Three-point contact model

An Early Three-dimensional Approach the Three-point Contact Model... 

The low energy sweetening properties of aspartame have been discussed on the basis of structural relationships [1, 83] within the context of the three point contact model of the sweet taste receptor. This model involves a hydrogen bond donor, a hydrogen bond acceptor, and a hydrophobic region with specific geometric relationships. The model accounts for the fact that only one of the four diastereomers of aspartylphenylalanyl methyl ester is sweet. 

The three-point contact model Diastereoisomers Stereoselectivity ratios Pfeiffer s rule... 

A new approach to stereoselective transfer hydrogenation of imines was the application of chiral phosphoric acid esters as organocatalysts [50-52]. The mechanism is based on the assumption that the imine is protonated by a chiral Bronsted acid, which acts as the catalyst. The resulting diastereomeric iminium ion pairs, which may be stabilized by hydrogen bonding, react with the Hantzsch dihydropyridine at different rates to give an enantiomerically enriched amine and a pyridine derivative [50-52]. The exact mechanism is still under discussion however computational density functional theory (DFT) studies ]53, 54] suggest a three-point contact model. ... 

It is interesting to note that the interaction of a prochiral reactant with a chiral step-kink site and with an adsorbed chiral modifier each fiilfills the three-point contact model required for chiral recognition [102-104]. At a step-kink site, the reactant adsorbed in the pro-(l ) geometry contacts the surface plane, the step, and the kink in a way that is not equivalent to the pro-(S) adsorbate. Similarly, in the modifier-reactant interaction between methyiacetoacetate and pyroglutamate, it is proposed that the reactant has three key points of contact comprising the metal-molecule interaction and two intermolecular H-bonds with the modifier [101]. 

When an asymmetric center is present in a compound, it is thought that the substituents on the chiral carbon atom make a three-point contact with the receptor. Such a fit insures a very specific molecular orientation which can only be obtained for one of the two isomers (Fig. 1.3). A three-point fit of this type was first suggested by Easson and Stedman [23], and the corresponding model proposed by Beckett [24] in the case of (R)-( )-adrenaline [= (R)-( )-epinephrine]. The more active natural (R)-( )-adrenaline establishes contacts with its receptor through the three interactions shown in Fig. 1.3. 

In discussing the appearence of diastereoselectivity in natural systems, the rule of three-point contact is frequently used11). This rule implies that a prochiral substrate becomes a chiral one, when it is fixed on three nonidentical points on the enzyme system. On the other hand, the well-known lock-key model is mainly based on a specific spacial arrangement and does not take into account specific types of bonding to the matrix12). For a discussion of diastereoselectivity in reactions with metal complexes, it seems more appropriate to use such a lock-key model. 

Steroidal podands similar to 3 are, in fact, quite well-adapted for enantioselective recognition. The three functionalised sites on the steroid nucleus may be modified to give asymmetrical derivatives of general form 10, capable in principle of the three-point contact required for the classical model of enantioselectivity. Whether or not this idealised situation is ever achieved in practice, the chiral, functionalised steroidal a-face appears to provide an excellent environment for enantioselection. 

Felix, Q. A., Ehret, R, and Summers, J. L., On Three- [66] Dimensional Flat-Top Defects Passing Through an EFIL Point Contact A Comparison of Modeling with Experiments, ... 

There are numerous chiral stationary phases available commercially, which is a reflection of how difficult chiral separations can be and there is no universal phase which will separate all types of enantiomeric pair. Perhaps the most versatile phases are the Pirkle phases, which are based on an amino acid linked to aminopropyl silica gel via its carboxyl group and via its amino group to (a-naphthyl)ethylamine in the process of the condensation a substituted urea is generated. There is a range of these type of phases. As can be seen in Figure 12.23, the interactions with phase are complex but are essentially related to the three points of contact model. Figure 12.24 shows the separation of the two pairs of enantiomers (RR, SS, and RS, S,R) present in labetalol (see Ch. 2 p. 36) on Chirex 3020. 

In order to elucidate the mechanism of moisture and water absorption by phenolic foams, Lowe et al." calculated the wall thickness of cells using three simplified models of cell packing 1) spheres with point contacts 2) den packing of cubes without distortion of faces 3) fused cubes with distorted faces. 

In order to avoid geometrical difficulties an ideal model of the packed bed will be employed to evaluate the heat transfer through the particle. The methods by which heat can enter a particle from its inner side are radiation, convection from the gas stream, and conduction through point contacts and stagnant fillets, as indicated in Fig. 13-8. Heat is transferred Through the particle and leaves the other side by the same three mechanisms. The three processes are in series, and the whole will be designated as the series mechanism. Hence... 

One important finding of this study is that, for the case of full-sphere geometry (Figure 6.11b), the planar diffusion model is not an appropriate approximation. Rather, the microparticle sphere exhibits a region, where the reverse scan current decreases with scan rate, as the geometry favors a convergent diffusion of charge -that is, towards the equatorial point of the three-phase contact. This causes a dilution of the product species near this point at scan rates above the thin layer... 

Fig. 17.1 Interaction capacities of the natural R +)-epinephrine and its S(-)-antipode. In simply assuming that the natural R(+)-epinephrine establishes a three point interaction with its receptor (A) the combination of the donor-acceptor interaction, the hydrogen bond and the ionic interaction will be able to generate energies in the order 12 to 17 kcal mole that corresponds to binding constants of 10 to 10 The less active isomer, S(+)-epinephrine, may establish only a two point contact (B). The loss of the hydrogen bond interaction equals to approximately 3 kcal mole this isomer should therefore possess an approximately 100-fold lesser affinity. The experience confirms this estimate. If we consider less abstract models it becomes apparent that the less potent enantiomer is also able to develop three intermolecular bonds to the receptor, provided that it approaches the receptor in a different manner. However, the probability of this alternative binding mode to trigger the same biological response is close to null.

In order to validate the point-process model introduced in Section 24.3.2, we consider three different characteristics of stationary point processes the distribution function of (spherical) contact distances (0, oo) (0,1], the nearest-neighbor distance distribution function D [0, OO) (0,1], and the pair-correlation function g [0, oo) [0, oo), which can be found, for example, in Illian et al. (26). 

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