Association constants, monomer-template

Also of considerable importance is the mechanism of site formation. At what stage in the polymerisation are the high energy sites formed and stabilised Does the solution structure of the monomer-template assemblies reflect the disposition of functional groups at the binding sites [24] (See Chapter 5 for a further discussion.) Attempts to correlate the association constants determined for the monomer-template interactions in homogeneous solution with the rebinding association... 

Knowledge of the association constants between the template and the monomer in solution may thus be used to predict suitable starting concentrations of monomer and template, leading to a larger fraction of high affinity sites. The latter parameter is of particular importance when the template is available only in small amounts, when it is poorly soluble in the common diluents or for analytical applications, when only a limited saturation capacity is required. In such cases it has proven possible to reduce the template coneentration by more than a factor of 10 without significant loss of the recognition properties of the material [106]. 

However, most of these difficulties should not arise in the case of template-monomer complexes formed with very high association constants. Consequently a number of laboratories, including our own, are currently involved in the design of new polymerisable monomers which can form strong complexes with some of the common structural motifs possessed by many template molecules [5-8], (See also Chapter 4 for further examples.) The use of transition metal co-ordination complexes as templates in imprinting is another particularly promising approach (see Chapter 6 for more detail). 

It is, therefore, not necessary to use any excess of binding site monomers in order to saturate the template nearly completely. In order to reach in equimolar concentrations (0.1 molar) during imprinting 90% or better 95% degree of association, association constants of 900 (or better 3800) are necessary. Usual noncovalent interactions show much lower values. Due to this stoichiometric interaction, we have called this type stoichiometric noncovalent interaction [18,26,27]. 

A primary indication on how well the monomers have been chosen is to simply see whether they are capable of assisting solubilization of the template in the prepolymerization mixture. A small-scale solubility test may thus be a good way to initially screen the monomers for strong monomer-template interactions. Weak interactions may be revealed by complexation induced spectral changes (in NMR,UVor fluorescence spectra). The complexation induced shifts of the characteristic H-NMR signals of the template upon increasing monomer concentrations are often used to estimate the monomer-template association constants. Prior to this, however, knowledge about the stoichiometry of the monomer-template complexation and the tendency of the monomer and template to self-associate are required.The former can be obtained by means of a so-called Job s plot whereas the latter by a dilution experiment. 

A similar approach has been adopted by Whitcombe et al. [15], where NMR chemical shift studies allowed the calculation of dissociation constants and a potential means for predicting the binding capacities of MIPs. The NMR characterization of functional monomer-template interactions has also been applied to the study of the interaction of 2,6-bis(acrylamido) pyridine and barbiturates [16], and of 2-aminopyridine and methacrylic acid [17]. Recent NMR work in our laboratory [18] has involved the determination of template-monomer interactions for a nicotine-methacrylic acid system. Significantly, it was shown in this study that template self-association complexes are present in the prepolymerization mixture and that the extent template self-association is dependent both upon solvent and the presence of monomer. 

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