Noncovalent assemblies

The pyruvate dehydrogenase complex (PDC) is a noncovalent assembly of three different enzymes operating in concert to catalyze successive steps in the conversion of pyruvate to acetyl-CoA. The active sites of ail three enzymes are not far removed from one another, and the product of the first enzyme is passed directly to the second enzyme and so on, without diffusion of substrates and products through the solution. The overall reaction (see A Deeper Look Reaction Mechanism of the Pyruvate Dehydrogenase Complex ) involves a total of five coenzymes thiamine pyrophosphate, coenzyme A, lipoic acid, NAD+, and FAD. 

In recent years, with increasing recognition of the roles played by specific noncovalent interactions in biological systems and chemical processes, the science of noncovalent assemblies- often called supramolecular science- has aroused considerable interest [76], The remaining part of this article reviews some important studies made on rotaxane and catenane, two classic types of supramolecular structure. 

Catenanes are another example of noncovalent assemblies, consisting of two or more ring molecules interlocked together (see Fig. 21). They offer chemists an equally intriguing object of research. In Sect. 4.2, some recent attempts to synthesize them are reviewed. 

Long-range electron transfer reactions have attracted attention relative to the initial stages of photosynthesis, and special interest has been focused on the design of noncovalently assembled donor-acceptor arrays. Dendrimers with spatially isolated porphyrin cores are potential motifs for investigating such long-range electron transfer reactions [12],... 

Jolliffe, K. A. Timmerman, R Reinhoudt, D. N. Noncovalent assembly of a fifteen-component hydrogen-bonded nanostructure. Angew. Chem. Int. Ed. 

A Cu(II)-induced perturbation of pyrene fluorescence has been utilized to create a sensor for glutamate [388], A 2 2 1 Cu2+ 3-CD pyrene complex is formed by the noncovalent assembly of the constituents the site of Cu(II) binding is unknown. The pyrene emission resulting from complexation of the lu-mophore to 3-CD is effectively quenched by the addition of Cu(II). A 500-fold enhancement in pyrene intensity is observed upon the addition of 1.87 M glutamate, which is presumed to extract Cu(II) from the 2 2 1 complex. The precise nature of the quenching and restoration mechanisms is currently unknown. 

Figure 11.12 Novel noncovalent assemblies, employing hydrogen bonding to mediate ET processes.

Timmerman P et al (1997) Noncovalent assembly of functional groups on calix[4]arene molecular boxes. Chem Eur J 3 1823-1832... 

Segre, D., Ben-Eli, D., and Lancet, D. 2000. Compositional genomes Prebiotic information transfer in mutually catalytic noncovalent assemblies. Proc. Natl. Acad. Sci. U.S.A. 97 4112-4117, p. 4112. 

Chambers 21st Century Dictionary defines material as any substance out of which something is, or may be, made [1]. For the purpose of this chapter a somewhat different statement will be used any organic substance that may have potential applications will be termed an organic material. Superimposed on this definition we would like to have another constraint such materials should be supramolecularly synthesizable (i.e. noncovalently assembled from molecular components). 

For dynamic processes involving hydrogen-bonded entities, see (a) Calama MC et al (1998) Libraries of non-covalent hydrogen-bonded assemblies combinatorial synthesis of supramolecular systems. Chem Commun 1021-1022. (b) Timmerman P et al. (1997) Noncovalent assembly of functional groups on Calix[4]arene molecular boxes. Chem Eur J 3 1823-1832. (c) Cai MM et al. (2002) Cation-directed self-assembly of lipophilic nucleosides the cation s central role in the structure and dynamics of a hydrogen-honded assembly. Tetrahedron 58 661-671... 

Membranes are noncovalent assemblies. The constituent protein and lipid molecules are held together by many noncovalent interactions, which are cooperative. 

However, lipid bilayers are impermeable to ions and most polar molecules, with the exception of water, so they cannot, on their own, confer the multiple dynamic processes which we see in the function of biological membranes. All of this comes from proteins, inserted into the essentially inert backbone of the phospholipid bilayer (Figure 3.27), which mediate the multiple functions which we associate with biological membranes, such as molecular recognition by receptors, transport via pumps and channels, energy transduction, enzymes, and many more. Biomembranes are noncovalent assemblies of proteins and hpids, which can best be described as a fluid matrix, in which lipid (and protein molecules) can diffuse rapidly in the plane of the membrane, but not across it. 

Columnar stacks of 3,4,5-trialkoxy-benzoylated 3,3 -diamino-2,2 -bipyridine triamide derivatives of benzene-1,3,5-carboxylic acid (87, 88) have recently been studied in dilute alkane (dodecane) and chloroform solution (Chart 14). In chloroform, neither the derivative with hexyloxy tails (88) nor the one with chiral citronellol tails (87) showed Cotton effects in the CD spectrum. TTie chiral derivative 87 showed such effects in dodecane, whereas the achiral derivative 88 showed them in the chiral solvent, (/7)-2,6-dimethyloctane. Interestingly, addition of a small amount (2.5%) of chiral 87 to stacks of achiral 88 in hexane induced a strong Cotton effect, similar in magnitude to that of the pure chiral derivative in hexane. This is an example of the sergeant and soldiers effect in a noncovalent assembly, and it is similar to the effects already established for stiff helical polymers like polyiso-... 

In contrast, phospholipid monomers noncovalently assemble into bilayer structure, which forms the basis of all cellular membranes (Chapter 5). 

Noncovalent assemblies of covalent or noncovalent polymers, e.g., the gel of the vitreous humor (Figure 1.5.1), may remain unchanged for several decades under favorable conditions but may also decompose upon simple heating to relatively low temperatures (e.g. 60°C) or application of modest mechanical pressure. 

Noncovalent fibers do not occur in nature. They are of synkinetic origin. Nature only uses covalent polymers. The most simple synkinetic route to linear noncovalent assemblies applies fatty acids of intermediate solubility and variations of pH and counterions. Myristic acid with 14 carbon atoms is most suitable. An acid soap consisting of 50% sodium myristate and 50% myristic acid crystal-... 

Supramolecular chirality is widely manifested in nature for example, the DNA double helix,the protein single helices and, in humans, rhinovirus 14, a member of the major rhinoviras receptor class, possesses a protein capsid that is composed of 60 protomers arranged in an icosahedrally symmetric arrayJ As shown previously in this book, at the molecular level, chirality is very important in asymmetric catalysis for the creation of novel chiral molecules however, more recently an increasing amount of attention has been drawn to chiral supramolecular assemblies. On the supramolecular level, chirality involves the nonsymmetric arrangement of molecular subunits in a noncovalent assembly via weak interactions such as hydrogen bonding, metal coordination and n-n interaction. 

Exciting supramolecular architectures Light-induced processes and synthetic transformations in noncovalent assemblies 05EJ04041. 

---