Monomer units, assembly

Diblock copolymers consist of contiguous sequences of two different covalently bound monomer units, arranged in an -A-A-A-B-B-B-B- structure. In an appropriate solvent, the diblock copolymers spontaneously self-assemble into micelles with cores which are essentially pure in one component and a diameter... 

A concept of amphiphilicity, as applied to single monomer units of designed water-soluble polymers, is presented in the third chapter by Okhapkin, Makhaeva, and Khokhlov. The concept is relevant to biomolecular structures and assemblies in aqueous solution. The authors consider the substantial body of information obtained experimentally and theoretically on surface molecular chemical structures, including those that are prospective for surface catalysis. Unusual conformational behaviors of single amphiphilic polymers recently observed in simulations are also discussed in detail. 

Block and graft copolymers are composed of significant sequences of different monomer units, normally in a nonstatistical fashion. For block copolymers, these sequences are assembled in a linear fashion whereas in the case of graft copolymers, blocks are grown from or attached to the backbone of another block as branches. In the case of block and graft copolymer PEMs, these sequences may be composed of significantly different chemical units or units that are chemically identical except that one block is sulfonated and the other is not. 

If C-(t) indicates the number of free monomer units in the assembly (e.g., G-actin), a kinetic equation for the depletion of free monomer may be written, in accordance with the scheme shown in Figure 1, as... 

There are three chemical problems associated with the assembly of a protein, nucleic acid, or other biopolymer. The first is to overcome thermodynamic barriers. The second is to control the rate of synthesis, and the third is to establish the pattern or sequence in which the monomer units are linked together. Let us look briefly at how these three problems are dealt with by living cells. 

The equilibrium constant for the self-assembly of the cyclic oligomer containing n monomer units. 

J van Heijenoort. Assembly of the monomer unit of bacterial peptidoglycan. Cell Mol Life Sci 54 300-304, 1998. 

Attention will focus in two primary modes of network assembly (1) random, uncontrolled connectivity analogous to classical polymer preparation, whereby monomer units or building blocks are essentially positioned relatively unsystematically via single-pot-type reactions and (2) ordered, controlled connectivity analogous to tessellated dendritic polymer construction whereby elements, or building blocks, are precisely juxtaposed into a coherent motif. 

Figure I Schematic representation of an example of hierarchical self-assembly at microscopic, mesoscopic, and macroscopic levels. At the microscopic level, molecules assemble into supramolecular polymer-like assemblies. This involves conformational changes to the monomer units that themselves are complex molecules. The polymers assemble into bundles at mesoscopic levels that under appropriate conditions spontaneously align macroscopically along some preferred direction to form a uniaxial nematic liquid-crystalline phase (after Aggeli et al., 2001).

Figure 2 Chemical reaction models for (a) isodesmic and (b) nucleated supramolecular assembly. 1 and K 2>1 are equilibrium constants for the elongation reactions, and Ka 1 and K a those for the conversion between assembly active and inactive forms of the monomer units. If /f aKa, then the nucleated assembly is self-catalyzed ( autosteric ) and if K a = Ka this is not so.

Figure 10 Assembly diagram of compound 1 of Figure 6 in the solvent n-butanol (adapted from van Gestel, 2004a Weiss and Terech, 2005). Symbols represent results from UV-vis absorption, UV, fluorescence decay, FD, and circular dichroim spectroscopy, CD, and the drawn line the theoretical fits to the data. There are three types of transition I, isodesmic polymerization II, helical transition of long supramolecular polymers and III, nucleated helical assembly of the monomer units.

Alternatively, dendrimers may be synthesized directly by our original II Divergent Core Proliferation method. This method may involve the exponential covalent assembly of monomer units around a multi-valent core to produce branch cells in situ or it may involve the direct use of pre-formed branch cell reagents. In either case the resulting covalent structure consists of precise numbers of dendrons organized around the initiator core. 

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