Noncovalent cross-linking

Noncovalent Cross-Linking of Casein by Epigallocatechin Collate Characterized by Single Molecule Force Microscopy (from Jobstl et al., 2006)... 

Jobstl, E. Howse, J. R. Fairclough, J. P. A. Williamson, M. P. Noncovalent Cross-Linking of Casein by Epigallocatechin Gallate Characterized by Single Molecule Force Microscopy. J. Agric. Food Chem. 2006, 34, 4077-4081. 

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. 

Figure 5.17 Noncovalent cross-linking strategies of cyanuric acid functionalized poly-CA to form (a) poly-CA-triazine by using a diaminotriazine cross-linking agent or (b) poly-CA-wedge by using an isophthalamide cross-linking agent.

Figure 5.21 Orthogonal noncovalent cross-linking as well as functionalization strategy of terpolymer using hydrogen bonding and metal coordination interactions.

Oakenful, D. 1984. A method for using measurements of shear modulus to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. J. Food Sci. 49 1103-1104,1110. 

A variety of rheological tests can be used to evaluate the nature and properties of different network structures in foods. The strength of bonds in a fat crystal network can be evaluated by stress relaxation and by the decrease in elastic recovery in creep tests as a function of loading time (deMan et al. 1985). Van Kleef et al. (1978) have reported on the determination of the number of crosslinks in a protein gel from its mechanical and swelling properties. Oakenfull (1984) used shear modulus measurements to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. 

The tenet of classical rubber theory has been that the chains are in random networks and the networks comprise a Gaussian distribution of end-to-end chain lengths. However, the mechanisms and molecular bases for the elasticity of proteins are more complex than that of natural rubber. In biological systems elastomeric proteins consist of domains with blocks of repeated sequences that imply the formation of regular stmctures and domains where covalent or noncovalent cross-linking occurs. Although characterised elastomeric proteins differ considerably in their precise amino acid sequences they all contain elastomeric domains comprised of repeated sequences. It has also been suggested that several of these proteins contain p-tums as a structural motif (Tatham and Shewry 2000). 

Oakenftill (1984) developed an extension of Equation 6.1 for estimating the size of junction zones in noncovalently cross-linked gels subject to the assumptions (Oakenfull, 1987) (1) The shear modulus can be obtained for very weak gels whose polymer concentration is very low and close to the gel threshold, that is, the polymer chains are at or near to maximum Gaussian behavior. (2) The formation of junction zones is an equilibrium process that is subject to the law of mass action. Oakenfull s expression for the modulus is (Oakenfull, 1984) ... 

El-ghayoury A, Hofmeier H, de Ruiter B, Schubert US (2003) Combining covalent and noncovalent cross-linking a novel terpolymer for two-step curing applications. Macromolecules 36(11) 3955—3959... 

To explain the behavior of these rubbery solutions, it is assumed that the molecules are linked by noncovalent cross-links to a network which extends throughout the solution. Such a solution has the properties of a... 

S.5.2.2 Noncovalent Cross-Linking by DNA Hybridization Nagahara and Matsuda [97] used the second approach to form a polyacryl-amide/DNA hybrid hydrogel. They grafted complementary ssDNA onto polyacrylamide. Upon mixing, cross-linking based on duplex formation due to DNA hybridization occurred. 

This article provides an overview of the recent progress in fundamental studies, preparation, characterization, and application of nanofibrillar hydrogels formed by physical (noncovalent) cross-linking of one-dimensional (ID) supramolecular building blocks of synthetic and biological polymers. High-aspect-ratio nanofibrils, as focused on in the present review, are formed by autonomous association (self-assembly) of individual molecules and not by their forced fabrication (directed... 

Extension of this self-assembly approach to supramolecular engineering has led to an alternate motif for noncovalent cross-linking, a series of bisthymines that are complementary to the diamidopyridine side chains of polymer 6 (33). Upon combination in non-polar media thermally reversible, micrometer scale spherical aggregates were formed (Figs. 13b and 13c). 

The commercial importance of cross-linked polymers has already been stressed. Noncovalent cross-links introduce new properties, such as reversibility and enhanced control over the network architecture, aud lead to tunable and responsive material properties, for example, based on the thermal sensitivity of hydrogen-bonding motifs or based on redox reactions for metal-ligand coordination complexes. 

Figure 10 Schematic representation of the lateral interactions that lead to noncovalent cross-links in supramolecular materials. Strong tendency for nanofiber formation in the case of U-UPy was visualized by tapping-mode AFM images. (Reproduced from Ref. 38. American Chemical Society, 2006.)...

Figure 13 Schematic representation of networks with covalent and noncovalent cross-links (left), based on the star polyester and polyol depicted on the right. (Reproduced from Ref. 43. American Chanical Society, 2009.)...

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