Adsorbate conformation

These calculated intracrystalline diffusion coefficients are particularly appropriate for comparison with those determined from pulsed field gradient (PFG) NMR experiments. Time-independent equilibrium properties such as adsorbate conformations are also readily accessible. The classical nature of the simulations allows a particle s trajectory to be followed, and from this it is possible to determine all kinds of information, such as how often a particle diffuses through a certain region. 

Adsorbate conformations Adsorption energy/eV Charge of Hl Charge of H2b... 

Plasma proteins organize on polymer substrates in different ways. Adsorbates are influenced by substrate physicochemical properties and by environmental factors, especially fluid shear and bulk protein distribution. Different types of binding interactions and more than one conformation for adsorbed protein are observed. In the case of albumin, the irreversibly adsorbed conformation, as measured by pulse intrinsic fluorescence, appears to be substantially altered from that of bulk albumin. Microaggregated albumin and undenatured forms are seen at the polymer interface, which are readily desorbed by viscous drag. 

Figure 8 Simulated adsorption by molecular dynamics of (A) poly (p-phenylene) oxide (PRO), (B) poly(methyl-l,4-phenylene) (PMP) and (C) poly(iso-propyl-l,4-phenylene) (PIPP) on an alumina surface showing substantial structural changes during the adsorption process (upper figure initial conformation lower figure = adsorbed conformation) (From Ref. 71.) Note the formation or absence of adsorption loops in the polymer chains for the different cases.

Fig. 15 (a) Molecular stmcture of PNIPAM-seg-PS. (b) Schematic of the adsorbed conformation of PNIPAM-seg-PS on PS substrate, (c) Typical force curves obtained in DI water. Figure reproduced with permission from [102]... 

A major experimental complication for adsorption at a soUd/liquid interface is that such interfaces can be very variable due to differences in cleaning procedures. Additionally, because solid surfaces are rigid, the attainment of an equilibrium adsorbed conformation may be difficult. Furthermore, adsorption may be located in regions of high specificity and thus the experimental observation may correspond to a thick layer (i.e. the hydrodynamic radius) whereas the adsorbed amount suggests a much flatter conformation. 

The adsorption process is affected by the presence of any background electrolyte in solution. An increase in ionic strength will shield both surface and polyelectrolyte charges with the consequence that the surface-polymer interaction will decrease. However, and more importantly, there will also be a decrease in the lateral interactions between segments in the adsorbed layer. This allows the adoption of a more compact adsorbed conformation with the result that the adsorbed amount will increase. In general, the adsorbed amount almost always increases with increasing ionic strength. 

Tribological Properties of Poly(L-lysine)-5roft-poly(ethylene glycol) Films Influence of Polymer Architecture and Adsorbed Conformation... 

Tribological Properties of Poly(L-lysine)-g-Poly(Ethylene Glycol) films Influence of Polymer Architecture and Adsorbed Conformation Scott S. Perry, X. Yan, E. T. Limpoco, Markus Muller, Seunghwan Lee, Nicholas D. Spencer... 

The transition temperature for adsorbed (presumably via multipoint attachment) poly(NIPAAM) molecules is lower than that in bulk solution, and the properties of the layer of collapsed macromolecules formed above the transition temperature depend strongly on the speed by which the temperature increases. At a low speed of temperature increase, the liquid-like polymer layer is formed, whereas at high speeds, the polymer layer has more solid-like properties (55). When cooling, the collapsed polymer molecules return to the initial loopy adsorbed conformation via transitional extended conformation. The relaxation process for the extended-to-loopy adsorbed conformational transition occurs slowly and depends on the temperature obeying an Arrhenius law. Kinetic constraints, it is proposed, play an important role in this transition (56). 

Understanding the general features of what happens when the surface undergoes reconstruction after adsorption or the adsorbate conformation is modified by the adsorption process is the goal of this work. In particular, section 3 will consider what happens when (HS) does not hold true and must be replaced by... 

The analysis of Avnir, Pfeifer and Farin was based on the implicit assumption that the adsorbate conformation remains imchanged after adsorption. The molecular shape, however, is expected to be modified by the intense fields in the vicinity of a surface. 

The experimental test of the theoretical models describing polymer adsorption has been slow in developing. While the situation is far from satisfactory, a number of techniques, including ellipso-metry, infrared spectroscopy, viscosity, attenuated total reflection, and radiotracer rate studies have provided information on the adsorbed conformation and conformational changes. We will summarize the information provided by these techniques for polystyrene. 

Competitive rates of adsorption provide information not only on the kinetics of the adsorption process, but also on the adsorbed polymer conformation. The overall rate of adsorption of a pol3rmer molecule is probably comprised of two separate rates the rate of initial attachment and the rate of reorientation of an attached molecule until it achieves its equilibrium conformation. The rates of desorption and exchange can also yield information on the adsorbed conformation since removal of a molecule from the surface is a function of the number of attached segments. 

In order to get rid of undesired lattice effects like the almost cuboid form of the most compact adsorbed conformations in the subphases of AC2 in the phase diagram of a polymer on a simple-cubic lattice (see Fig, 13.2), we now investigate the stmcture of conformational phases of a semiflexible off-lattice polymer near an attractive substrate [304,307,308]. 

Decreasing the surface attraction at low temperatures, layer after layer is added until the number of layers is the same as in the most compact conformation. A lattice polymer has no other choice than forming layers in this regime. The layering transition from ACl to AC2 is very sharp in both models. Also the shape of the transition region from topologically two-dimensional adsorbed to three-dimensional adsorbed conformations looks very similar. 

The adsorption of the peptides at the semiconductor surface is a conformational pseudophase transition and accompanied by structural changes of the peptides during the adsorption process. The energetic response of the peptides upon binding can be obtained from Fig. 14.9, where the specific heat curves are plotted for each of the peptides. The peaks for SI and S3 and the increase toward lower temperatures for S3 and SI indicate energetic activity that signals the onset of a crossover between random-coil structures in solvent and adsorbed conformations at the substrate. 

Considering the previous argument about structural properties of these peptides in solvent only, we also expect that the adsorbed conformations are not supposed to exhibit clear symmetries. However, good binding properties are only possible if the peptide forms comparatively flat structures, enabling it to maximize the number of substrate contacts. Thus, at room temperature, surface-attached peptides are expected to be compact without noticeable internal structure. The deformation of the peptides due to adsorption is apparent if we analyze the gyration tensor components perpendicular and parallel to the substrate separately. Let the gyration radius of the heavy atoms be... 

As expected, adsorbed peptides do not exhibit clear structures at room temperature. In Fig. 14,11, the respective a-helix and /3-stand contents of the adsorbed conformations, ria)b and ( )b, respectively, are shown. Although noticeable differences for the mentioned peptide groups are found, there is no significant population of a-helical or /3-sheet structures, at least at room temperature. Nonetheless, there is a tendency that residues of SI and S3 are rather in a and residues of S3 and SI in /3 state. The small secondary-structure contents are quite similar to what was observed for the peptides in solution (without substrate see Fig. 14,5), It is obvious that the presence of the Si(lOO) substrate does not lead to a stabilization of secondary stmctures here. Such stabilization can occur, however, and has been reported, for example, for a synthetic peptide binding at silica nanoparticles [358]. 

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