecular weight polymer when the tetrabiotinylated ligand and active protein are mixed.

Self-assembly of streptavidin/bisbiotin monolayers and multilayers

A range of different bifunctional bisbiotin ligands have been synthesised and their ability to form polymers with the proteins avidin and streptavidin investigated. Polymers were characterised by gel chromatography and transmission electron microscopy