Formation of Supramolecular Assemblies

Fig. 3 Mechanisms for enzymatic supramolecular polymerisation (a) Formation of supramolecular assembly via bond cleavage, (b) Formation of supramolecular assemblies via bond formation. Examples are shown of biocatalytic supramolecular polymerisation of aromatic peptide amphiphiles via (i) phosphate ester hydrolysis, (ri) alkyl ester hydrolysis, and (iii) amide condensation or reversed hydrolysis using protease...

Table 1 Examples of enzymatic formation of supramolecular assemblies...

Fig. 4 Free energy diagram for the two possible situations in enzyme-triggered formation of supramolecular assembly. Left. The enzyme-catalysed reaction and self-assembly process are both favoured independently and therefore uncoupled. Right. Enzyme-triggered self-assembly under thermodynamic control formation of the building blocks is thermodynamically unfavoured in isolation and occurs in reversible fashion when coupled to a sufficiently stable self-assembled structure formation...

Fig. 9 Intracellular cleavage of an esterase substrate (14) by an endogenous esterase to the hydro-gelator (15) and corresponding formation of supramolecular assembly within the cells. Reproduced with permission from [75] Copyright WUey-VCH Verlag GmbH Co. KGaA...

The introduction at the C- or N-terminal position of a crown ether unit has been used as a strategy to control the aggregation of poly(benzyl glutamate) derivatives 19 The incorporation of the crown unit at the C-terminal position is performed using (benzo-15-crown-5)-4-amine as initiator of the polymerization of l-G1u(OBz1)-NCA. Physical properties of such crown derivatives can be modulated by the formation of sandwich 2 1 complexes driven by the addition of specific alkali metal ions. In the reported case, the formation of K+ sandwich complex between two C-terminal benzo-15-crown-5 modified helical polypeptides induced aggregation. In a similar approach,f20 addition of Cs+ to 18-crown-6 terminated helical peptides results in the formation of supramolecular assemblies having membrane ion conductivity activities. 

Amide groups with long hydrophobic chains have been attached to the thiophene rings in order to promote the formation of supramolecular assemblies in solution. The reversible photoswitching of 13 from the open form to the closed form causes a change in the extent of aggregation this in turn causes a decrease in viscosity. Thus the viscosity could be modulated by appropriate illumination <2004CSR85>. 

Coordinate bonding is another type of direction-specific interaction. This type of interaction occurs between metal ions and electron-rich atoms and is of moderate strength. Such interactions have also been utihzed in the formation of supramolecular assemblies, and several examples are given in Chap. 3. 

Although solubility is an important factor in determining nucleation rates, the dependence of nucleation rate on solubility may not be followed in the presence of strong solvent-solute interactions that interfere with the formation of molecular assemblies compatible with those in the crystalline state. Prior to nucleation, solute molecules in solution are closely associated with solvent molecules. These solvated solute molecules must de-solvate such that the necessary solute-solute interactions may occur to bring about the formation of supramolecular assemblies that will lead to crystallization. When strong interactions exist between solvent and solute molecules in solution, nucleation kinetics may be significantly altered. On the other hand, the nucleation rate is expected to be fastest in solvents providing an optimal compromise between... 

From the point of view of rational design, there is still much to do in order to understand the principles which control the formation of supramolecular assemblies. However, in this field much progress has been made. Symmetry considerations in combination with the basic standards of coordination chemistry have made perceptible the construction of a variety of nanoscale systems which procure shape, size and, ultimately, function. It has been demonstrated that two types of building blocks are required. Rigid complexes... 

Harada A, Kataoka K. On-off control of enzymatic activity synchronizing with reversible formation of supramolecular assembly from enzyme and charged block copolymer. J Am Chem Soc 1999 121 9241-9242. 

Self-assembly processes can today be found in a variety of fields ranging from biology to materials science and electronics. While self-assembly has proven to be a valuable tool in the formation of supramolecular assemblies, it remains less a discipline in its own right than a compilation of principles that have been applied to an apparently disparate collection of research fields. One outcome of this historical development is that the concepts of self-assembly were typically defined in discipline-specific and sometimes overlapping ways. This fragmentation obscures their underlying coherence and universality. It also hinders a widespread understanding of self-assembly. 

The use of lower coordinate (two or three) metal ions in the formation of supramolecular assemblies is limited by the linkages possible using such metal centers. Nonetheless, there are some notable examples. 

Lehn has outlined a hierarchical order of the terms tern-plating, self-assembly, and self-organization to describe the spontaneous formation of supramolecular assemblies composed of preprogrammed molecular components (i.e., tectons) Self-assembly concerns the antientropic aggregation of individual components to yield an ordered multi-molecular entity. Intermolecnlar interactions that are intrinsically preprogrammed (see synthon) into the bnilding... 

Formation of supramolecular assemblies using leucine-zipper displaying dendrimers... 

Self-assembly The generation of a well-defined supermolecule from a given set of components, under appropriate conditions. The process involves three stages recognition between the components appropriate orientation of the components to allow growth of the structure termination of the growth to give a finite supramolecular species. The term also applies to the formation of supramolecular assemblies from their components. 

DOSY NMR and DLS analysis were performed to determine the hydrodynamic diameters of dendrimers 1 and 2. Interestingly, very large values of DLS were obtained, which indicates the aggregation of a number of dendrimers and the formation of supramolecular assemblies of dendrimers. This observation was attributed to interdendritical interactions of TEG-termini. 

If the copolymer is water soluble, the easiest method for generating polymersomes is direct dissolution of the dry polymer in aqueous media. However, this method cannot be used for insoluble water amphiphilic block-copolymers, for which a solvent-free or a solvent displacement method must be used. With the solvent-free method, a homogeneous amphiphilic polymer film is first deposited on a solid surface, and then subsequently hydrated in an aqueous buffer solution to form supramolecular assemblies upon desorption from the support. This is the method of choice when working under organic solvent-free conditions is mandatory, as for example for the combination with biomolecules. " When applying the solvent displacement method, the copolymer is first dissolved in an appropriate solvent, and then slowly mixed with water the reduced solubility of the copolymer drives the self-assembly process and formation of supramolecular assemblies, such as polymersomes. The organic solvent is eventually extracted, but complete removal of solvent traces is not possible and traces of organic solvents could affect sensitive biomolecules. 

The formation of supramolecular assemblies of ECM biopolymers defines an obvious and highly important interconnection of physical and biospecific characteristics of the matrices. In general, multiple specific interactions between individual biopolymers, such as collagen or fibronectin, result in the formation of large fibrillar or meshwork-like structures segmenting tissues and guiding cells by microstructural features and mechanical characteristics. With those structures, often referred to as connective tissue, basement membrane, and several others, the unique functionality of ECM and the fact that it is based on both the provision of specific binding sites and physical forces is best represented. 

In addition, when ILs are used as solvents, how will the supramolecular structures of ILs themselves affect the formation of supramolecular assemblies. For example, when the surfactant is added into the IL/oil mixture, two cases would happen. If the hydrophobic interactions between oil and surfactant are stronger than the interactions between IL and oils, liquid clathrate can be destroyed and O/IL microemulsions form. Otherwise, a new state would exist instead of O/IL microemulsions. However, in the reference, this field has not been mentioned. Moreover, because trivial water is difficult to be removed and could be included in the supramolecular framework of imidazolium ILs, ILs are not pure, which makes the system more complicated. This supramolecular structure may also affect the formation of supramolecular assemblies. Recently, we noticed that water plays the key role in the formation of IL based microemulsions. A small quantity of water can lead to great change in the phase diagram of IL/TX-100/ oil ternary systems. Therefore, the effect of the supramolecular structures of ILs on the formation of supa-amolecular assemblies is another valuable subject. 

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