Self-assembly irreversible

Veerman, C., Sagis, L.M.C., van der Linden, E. (2003). Gels at extremely low weight fractions formed by irreversible self-assembly of proteins. Macromolecular Bioscience, 3,243-247. 

Irreversible self-assembly involves the formation of a product through a series of irreversible (usually covalent) bond-forming steps under kinetic control. There is no margin for self-correction and any mistakes are fatal to the formation of the final assembly. This class of reaction is not of significant interest in supramolecular chemistry but is topical in organic synthesis as tandem or domino reactions.2... 

Self-assembly can be divided into a number of classes 1. strict self-assembly, 2. irreversible self-assembly, 3. precursor modification followed by self-assembly, 4. self-assembly with post-modification, 5. assisted self-assembly, 6. directed self-assembly, 7. self-assembly with intermittent processing. 

Several types of self-assembly have been identified from the biological literature 7 (i) thermodynamic self-assembly (ii) irreversible self-assembly (iii) assisted and directed self-assembly and (iv) self-assembly with pre-, post-, or intermittent modification. 

Irreversible self-assembly involves a series of steps which cascade down a particular pathway, guided by kinetic influences. The products of each step in the assembly are kinetically stable, so that each coordinate bond must be formed correctly the first time for the assembly to proceed successfully. While several designed self-assembly processes appear to occur irreversibly, the generality of such procedures must be considered uncertain. 

Assisted and directed self-assembly are derivatives of thermodynamic and irreversible self-assembly in which an external agent or template either prevents the formation of non-functional intermediates (assisted self-assembly), or stabilizes key intermediates or products (directed self-assembly). The external agent need not appear in the final product. Numerous examples of directed self-assembly, in particular, have been described. Such systems provide important new pathways to novel structures. However, the factors controlling such processes are often not easily rationalized. 

Unlike most self-assembly processes (vide infra), irreversible self-assembly is under kinetic control. Consequently. in most irreversible multistep assembly processes, the initial architectures must be precisely located and aligned.However, in some cases, highly complex architectures can result from apparently nontemplated, irreversible self-assembly... 

One addition to the assembly lexicon added a layer of complexity to the above definition. Thus, one of the seven different classes of self-assembly originally proposed by Lindsey. — which are strict self-assembly processes (with or without a template) positioned in different chemical settings—is commonly known as "irreversible self-assembly." This term is used to describe two-step processes, whereby a strict self-assembly processes is followed by irreversible reactions that covalently knit together the array of subunits. As Whitesides noted, strictly speaking this term is a misnomer. Hence, along with other types of post-assembly modified self-assemblies, we categorize these processes as "self-assembly with covalent modification." Postmodification generally comes in the form of a series of covalent bond formation steps and is of less interest to us here. The crux of any self-assembly process is the self-assembly. 

While most examples of self-assembly involve labile metal ions, a racemic triangle/square mixture synthesized from inert Co ions has been isolated. The triangles and squares, as well as the enantiomers of each respective structure, can be isolated because of kinetic inertness of the metal-ligand interaction. Formation of the square and triangle likely occurs via an irreversible self-assembly process in which the ratio of the triangle to square is kinetically determined. In this and other examples, inert ions have been used in self-assembly for the purpose of isolating kinetic products and potentially elucidating the self-assembly mechanism. 

Irreversible self-assembly. The final product of the self-assembly process cannot revert into its component pieces without covalent bonds being broken. The last stage in the assembly process is therefore usually the formation of a covalent bond in a process that is under kinetic control. 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

In order to prevent the irrevisible adhesion of MEMS microstructures, several studies have been performed to alter the surface of MEMS, either chemically or physically. Chemical alterations have focused on the use of organosilane self-assembled monolayers (SAMs), which prevent the adsorption of ambient moisture and also reduce the inherent attractive forces between the microstructures. Although SAMs are very effective at reducing irreversible adhesion in MEMs, drawbacks include irreproducibility, excess solvent use, and thermal stability. More recent efforts have shifted towards physical alterations in order to increase the surface roughness of MEMS devices. 

C. Goubault, E. Leal-Calderon, J.-L. Viovy, and J. Bibette Self-Assembled Magnetic Nanowires Made Irreversible by Polymer Bridging. Langmuir 21, 3725 (2005). 

IRREVERSIBLE POLYMERIZATION Template-directed self-assembly, BIOCHEMICAL SELF-ASSEMBLY Template-independent irreversible polymerization,... 

Preferential adsorption of a surfactant from a mixture depends on the structures of the amphiphiles and the substrate. Self assembly by physisorption is reversible, while that by chemisorption is irreversible. Thus, surfactants physisorbed in monolayers can be replaced by surfactants which are able to chemisorb. Such behavior was demonstrated by allowing a donor cyanine surfactant, D (capable of physisorption), and OTS (known to chemisorb) to... 

Scheme 3. Schematic representation of the irreversible formation of catenane 15. Processes (a) release (salt + heating) (b) self-assembly, (c) locking (salt + cooling).

This process involves the covalent locking in of structures formed by reversible self-assembly. The irreversible, post-assembly step switches off the equilibrium process involved in the self-assembly. As we will see in the following sections, self assembly with covalent postmodification is involved in a range of biochemistry (e.g. insulin synthesis) and elegant abiotic supramolecular synthesis as in the formation of catenanes and knots. 

This class of self-assembly incorporates elements from all of the preceeding classes and involved complex processes in which there are sequential steps involving self-assembly and covalent or irreversible modification. In general such processes are only found in biology at the present state of the field. 

Efficient and defect-free folding of the polymer will, at least, require a control of the regioselectivity of monomer incorporation and polymer tacticity. If, for example, the degree of tacticity control is poor, the stereochemical defects are irreversibly incorporated into the polymer, and the subsequent folding may fail or produce defective structures. By contrast, dynamic self-assembly may allow defects to be corrected and the hierarchical structure to be controlled or fine-tuned using external parameters (solvent, additives, temperature) prior to covalent fixation by polymerization. 

A CO Sensor Based Upon Self-assembled Ferrocenyl Ferraazetine. Having demonstrated the CO dependent solid-state electrochemistry of ferrocenyl ferraazetine, we synthesized a ferrocenyl ferraazetine molecule with disulfide functionality (Id). Scheme III. The specific aim was to design a CO sensitive molecule that could be confined to the working electrode of a two-terminal device via monolayer self-assembly techniques. Disulfides have been shown to irreversibly adsorb to Au and Pt surfaces (3-10). NMR and mass spectrometry are consistent with the proposed structure for compound Id. The FTIR spectrum of Id in THF exhibits metal carbonyl bands at 2067,2024,1989,1985 cm similar to the spectra for other ferraazetine derivatives la-c (2,5-6). Like derivatives la-c. Id reacts with CO (1 atm) at 298 K in CH2C12 to form a ferrapyrrolinone complex 2d, equation (3). 

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