Stoichiometric noncovalent interaction

Wulff G, Knorr K. Stoichiometric noncovalent interaction in molecular imprinting. Bioseparation 2001 10 257-276. 

R 635 G. Wulff and K. Knorr, Stoichiometric Noncovalent Interaction in Molecular Imprinting , Bioseparation, 2001,10, 257 R 636 C. Wunderlich and C. Balzer, Quantum Measurements and New Concepts for Experiments with Trapped Ions , Los Alamos National Laboratory, Preprint Archive, Quantum Physics, Avail. URL http //xxx. lanl. gov/pdf/quant-ph/0305129... 

As already outlined in Section IV, stoichiometric noncovalent interactions combine the advantages of covalency and noncovalency without suffering from their disadvantages. Stoichiometric interaction in our sense means, if a 1 1 equimolar mixture of template and binding site monomer is used, more than 90% (or even better 95%) of the template should be bound to the binding site monomer. 

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]. 

Reviews on stoichiometric noncovalent interactions can be found in Refs. 18,28. In Ref 28 also methods for the determination of the association constants are described in some detail. 

The polymer is saturated. This is a favorable example of an imprinted polymer using stoichiometric noncovalent interactions re-uptake is quantitative, but nevertheless no unspecific binding is observed. This stands in contrast to conventional noncovalent imprinted polymers which only show poor re-uptake. 

Though quantitative determinations are lacking for the monomer, from other model substances like 2,6-bis(acetamido)pyridine, association constants with barbiturates like hexobarbital 21 are known [22]. In this case, the association constant is ass = 200 (Table l).This shows that this group represents an intermediate between nonstoichiometric and stoichiometric noncovalent interactions. At 0.1 M concentration for both partners in CHCI3, 80% complex formation is expected to occur. 

In case of estradiol, the association constant with P-cyclodextrin in Me0H/H20 (45 55 v/v) oiK= 4.7x10 has been measured in solution. So, in principle, this is a further possibility of a stoichiometric noncovalent interaction. 

Whitcombe and coworkers [25] reported an interesting example of two independent types of stoichiometric noncovalent interaction for the template ampiciiiin (a penicillin derivative). 

These two new functional monomers demonstrate the high potential of molecular imprinting employing stoichiometric noncovalent interactions. 

Scheme 4 A hybrid method of imprinting by Whitcombe et al. uses stoichiometric amounts of monomer and template during polymerization and then relies on noncovalent interactions during the rebinding phase.

WulflF sees the polydispersity of the imprinted receptor sites, nonspecific binding problems and the poor mass transfer properties of typical imprinted materials as key problems. He has recently published work on imprinting using strong noncovalent interactions to achieve near-stoichiometric association of templates and functional monomers in an attempt to improve the homogeneity of receptor sites and minimize the fraction of functional monomers dispersed non-specifically in the polymer structure. Continuation of this work, along with related work from other groups, will no doubt lead to further advances in this area. 

The first point implies the need for thermodynamically and kinetically stable interactions, whereas readily reversible interactions are demanded to satisfy the other three points. Mostly covalent (a), noncovalent (b), stoichiometric noncovalent (c). 

This conception of a dynamic structure originates from elementary thermodynamic considerations. All components of isolated lipoproteins exist in equilibrium between the lipoprotein phase and the surrounding aqueous phase. Historically, lipoproteins have been viewed as static stoichiometric complexes of lipids and apoproteins, with emphasis on the fixed structural features. The lipoprotein structure has been considered to be the result of noncovalent interactions of oriented apoproteins in lipids with fixed arrangements, stoichiometries and distances in these complexes. This conceptualization of lipoproteins and membrane structure does not, however, recognize the dynamic aspects of lipoprotein and membrane structure. 

In the first part of this chapter we will briefly discuss the nature of noncovalent interactions, followed by an overview of standard methods for probing them (optical and fluorescence spectroscopy, isothermal titration calorimetry/microcalorimetry, differential scanning calorimetry, and surface plasmon resonance), with particular attention to their application to biomolecules. An overview of mass spectrometric methods suitable for detecting noncovalent complexes will then be presented. The advantages of mass spectrometry compared with conventional analytical methods are sensitivity, speed, and the ability to obtain stoichiometric information directly. 

Catalase reacts reversibly with some weak acids forming spectroscopically and magnetically distinct noncovalent derivatives. Of these, catalase-cyanide, -azide, -fluoride, -formate, and -acetate complexes have been extensively studied (37, 135, 136) and reviewed in some detail (16-18). Briefly, there is a consensus that such reactions do not involve heme-heme interaction and, with the possible exception of carboxylate ligands (102), all presumably result in replacement of the proximal aquo ligand at Ls in a stoichiometric reaction shown in Eq. (11) ... 

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