In this paper, we study the polymer conformation of chemically
end-grafted polymer chains
in bad-solvent conditions using a scanning force microscope. The
polymer layers were prepared by exposing
a gold substrate in a dilute toluene solution of thiol-terminated
polystyrene (PS-SH). Although the
adsorption process took place in good-solvent conditions, the imaging
was performed in water (bad solvent
for polystyrene chains). For short incubation times and low
concentrations, we identified individual
polymer chains. For longer incubation times (higher surface
coverage), we observed microphase separation
of the polymer monolayer into globular clusters. Using different
molecular weights of PS-SH, we showed
that the sizes of these clusters satisfy the scaling laws that were
predicted for pinned micelles.
The main function of the transmembrane light-harvesting complexes in photosynthetic organisms is the absorption of a light quantum and its subsequent rapid transfer to a reaction center where a charge separation occurs. A combination of freeze-thaw and dialysis methods were used to reconstitute the detergent-solubilized Light Harvesting 2 complex (LH2) of the purple bacterium Rhodopseudomonas acidophila strain 10050 into preformed egg phosphatidylcholine liposomes, without the need for extra chemical agents. The LH2-containing liposomes opened up to a flat bilayer, which were imaged with tapping and contact mode atomic force microscopy under ambient and physiological conditions, respectively. The LH2 complexes were packed in quasicrystalline domains. The endoplasmic and periplasmic sides of the LH2 complexes could be distinguished by the difference in height of the protrusions from the lipid bilayer. The results indicate that the complexes entered in intact liposomes. In addition, it was observed that the most hydrophilic side, the periplasmic, enters first in the membrane. In contact mode the molecular structure of the periplasmic side of the transmembrane pigment-protein complex was observed. Using Föster's theory for describing the distance dependent energy transfer, we estimate the dipole strength for energy transfer between two neighboring LH2s, based on the architecture of the imaged unit cell.
The conductivity of two photosynthetic protein^pig-ment complexes, a light harvesting 2 complex and a reaction center, was measured with an atomic force microscope capable of performing electrical measurements. Current^voltage measurements were performed on complexes embedded in their natural environment. Embedding the complexes in lipid bilayers made it possible to discuss the di¡erent conduction behaviors of the two complexes in light of their atomic structure.
Atomic force microscopy (AFM) is used to study the behavior of a diblock copolymer onto a solid surface while the solvent quality is changed. In a first step, the copolymer poly(2-vinylpyridine)/polystyrene (P2VP/PS) is adsorbed onto mica from a selective solvent (the PS block is well solvated and the P2VP is not solvated). In such a case, the P2VP block adsorbs preferentially on the substrate and anchors the PS block to the surface. In a second step, the solvent quality is reduced to such an extent that the PS block is no longer solvated. This procedure allows the formation of octopus surface "micelles". The parameters characterizing this regime are measured using AFM. In addition, a direct determination of the adsorbance is performed. It reveals that the PS blocks when immersed in good solvent do not overlap. A mushroom surface regime instead of a brush one is thus created at the surface.
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