Convex Substrates

By my remarks I wanted to emphasize the importance of concavity of enzymes as a feature that determines the specificity and stereospecificity. I didn t want to say that it is a feature that is necessary for all enzymes. However, starting with a globular convex substrate, a concave active site allows a strategic distribution of a number of small and large interactions that make the enzyme specific and stereospecific. 

Surface curvature plays an important role in determining initiation efficiency. The initiation efficiency of SI-ATRP from a flat substrate has been estimated to be around 10% [83-85]. On the other hand, the initiation efficiency that could be obtained from a convex substrate is close to, or even more than, 30% [73-76, 79, 86]. Even higher initiation efficiency values of approximately 80% for particles have also been reported in several studies [77, 78, 87]. In particle systems, some studies have reported a constant increase in initiation efficiency with time [73,75], whereas others reported it to increase as polymerization progresses to higher conversion [77, 78, 86]. This again shows the uncertainty in predicting initiation efficiency. 

Termination is unavoidable in ATRP systems because of the very nature of radicals. Termination in SI-ATRP is highly dependent on the geometry of the substrates. For example, the confined environment of concave substrate leads to closer proximity of polymer chains, which could lead to higher possibility of termination. On flat or convex substrates, the termination could occur via multiple modes. The modes of termination and experimental data supporting the role of termination in kinetics of SI-ATRP are discussed in the following sections. 

Figure 10.53 shows the measured surface shape of the warped cell. The cathode side is a convex shape and the top of the cell is warped by more than 2 mm from the comer position in a perpendicular direction to the plane for the 10 mm x 10 mm sample. From the simulation, it is found that the magnitude of the warp depends on the relative thickness between the anode substrate and electrolyte, as shown in Figure 10.54. The warp increases with the increase in the ratio of the anode thickness to electrolyte thickness. The calculated value of the warp is very close to the measured one indicating that this simulation is reliable in predicting the warp with various combinations of the thickness between the anode and electrolyte.

Substrate molecules with convex / docking sites complementary to receptors... 

In many cyclic or bicyclic molecules a stereo structure is present in which one can identify a convex and a concave side. Because reactions usually take place in such a way that the reacting reagent is exposed to the least possible steric hindrance, convex/concave substrates are generally react on their convex side. 

Other cyclic or bicyclic ketones do not have a convex side but only a less concave and a more concave side. Thus, a hydride donor can add to such a carbonyl group only from a concave side. Because of the steric hindrance, this normally results in a decrease in the reactivity. However, the addition of this hydride donor is still less disfavored when it takes place from the less concave (i.e., the less hindered) side. As shown in Figure 10.10 (top) by means of the comparison of two reductions of norbomanone, this effect is more noticeable for a bulky hydride donor such as L-Selectride than for a small hydride donor such as NaBH4. As can be seen from Figure 10.10 (bottom), the additions of all hydride donors to the norbomanone derivative B (camphor) take place with the opposite diastereoselectivity. As indicated for each substrate, the common selectivity-determining factor remains the principle that the reaction with hydride takes place preferentially from the less hindered side of the molecule. 

Figure 8.4 shows the influence of e on the x (r) shape. For fixed (k, A), we simulated the time courses for e = 0.5, 1, 2, 5. It is noted that the shape of the substrate profiles varies remarkably with the values of e thus profiles of biphasic, power-law, and nonlinear type are observed. So, the sensitivity of the kinetic profile regarding the available substrate and enzyme amounts is studied by using several e values for low substrate or high enzyme amounts the process behaves according to two decaying convex phases, in the reverse situation the kinetic profile is concave, revealing nonlinear behavior. 

As observable from the SFM topography measurements, the surface of the epoxy inside the holes of the copper film exhibits a slight convex curvature, probably due to the surface tension of the epoxy formulation. Hence, the epoxy surface nearby the Cu/epoxy interface was not in contact with the mica substrate when curing the mixture of epoxy resin and curing agent, and any interference on the curing reaction from the presence of the mica can be ruled out. On the other hand, the local slope of the curved epoxy surface is... 

At mechanical equilibrium, the film/ substrate system is therefore curved. In the example shown in Figure 1, the curve is concave and corresponds to a residual tensile stress in the film. A stress gradient is formed in the substrate, with compression occurring at the interface. Conversely, the system would be convex if the film were subjected to a residual compressive stress. 

The expansive internal stress in a plasma polymer is a characteristic property that should be considered in general plasma polymers and is not found in most conventional polymers. It is important to recognize that the internal stress in a plasma polymer layer exists in as-deposited plasma polymer layer, i.e., the internal stress does not develop when the coated film is exposed to ambient conditions. Because of the vast differences in many characteristics (e.g., modulus and thermal expansion coefficient of two layers of materials), the coated composite materials behave like a bimetal. Of course, the extent of this behavior is largely dependent on the nature of the substrate, particularly its thickness and shape, and also on the thickness of the plasma polymer layer. This aspect may be a crucial factor in some applications of plasma polymers. It is anticipated that the same plasma coating applied on the concave surface has the lower threshold thickness than that applied on a convex surface, and its extent depends on the radius of curvature. 

Figure 1.40. The Philips photopolymerization process for replicating video disks, (a) The liquid layer L is spread over the mold Mo by deforming the substrate S to make it slightly convex, (b) Exposure to UV light polymerizes the liquid coating, (c) The substrate unth coating is separated from the mold, (d) The information layer is coated with a mirror M and protective layer P. Reproduced with permission from reference 55. Copyright 1982 Philips Re-search Laboratories.)...

This is a LiAlH4 reduction of the a,(3-unsaturated ketone of the seven-membered ring. The low temperature and the use of only 0.25 eq. of LiAlH4 ensures that only the fastest reaction takes place and no reduction of the ketone in the five-membered ring or of the double bonds is observed. This reduction proceeds with substrate control of the diastereoselectivity, because the hydride attacks the molecule mainly from its convex and not from its concave face. This becomes clear when looking at 44 which is a three-dimensional representation of 31. Whether the diastereomeric ratio of 10 1 is important, will become clear in the further synthesis. 

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