Steric forces between polymer brushes and atomic force microscope tips were investigated. We studied two systems:  polystyrene (PS) grafted to silicon in cyclohexane and poly(ethylene oxide)/poly(methacrylic acid) (PEO/PMAA) diblock copolymer adsorbed with the PMAA block to aluminum oxide in aqueous medium. On approach exponentially decaying repulsive forces were observed in both systems. With a homemade heat stage we could adjust the temperature. Increasing the temperature between 19 and 53 °C led to a linear increase of the decay length for PS in cyclohexane. Also the work required to bring the tip to a certain distance increased roughly linearly with temperature. This supports the view that the repulsion is of entropic origin. At the same time this demonstrates that the temperature dependence of surface forces could be routinely measured. For PEO in water the repulsive force was not significantly affected by a change in temperature. Approaching and retracting parts of force curves measured with PS in cyclohexane were in most cases indistinguishable. In contrast, for PEO in water a significant hysteresis was observed. This might be caused by an escape of polymers underneath the tip of the atomic force microscope. When retracting the tip in some cases the stretching of individual polymers was observed in both systems. Stretching force vs distance curves could be described by a wormlike chain model with typical persistence lengths of 1 nm.

Atomic force microscopy was used to study the structure of two membrane-associated proteins adsorbed to various supported phospholipid bilayers in physiological buffer. The aim was (a) to develop a preparation for the investigation of membrane-associated proteins at high resolution under native conditions and (b) to obtain information about the factors that determine the adsorption process and the structure of adsorbed proteins. Therefore, solid-supported membranes were formed on mica by spontaneous vesicle adsorption and spreading. Once a homogeneous, pinhole-free bilayer was formed, solutions containing the proteins at appropriate concentrations were applied. The two positively charged proteins chosen were myelin basic protein (MBP), which plays an essential role in the formation of functional myelin, and cytochrome c. On charged bilayers, MBP applied at concentrations of 0.5-50 µg/mL formed aggregates of defined height (1.9 ( 0.2 nm on negatively and 2.7 ( 0.2 nm on positively charged lipids), which at high concentration covered the entire bilayer. These aggregates are probably monomolecular layers of MBP. On neutral lipid adsorbed MBP formed irregular aggregates. Cytochrome c showed a different adsorption: On negatively charged lipid it formed aggregates of defined, monomolecular height (3.3 ( 0.2 nm). On neutral bilayers small aggregates were observed. On positively charged lipid no adsorption was observed at all. These results indicate that (a) the adsorption of cytomchrome c can be interpreted in terms of a dominating electrostatic interaction; (b) MBP adsorption to lipid bilayers is not exclusively electrostatically driven and depends on the specific lipid bilayer composition; (c) the structure of adsorbed aggregates indicates a strong protein-protein interaction.

The mechanical and adhesion properties of myelin basic protein (MBP) are important for its function, namely the compaction of the myelin sheath. To get more information about these properties we used atomic force microscopy to study tip-sample interaction of mica and mixed dioleoylphosphatidylserine (DOPS) (20%)/egg phosphatidylcholine (EPC) (80%) lipid bilayer surfaces in the absence and presence of bovine MBP. On mica or DOPS/EPC bilayers a short-range repulsive force (decay length 1.0-1.3 nm) was observed during the approach. The presence of MBP always led to an attractive force between tip and sample. When retracting the tip again, force curves on mica and on lipid layers were different. While attached to the mica surface, the MBP molecules exhibited elastic stretching behavior that agreed with the worm-like chain model, yielding a persistence length of 0.5 +/- 0.25 nm and an average contour length of 53 +/- 19 nm. MBP attached to a lipid bilayer did not show elastic stretching behavior. This shows that the protein adopts a different conformation when in contact with lipids. The lipid bilayer is strongly modified by MBP attachment, indicating formation of MBP-lipid complexes and possibly disruption of the original bilayer structure.

Creating patterns of microstructured arrays of functional material is of interest in many areas of current surface science and microsensing techniques. We have used the atomic force microscope to create micrometersized structures of poly-L-lysine on lipid bilayers. The entire process was performed in aqueous solution. The created structures were 0.8 ( 0.1 nm high and as small as 100 nm in diameter. Our results suggest that at least part of the poly-L-lysine molecules in these structures are anchored to the mica surface below the lipid bilayer. Thus, the presented method might also be used to design structures of poly-L-lysine and other biomolecules on mica.