Cyclic assemblies, thermodynamics

Amorin, M., Castedo, L., and Granja, J. R. (2003). New cyclic peptide assemblies with hydrophobic cavities The structural and thermodynamic basis of a new class of peptide nanotubues./. Am. Chem. Soc. 125, 2844-2845. 

Voltammetry can provide a powerful insight into the thermodynamics and kinetics of electron transfer across the electrode/adsorbate interface and within the interfacial supramolecular assembly. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) are the most commonly used techniques to study the equilibrium behavior and kinetics of redox-active interfacial supramolecular... 

This chapter deals with formation of cyclic 2D and 3D structures in solution by self-assembly of two or more components using hydrogen bonds as the major interactions. Obviously, the formation of a defined supramolecular aggregate stabilized by non-covalent forces is a thermodynamically driven process which reflects a balance between enthalpy and entropy. Consequently, the product of a non-covalent macrocyclic synthesis must be evaluated and predicted in terms of thermodynamic minima in an equilibrium mixture. 

The theoretical treatment for the self-assembly of a molecular entity into rings or cages occurring under thermodynamic control is based on two fundamental physicochemical quantities the effective molarity (EM) of the intramolecular interaction of the cyclic n-mer and the value of the stability constant of the intermolecular interaction (fCinter) [5]. The following formula has been derived by Ercolani and allows calculation of the minimum value of the product fCinterEM to obtain a virtually complete self-assembled macrocycle at certain monomer concentration ... 

As to the thermodynamically controlled formation of acyclic polymeric assemblies in solution, there are only a few examples in the literature. The main problems are due to a) competition with cyclization processes b) low association constants c) low solubility of the oligomers. In fact the first reported polymeric assembly in solution made of zinc 5-(4-pyridyl)-10,15,20-triphenylporphyrin (ZnPyTPP) units [47], actually consists, as successively demonstrated, of a cyclic tetramer (Sect. 3.3). The linear polymer, however, was unambiguously detected in the solid state by X-ray analysis of a ZnPyTPP single crystal [47]. This is one of the first examples in the literature of met-alloporphyrins, illustrating the fact that the stability of an assembly may be strongly dependent on the aggregation state. 

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. 

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