62nd Annual Report on Research 2017

The objectives of the project are to explore energy dissipation and transport of highly hydrated macromolecular vectors, and explicitly those that are expose highly-hydrated phosphocholine groups, as are relevant in phospholipid headgroups (the most common in the body, so called phosphatidylcholine or PC lipids). Several sub-projects comprise the overall aims, including the preparation of the different phosphocholinated vectors and their examination using a number of techniques. These include standard characterization methods such as zeta potential and dynamic light scattering methods, but especially the surface-force balance which can measure frictional energy dissipation with unique sensitivity and resolution.

Some of our achievements towards these sub-aims were described in last year’s narrative report and in the 6 published papers resulting from it as appear in the online report. The final period of the project continued to address its objectives and was also prolific, with 4 additional papers acknowledging PRF support as are listed below for reference purposes and in the online report. Here we summarize some of the main outcomes. These derive from our studies on several aspects of energy dissipation when phosphocholine-exposing vectors (primarily phosphphocholinated nanobrushes and phophocholine-exposing PC lipid vesicles) undergo dynamic perturbation. In addition related studies were also carried out as these shed further light on these basic phenomena.

The underlying dynamics and energy dissipation of hydrated nanovectors arise from the shear and dissipation within the hydration layers themselves, which are a sensitive function of the shear environment and local electric fields arising from the charges enclosed by the hydration layers. Thus we extended our earlier study (see last year’s narrative report) to examine how modifying surface potentials, and thus surface fields, affect the energy dissipation in aqueous thin films (Tivony et al. 2017, below). These revealed that surface interactions across such (nm) thin layers could be strongly affected by the fields, whose values could reach magnitudes (ca. 10⁸ V/m) comparable to those in the hydration shells surrounding charges in water.

It is of special interest to examine also energy dissipation in analogous hydrated groups, and this was comprehensively done by measuring the effect of trimeric surfactants forming surface assemblies (monolayers, bilayers and surface micelles) on mica substrates (Kampf et al. 2016, below). The frictional dissipation was affected both by the nature of the groups exposed and by the structure of the surface complex. Thus the exposed hydrated N⁺ groups at the head of such trimeric surfactants provided efficient boundary lubrication through friction reduction via shear of their fluid hydrations shells, while exposing their hydrophobic tails resulted in behavior characteristic of hydrophobic interactions between the surface-attached trimeric-complex vectors. Due to the nature of the surface structure – including bilayers and hemimicellar rod-like structure – the overall friction, which is a sum of that arising from different energy dissipation pathways as the vectors slide past each other – could vary with the conditions. This was revealed in the higher friction measured when the surfaces were rougher due to structuring of the surface attached surfactant complexes.

A more direct investigation of the dissipative interactions between phosphocholine-exposing nanoparticles (NPs) composed of PC lipid vesicles and macromolecular surface phases was carried out (Zhu et al. 2017). This was done by forming surface layers of hyaluronic acid (HA) molecules on a solid substrate and then interacting them with the PC vesicles. The HA molecules is negatively charged and so interacts with the exposed phosphocholine groups of the PC vesicles via an attractive electrostatic dipole-charge force, forming HA-lipid complexes at the surface. This investigation revealed clear differences in the way the vectors interacted with the surface-attachedd macromolecules, depending cruicially on their tail structure. Thus unsaturated tails (of the lipid palmitoyloleoylphosphatidylcholine, POPC) or shorter tails (of the lipid dimyristoylphosphatidylcholine, DMPC), both of which are associated with a rather low gel-to-liquid phase transition temperature T_M, were far less robust and so provided rather pooor dissipation-reduction properties at high pressures than the hydrogenated soy phosphatidylcholine (HSPC) lipid vescicles complexed with the same HA on the surfaces. Hoever, at low pressures the higher hydration levels of the POPC and DMPC vesicle surfaces resulted in efficient reduction in the sliding friction.

An important ingredient within biological systems that affects friction reduction by PC vesicles is the lipid cholesterol (chol), which incorporates itself within the hydrophobic tail region of the bilayer membranes constituting the liposomes. We examined directly, using the surface force balance (SFB), how incorporation of chol within different PC vesicles would influence their lubricating ability (Sorkin et al., 2017). Several levels of chol incorporation in the PC vesicle walls were used, as well as a systematic variation of the lipid types (saturated (C₁₄)₂, (C₁₆)₂ and (C₁₈)₂ lipid tails on the phosphocholine headgroups) and these showed clearly the effect of the added chol, as well as the difference between the different-tailed PC lipids. Broadly, the results revealed that chol strongly reduces the robustness against compression and shear, but that – prior to vesicle disruption - the frictional dissipation was only slightly affected, likely because the incorporation of chol maintained a high hydration level.

Overall therefore we have achieved most of of the objectives of this project.

260. Kampf, N., Wu, C., Wang, Y., Klein, J., ‘A Trimeric Surfactant: Surface Micelles, Hydration lubrication, and Formation of a Stable, Charged Hydrophobic Monolayer’, Langmuir, 32 (45), pp 11754–11762 (2016) DOI: 10.1021/acs.langmuir.6b02657

262. Tivony, R. and Klein J., ‘Modifying surface forces through control of surface potentials’, Faraday Discussions, 2017, 199, 261 – 277 (Chemical Physics of Electroactive Materials), DOI: 10.1039/c6fd00255b

263. Zhu, L., Seror, J., Day, A.J., Kampf, N. and Klein J., ‘Ultra-low friction between boundary layers of hyaluronan-phosphatidylcholine complexes’, Acta Biomaterialia, 59, 283-292 (2017) doi: 10.1016/j.actbio.2017.06.043

265. Sorkin, R., Kampf N. and Klein, J., ‘Effect of cholesterol on the stability and lubrication efficiency of Phosphatidylcholine surface layers’, Langmuir, 33(30), 7459-7467 (2017). doi: 10.1021/acs.langmuir.7b01521