Noncovalent bonding approach

Molecularly imprinted polymers (MIPs) can be prepared according to a number of approaches that are different in the way the template is linked to the functional monomer and subsequently to the polymeric binding sites (Fig. 6-1). Thus, the template can be linked and subsequently recognized by virtually any combination of cleavable covalent bonds, metal ion co-ordination or noncovalent bonds. The first example of molecular imprinting of organic network polymers introduced by Wulff was based on a covalent attachment strategy i.e. covalent monomer-template, covalent polymer-template [12]. 

At pH 12, the disulfide and noncovalent bonds are both broken, and the monomer with a sedimentation constant of 1.45 Svedberg units is released. From frictional ratios, the monomer appears to exist as a coil with a diameter of 16 A and a length of 150 A. Analysis of the primary structure of K-casein (Loucheux-Lefebvre et al. 1978) suggests considerable secondary structure in the monomer. 23% a-helix, 31% /3-sheets, and 24% 0-turns. In contrast, other investigators, using several different approaches, obtained a-helix contents ranging from 0 to 20.8% (Bloomfield and Mead 1975). Circular dichroism spectra on the monomer indicated 14 and 31% for a-helix and / -sheet, respectively (Loucheux-Lefebvre et al 1978). An earlier study of the optical rotatory dispersion of the K-casein monomer yielded values for the a-helix content ranging from 2 to 16% (Herskovits 1966). 

Figure 10.64 The auxiliary linkage approach to the synthesis of [2] and [3] catenanes. The auxiliary linkage may be a covalent, coordinate or noncovalent bond.

Compound mixtures of structures, which include salts, may be encoded using SMILES. A period between two SMILES means that the compound SMILES represents two or more noncovalently bonded structures associated with each other, such as in a salt. For example, sodium benzoate can be represented as clccccclC(=0)0.[Na], or possibly clccccclC(=0)[0-]. [Na+]. It may be necessary to define a set of rules about whether to represent salts using charged atoms or neutral atoms. Even with such a rule in place, one component of this mixture may be considered the important compound and the other component the counter-ion or secondary component. In some cases, the counter-ion is obviously the smaller of the two components. This is not always true. Another approach is to define a set of typical counter-ions. This set may include large groups, such as acetate or even bigger ions. Creating a table of typical counter-ions can help identify the primary and secondary components in mixtures. 

The idea of benchmarking quantum chemical methods by introducing databases covering a wide variety of different properties, for example, atomization energies, spectroscopic properties, barrier heights and reaction energies of diverse reactions, proton affinities, interaction energies of noncovalent bond systems, transition metal systems, and catalytic processes, was extended by Truhlar and coworkers [51]. They were the first to carry out overall statistical analyses of combinations of different test sets to obtain an overall mean absolute deviation (MAD) number for each tested quantum chemical method, which made a comparison with other approaches more feasible. 

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