Surface modification, anchoring molecules

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

A qualitatively new approach to the surface pretreatment of solid electrodes is their chemical modification, which means a controlled attachment of suitable redox-active molecules to the electrode surface. The anchored surface molecules act as charge mediators between the elctrode and a substance in the electrolyte. A great effort in this respect was triggered in 1975 when Miller et al. attached the optically active methylester of phenylalanine by covalent bonding to a carbon electrode via the surface oxygen functionalities (cf. Fig. 5.27). Thus prepared, so-called chiral electrode showed stereospecific reduction of 4-acetylpyridine and ethylph-enylglyoxylate (but the product actually contained only a slight excess of one enantiomer). 

All three surface modifications induced homeotropic anchoring of the liquid crystal molecules. Even in the isotropic phase, the surface-induced alignment can cause layering of the molecules close to the modified surfaces. The amplitude and range of this surface-induced alignment not only depends on temperature, but also... 

In the later part of the article, the effect of cell surface modification is considered. Namely, polyethylene glycol molecules are anchored to the membrane surface to provide a steric barrier to fusion. The purpose is to provide a means to reduce unwanted fusion (i.e. stealthiness). Giant liposomes provide an excellent model with which to test this theory, because their external, internal and membrane conductivities and dielectric constants can be controlled precisely, and their surfaces can be easily modified. 

Thus, lipoproteins could be injected over the surface of a lipid covered SPR sensor in a detergent free buffer solution and showed spontaneous insertion into the artificial membrane.171 Again two hydro-phobic modifications are necessary for stable insertion into the lipid layer, whereas lipoproteins with a farnesyl group only dissociate significantly faster out of the membrane. Therefore the isoprenylation of a protein is sufficient to allow interaction with membraneous structures, while trapping of the molecule at a particular location requires a second hydrophobic anchor. Interaction between the Ras protein and its effector Raf-kinase depends on complex formation of Ras with GTP (instead of the Ras GDP complex, present in the resting cell). If a synthetically modified Ras protein with a palmi-... 

This investigation aims for the first time at the stabilization of photoisomers of photochromic dye molecules in molecular sieves by modification of the MCM-41 surface and covalent bonding of dye molecules. Diphenyldichlorosilane is utilized for pre-silylation to warrant an anchoring of the dyes exclusively inside the MCM-41 channel structure. 

Another approach, artificial hydrophobization of polypeptides with a small number of fatty acid residues (e.g. stearate or palmitate) has been showm to enhance cellular uptake (Kabanov et al., 1989a). Specifically, this technique involves point modification of lysine or N-terminal amino groups with one or two fatty acid residues per protein molecule. As a result of such modification, the protein molecule remains water-soluble but also acquires hydrophobic anchors that can target even very hydrophilic proteins to cell surfaces (Slepnev et al., 1995). To obtain low and controlled degrees of modification, a system of reverse micelles of a surfactant, sodium bis-(2-ethyIhexyl)sulfosucciate (Aerosol OT) in octane was used as a reaction medium (Kabanov et al., 1987) (Figure 47.4). Over a... 

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