Supramolecular interaction

Supramolecular polymer networks combine the characteristics of chemical and physical networks and can be tailored to specific needs through the use of macro-molecular building blocks. Although they form strong materials tmder favorable 

Because of their transient and reversible cross-linking, supramolecular polymer networks are responsive [4] to external stimuli such as variation in temperature [31], pH [32], polarity of the solvent [33], redox reactions [34], and competitive ligation [35]. This tunability makes them useful for a plethora of applications. They can be used as drug delivery systems [36] and as matrixes in tissue engineering [37]. Drugs and cells can be encapsulated and protected within these materials and 

As an alternative, side-chain functionalized polymer chains can associate by (c) low molecular weight crosslinkers or (d) mutual heterocomplementary polymer-polymer binding 

The following sections describe the preparation and characterization of supramolecular polymer networks, particularly emphasizing their physical-chemical features with regard to the type and strength of physical chain cross-linking and the resulting macroscopic material properties. Furthermore, recent work on the formation and characterization of supramolecular hydrogels based on synthetic and natural precursors is summarized with a focus on their application and potential in biomedicine. 

Non-covalent interactions represent the energies that hold supramolecular species together. Non-covalent interactions are considerably weaker than covalent interactions, which can range between ca. 150 kj mol to 450 kj mol for single bonds. Non-covalent bonds range from 2 kJ mol for dispersion interactions to 300 kJ moP for ion-ion interactions. However, when these interactions are used in a co-operative manner a stable supramolecular complex can exist. The term non-covalent includes a wide range of attractions and repulsions which are summarised in Table 1.1 and will be described in more detail in the following sub-sections. 

Hydrophobic Related to solvent-solvent interaction energy Cyclodextrin inclusion compounds 

Anslyn, E. V. and Dougherty, D. A., Modern Physical Organic Chemistry, 

Electrostatic interactions play an important role in understanding the factors that influence high binding affinities, particularly in biological systems in which there 

Jeffery, G. A., An Introduction to Hydrogen Bonding, Oxford University 

Inspired by the remarkable efficiency of many enzymes, chemists have tried to prepare artificial systems that operate in form and function like enzymes do. Cyclodextrins are the most extensively used platforms for these efforts. The dominance of cyclodextrins stems from the pioneering observations of Breslow and Tabushi, who showed that simple organic compounds can display many of the hallmarks of enzymatic catalysis, such as binding, rate accelerations, and turnover. However, most artificial systems do not give the large rate enhancements that their natural counterparts impart. One example that does produce a large rate enhancement is based on a cyclodextrin dimer. 

The dimer of p-cyclodexfrin shown below (refer back to Section 4.2.4 to see the structure of p-cyclodextrin) presents a rigid binding cleft possessing a metal as an electrophilic catalyst. The spacing between the two cyclodextrins leads to binding of the substrate in a manner that places 

A Vmax value of 2.24 X 10 - M s and a Km value of 4.69 X lO M were determined. This example is one of the very best artificial enzymes yet reported, and bodes well for continued efforts in this field. 

Breslow, R., and Dong, S. D. Biomimetic Reactions Catalyzed by Cyclodextrins and Their Derivatives. Chem. Ren, 98,1997-2011 (1998). Zhang, B., and Breslow, R. Ester Hydrolysis by a Catalytic Cyclodextrin Dimer Enzyme Mimic with a Metallobipyridyl Linking Group. /. Am. Chem. Soc., 119,1676-1681 (1997). 

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Fig. 1. Schematic representation of a receptor—substrate (host—guest) complex involving cavity inclusion of the substrate and the formation of different types of weak supramolecular interactions between receptor (hatched) and substrate (dotted).

Bipyridines were efficiently used in supramolecular chemistry [104], Since the molecule is symmetric no directed coupling procedure is possible. In addition, 2,2 6/,2//-terpyridine ligands can lead to several metal complexes, usually bis-complexes having octahedral coordination geometries [105,106], Lifetimes of the metal-polymeric ligand depend to a great extent on the metal ion used. Highly labile complexes as well as inert metal complexes have been reported. The latter case is very important since the complexes can be treated as conventional polymers, while the supramolecular interaction remains present as a dormant switch. 

The weak supramolecular interactions (H-bonds, coordination or van der Waals interactions, etc.) positioning the molecular components to give the supramolecular architectures are typically several orders of magnitude less robust than the cross-linked covalent bonds formed in a specific polymerization process. Accordingly, the sole solution to overcome these difficulties is to improve the binding (association) efficiency of the molecular components generating supramolecular assemblies. At least in theory, an increased number of interaction moieties and the selection of the... 

Besides the preparation of glyconanotube conjugates by supramolecular interactions, another synthetic pathway commonly used involved covalent attachment of the saccharidic units on the CNT scaffolds. The progress recently recorded concerning the derivatization of unfunctionalized CNTs, allowing introduction of suitable anchoring functions at their surface, are the basis of this synthetic alternative.255... 

The examples cited in this chapter are but a rather small and arbitrary selection from the richly varied possibilities for supramolecular bonding. Recognition of the intrinsic chemical (partially covalent, exchange-type) character of supramolecular interactions leads inevitably to an extended definition of chemistry that includes many aspects of nanoscale aggregation, structure, and function in the biophysical and material-science domains. From this viewpoint, the molecule is seen to be... 

But, we expect that the majority of readers will be those with only a rudimentary command of quantum chemistry and chemical bonding theory (e.g., at the level of junior-year physical chemistry course) who wish to learn more about the emerging ab initio and density-functional view of molecular and supramolecular interactions. While this is not a textbook in quantum chemistry per se, we believe that the book can serve as a supplement both in upper-level undergraduate courses and in graduate courses on modern computational chemistry and bonding theory. 

Balazs, G. Breunig, H. J. Supramolecular Interactions in Structures of Organic Antimony and Bismuth Compounds. In Unusual Stuctures and Physical Properties in Organometallic Chemistry Gielen, M., Willem, R., Wrackmeyer, B., Eds. Wiley Chichester, 2002 pp 387-111. 

The rhombic (4,4) networks differ in the supramolecular interactions linking the cations and anions. For eleven (Table 2), alternating cations and anions are linked directly through hydrogen-bonded interactions. For two (Table 2) the supramolecular hydrogen-bonding assemblies incorporate protic solvents and for a single example (Table 2) there are no molecular interactions. 

Furthermore, C-H - - O hydrogen bonds are formed between some of the polyether oxygen atoms and the a-bipyridinium protons. A second type of hydrogen bonding interaction, C-H- -77, is observed between the 1,4-dioxybenzene protons and thep-phenylene spacers. In most systems it is very difficult to measure their individual strength and importance in the assembly process. However, it is usually assumed that rr-stacking between complementary aromatic species is the main supramolecular interaction in these systems. 

Stoddart has employed the anion-directed [3]pseudorotaxane assemblies described above to control the outcome of solid-state photodimerisation reactions of olefins [116]. A combination of supramolecular interactions (one of them being hydrogen-bonding to PF,) has been employed to pre-organize bis(dialky-lammonium) salts containing fraws-stilbenoid units (89) into the [3]pseudoro-taxane assembly 90 shown in Scheme 45. 

This indicates that fixing of the torsion about the phenylacetylene bond leads to a AAG(CH3CN) of 1.2 kcal mol very close to the expected stabilization for AGnuc upon fixing of one torsion. This demonstrates that stabilization of the folding nucleus through supramolecular interactions leads to the stabilization of the folded-state structure. 

In this chapter, we present a brief overview of supramolecular polymers and polymerization and supramolecular interactions of polymer side chains. We provide examples of the control over the solution state polymer strucmre that can be achieved at the molecular level and then extended to micro- and macroscale assemblies. 

Dramatic examples have also been reported for main-chain supramolecular polymers (SPs Lehn 1993 Ciferri 2005 Fig. 3.1), in which specific and directional molecular recognition events between end groups define the main chain of a linear polymeric assembly. Although main-chain SPs had been created and characterized previously (Broze et al. 1983 Fouquey et al. 1990 Alexander et al. 1993 Bladon and Griffin 1993 St Pourcain and Griffin 1995), it was a groundbreaking paper in 1997 that demonstrated the mechanical potential of supramolecular interactions and catalyzed much of the current interest in the field (Sijbesma et al. 1997). 

The fields of polymer physics and the physical organic chemistry of supramolecular interactions therefore become intrinsically linked. To the extent that chemists are able to relate behavior at the level of pol5mer physics to characteristics of the isolated small molecular constituents, therefore, the control of SP properties becomes a rational and potentially very precise science. This theme permeates the field, and this... 

The overlaps between SPs in semidilute concentrations can be thought of in very similar terms to the entanglements defined above. Supramolecular interactions create large stmctures that physically interact to determine the mechanical response (in this case, viscous flow). The primary relaxation is the diffusion of an SP that is effectively intact on the timescale of the diffusion process. Thus, at a fixed concentration, the SP properties in dilute solution are therefore quite similar to those of covalent polymers of the same molecular weight and molecular weight distribution. 

The mechanical properties of SPs described in Sections 3.2-3.4 are, in general, suc-cessfiiUy interpreted, often quantitatively, in terms of thermal rate and equilibrium constants, but it is reasonable to expect that the underlying molecular behavior should be perturbed by the application of a mechanical stress. On the whole, the mechanical properties of supramolecular interactions are not well known, and their study constitutes a relatively new but burgeoning research area related to the field of SPs. 

Such effects are likely to be important. The use of SP interactions to create bioinspired material properties (e.g., see Chap. 9) implies that the ultimate yield behavior of SP materials could depend on the mechanical response of supramolecular interactions. Paulusse and Sijbesma (2004) have also shown that ultrasound-generated shear stresses can mechanically tear apart coordination SPs, damage that is subsequently repaired during dynamic equilibration once the shear stresses are removed. The mechanical response of supramolecular interactions within materials has potentially important consequences in the context of self-repairing materials, where the mpture of sacrificial supramolecular interactions protects a permanent, underlying materials architecture. The dynamic repair of the SP component in... 

TABLE 3.1 Selected Examples of Supramolecular Interactions, Relevant to Supramolecular Polymers, Stndied by Eorce Spectroscopy... 

Crespo-Biel O, Dordi B, Maury P, Peter M, Reinhoudt DN, Huskens J. Patterned, hybrid, multilayer nanostructmes based on multivalent supramolecular interactions. Chem Mater 2006 18 2545-2551. 

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