NONCOVALENT SIDE CHAIN MODIFICATION

In this chapter we will focus on side chain functionalized supramolecular polymers as well as main chain noncovalent functionalized polymers, which are the two main areas of supramolecular polymers. We will initially discuss the design principles and methodology of side chain functionalization, in particular, multifunctionalization. In the later part of the chapter, we will discuss in detail two important applications of side chain functionalized supramolecular polymers. The first application involves the use of noncovalent interactions to yield highly functionalized materials, whereas the second application involves the reversible noncovalent cross-linking of polymers to yield responsive materials. 

Molecular Recognition and Polymers Control of Polymer Structure and Self-Assembly. Edited by V. Rotello and S. Thayumanavan Copyright 2008 John Wiley Sons, Inc. 

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Ilhan F, Gray M, Rotello VM. Reversible side chain modification through noncovalent interactions. Plug and play polymers. Macromolecules 2001 34 2597-2601. 

Besides these modifications, certain proteins can acquire prosthetic groups (e.g., hemes, flavins, iron-sulfur centers, and others). The prosthetic groups may be attached covalently (usually to amino acid side chains) in some cases, noncovalently... 

Biotin s interaction with the proteins avidin and streptavidin is among the strongest noncovalent affinities known (K l = 1015 M l). The binding occurs between the bicyclic ring of biotin and a pocket within each of the four subunits of the proteins. The valeric acid portion is not involved or required for the interaction (Green, 1975 Wilchek and Bayer, 1988). This characteristic allows modification of the valeric acid side chain without affecting the binding potential toward avidin or streptavidin. 

Two mechanisms that are commonly employed in altering enzyme activity are covalent modification and allosteric regulation. Covalent modification is an enzymatically catalyzed reaction that involves the reversible formation of a covalent bond between a small molecule and a specific amino acid side chain(s) on an enzyme that affects its activity. Allosteric regulation of an enzyme s activity involves noncovalent binding of a small molecule at a site other than the active site that alters the enzyme s activity. Unlike the limited examples of covalent modification that have been discovered (see Table 15-1), a wide variety of small molecules have been found to regulate the activity of particular enzymes allosterically. 

The formation of peptide bonds between amino acid residues, which is directed by mRNA and catalyzed by ribosomes, is at the heart of protein biosynthesis. However, a wide variety of other processes are also necessary for proteins to achieve their biological function(7, ). All polypeptides must undergo noncovalent changes, such as folding of the polypeptide chain, association with other subunits, and translocation across membranes. Many also undergo covalent modifications of both the peptide backbone and amino acid side chains. These covalent modifications can drastically affect both protein structure and function. 

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