Cyclic assemblies

It was predicted that, in the limit of an infinite value of there is a critical monomer concentration erne = nEM below which the solution is virtually solely composed of the self-assembled macrocycle and above which the concentration of the latter remains constant, with the excess monomer then producing only non-cyclic species. 

Considerable interest has been shown in the formation of hydrogen bond-mediated (discrete) cyclic assemblies incorporating between three and ten component molecules. A few hydrogen bonding motifs have dominated these studies. These include cyanuric acid-melamine contacts, 2-aminopyridine-carboxylic acid contacts and carboxylic acid or pyridone dimer contacts. While individual hydrogen bonding interactions of this type are often weak (with association constants of 

Diederich, Cyclophanes, Royal Society of Chemistry, Cambridge, 1991 F.N. Diederich, in Supramolecular Chemistry, V. Balzani and L, De Cola, eds., Kluwer, Dordrecht, 1992, pp. 119-136  

Vogtle, Cyclophane Chemistry, John Wiley and Sons, Chichester, 1993. 

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As discussed above, fusion is driven by the assembly of SNAREs mediated by their SNARE motifs. Assembly is associated with a major release of energy, and consequently the SNAREs need to be refueled with energy by the generation of free SNAREs before they are reusable for another round of fusion. Thus, SNAREs undergo cyclic assembly and disassembly, and together the individual reactions involved are referred to as the conformational cycle of SNAREs (Figure 2). 

Linares F, Quartapelle Procopio E, Galindo MA, Angustias Romero M, Navarro JAR, Barea E (2010) Molecular architecture of redox-active half-sandwich Ru(II) cyclic assemblies. Interactions with biomolecules and anticancer activity. Cryst Eng Comm 12 2343-2346... 

Early work of Whitesides demonstrated the formation of heteromeric cyclic assemblies of barbital and ArW,-bis(4-tert-butylphenyl)-melamine that formed a 2D hydrogen-bonded rosette , of composition 3(barbital) 3(melamine), in the solid state.24 The components assembled via 18 hydrogen bonds based on alternating ADA and DAD sequences of a cyanuric acid-melamine lattice. Covalent... 

Following the work of Whitesides, Hamilton reported a cyclic assembly based on a biphenyl-3,3 -dicarboxylic acid and an isophthalpyl bis(aminopyridine) (Fig. 10).25 The four-component assembly, of composition 2(acid) 2(bipyridine), was held together by eight hydrogen bonds. The components adopted a figure-of-eight structure in the solid state, wherein the diacids and bipyridines participated in face-to-face 7i-7i forces. 

Fig. 12 Self-assembly of the four-component cyclic assembly of 2(oxine) 2(salicyclic acid) in the solid state.

As the number of monomeric units of a cyclic assembly increases, its formation in solution becomes unfavorable. Although there is a report regarding the formation of a cyclic hexamer in the solid state [94], to the best of our knowledge, there are no evidences for the complete assembly in solution of simple multiporphyrin rings of order greater than four. 

Multicyclic assemblies, as well as cyclic assemblies, are generally made either from a unique monomeric building block (homo-assemblies) or two monomeric building blocks (hetero-assemblies). The two types of architectures can be exemplified by the assembly 40 prepared by Osuka et al. [88,89]... 

An overview of the thermodynamic principles governing self-assembly in solution, with particular reference to multiporphyrin architectures, is presented by Gianfranco Ercolani in the fifth chapter. The topic is discussed in order of increasing complexity, from simple acyclic assembhes to multi-cyclic assemblies. The principles are illustrated by selected examples of metal-mediated assemblies of porphyrins, many of which have been described in the previous chapters. 

The plot of Eq. [21] is useful to evaluate the maximum yield of a cyclic assembly obtainable with a given driving force (Fig. 6). 

KEM = 1000 satisfies the condition in Eq. [23]. Fig. 5 also shows that the value of the monomer concentration [Mi]o is crucial to determine the stability of a cyclic assembly in solution. If the concentration is too low, the assembly lacks the driving force to form and thus only low-order linear oligomers wiU be present in solution on the other hand, if the concentration is too high, the cyclic assembly loses the competition with the linear polymer. The optimal monomer concentration is obtained by Eq. [22]. It is interesting to note that when the condition in Eq. [23] is satisfied, (C %)max 100% and the optimal monomer concentration reduces to ([Mi]o)max 0.1fiEM . From Eq. [21] and Fig. 6 it also appears that a cyclic assembly does not form to any significant extent at any concentration, that is, (C %)max < 1%. if the condition in Eq. [24] is satisfied ... 

The chelate cooperativity factor jd, defined in Fig. 15 for the case of a cyclic assembly, can be generalized to the case of a multicyclic assembly. For example, consider the equilibrium shown in Fig. 16 in which a trivalent receptor B3 saturated by three molecules of a trivalent Hgand A3 reacts to form a 1 1 bicycHc complex r-A3-B3. 

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