Selective molecular differentiation

Lavigne, J. J., Anslyn, E. V., Sensing a paradigm shift in the field of molecular recognition From selective to differential receptors. Angew. Chem., Int. Ed. 2001, 40, 3119-3130. 

Fig. 11 Cellulose (beech sulfite pulp) dissolved in NMMO (Lyocell dope). Left DSco and differential MWD of the starting pulp, molecular weights of DP = 50 and DP = 200 are indicated by vertical dashed lines. Right Time course of the overall carbonyl content for three selected molecular weight ranges. Reprinted with permission from Biomacromolecules (2002) 4 743. Copyright (2002) American Chemical Society...

C. De Duve (1987). Selection of differential molecular survival A possible mechanism of early chemical evolution. Proc. Natl. Acad. Sci. USA, 84, 8253-8256. 

J.J. Lavigne, E.V. Anslyn, Sensing a Paradigm Shift in the Field of Molecular Recognition From Selective to Differential Receptors , Angew. Chem. Int. Ed., 40, 3119 (2001)... 

Shape selective catalysis differentiates between reactants, products, or reaction intermediates according to their shape and size. If almost all catalytic sites are confined within the pore structure of a zeolite and if the pores are small, the fate of reactant molecules and the probability of forming product molecules are determined by molecular dimensions and configurations as well as by the types of catalytically active sites present. Only molecules whose dimensions are less than a critical size can enter the pores, have access to internal catalytic sites, and react there. Furthermore, only molecules that can leave the pores, appear in the final product. 

Most real samples that are analysed are, unless they have been deliberately purified (and even then they may still be), actually made up from a number of different chemicals this is certainly trae for most colorants. As has already been discussed when considering molecular spectroscopy techniques, analysis of mixtures can lead to complex, incomplete or even unrcsolvable data. The answer/solution to problems of this type normally involves separation science, where there is selective or differential interaction/behaviour of the different components in the separation system. The principle separation sciences are chromatography and electrophoresis. 

This is a drug-like score derived by a substructural analysis approach used to calculate biological activity profiles, which contain weights that describe the differential occurrences of selected molecular properties in active and inactive molecules [Gillet, Willett et al, 1998]. 

The theoretical methods reviewed in the next chapters produce all of these quantities, ranging from state and energy selected differential cross sections to temperature dependent rate coefficients. Thus the theory of chemical reaction dynamics provides a crucial link between the results obtained in detailed state selective molecular beam experiments and thermally averaged bulb measurements. 

The basic principles are described in many textbooks [24, 26]. They are thus only sketchily presented here. In a conventional classical molecular dynamics calculation, a system of particles is placed within a cell of fixed volume, most frequently cubic in size. A set of velocities is also assigned, usually drawn from a Maxwell-Boltzmann distribution appropriate to the temperature of interest and selected in a way so as to make the net linear momentum zero. The subsequent trajectories of the particles are then calculated using the Newton equations of motion. Employing the finite difference method, this set of differential equations is transformed into a set of algebraic equations, which are solved by computer. The particles are assumed to interact through some prescribed force law. The dispersion, dipole-dipole, and polarization forces are typically included whenever possible, they are taken from the literature. 

The critical sizes of the reactant molecules were estimated and are shown in Figure 5, where the figures for 2-hexanol, isopropylacetate, sec-butylacetate and cyclohexylacetate are estimated by MM2 from Pauling s atomic radius and molecular model [18]. Therefore, the unique catalysis of Cs2.2 is understood if one assumes that it is active only for small molecules. In other words, this catalyst exhibits "reactant shape selectivity", where the catalyst differentiates the reactants according to their size. 

At a more molecular level, the influences of the composition of the membrane domains, which are characteristic of a polarized cell, on diffusion are not specifically defined. These compositional effects include the differential distribution of molecular charges in the membrane domains and between the leaflets of the membrane lipid bilayer (Fig. 3). The membrane domains often have physical differences in surface area, especially in the surface area that is accessible for participation in transport. For example, the surface area in some cells is increased by the presence of membrane folds such as microvilli (see Figs. 2 and 6). The membrane domains also have differences in metabolic selectivity and capacity as well as in active transport due to the asymmetrical distribution of receptors and transporters. 

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