Substrate-surface interactions, steric

One should assume that one dye molecule involving two different acid centers (Fig. 1) can not be neutralized by the same Ca ion that is retained on the substrate surface, for sterical reasons. Indeed, the two negative ends of the molecule are too distant from each other so as to interact with the same Ca ion to form a chelate type complex. In contrast, such a complex is probably formed in the commercial pigment, that is basically the calcium salt of LRB. 

Influence of Substrate Surface Concentration. The influence of the substrate concentration on the kinetics was investigated by several authors (2,4,7). Generally, the rate of enzymic cleavage is greatly reduced at high and low surface concentrations. At high concentration, the diminution probably arises from steric hindrance of the enzyme-substrate interaction (2). 

The simulation results for this model that will be discussed in the following section were obtained by multithreaded, multicanonical, and parallel tempering Monte Carlo simulations imder the assumption that the peptide only interacts with the surface layer (/= 1). A simulation box of dimension [50 A] with periodic boundary conditions parallel to the substrate was used. In perpendicular direction, peptide mobility is restricted by the Si substrate residing by definition atz = 0. The influence of the wall parallel to the substrate is simply steric, i.e., the atoms experience hard-wall repulsion atz = Zmax = 50 A [340]. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

Preferred adsorption of the unsaturated bond of the substrate occurs at that face which presents the least steric interactions between the adsorbed substrate and the surface. Since some amazingly sterically hindered molecules can be hydrogenated, at least some active sites must look like corners or edges or some other protuberances. 

Sachtler [195] proposed a dual-site mechanism in which the hydrogen is dissociated on the Ni surface and then migrates to the substrate that is coordinated to the adsorbed dimeric nickel tartrate species. In their model, adsorption of modifier and reactants takes place on different surface atoms in contrast to Klabunovskii s proposal. Adsorbed modifier and reactant are presumed to interact through hydrogen bonding (Scheme 14.5). The unique orientation of adsorbed modifier molecules leads to a sterically favored adsorbed reactant configuration to achieve this bonding. 

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