Aggregation and Assembly

This book is all about materials that are soft in nature, but the way in which the constituent molecules in many soft materials are arranged is not random. Instead, they have some structure that can be measured, even if they do not exhibit long-range order like a crystalline solid. The molecules or particles that make up a material are subject to attractive and repulsive forces from each other and any solvent molecules in the 

We have already discussed the first of these by considering the solid, liquid, and gas phases. These phases, among others, spontaneously form as a result of the competing effects of thermal fluctuations and intermolecular forces, leading to a uniform material structure. A variety of interesting thermodynamically stable, reversible phases will be introduced throughout this book as we describe the diverse forms of soft matter. 

The second mechanism for self-assembled structures to form depends on exactly how the constituent particles come together (i.e., the method of assembly), and the structures that form as a result may be reversible or irreversible. Many materials can be thought of as aggregates, and these materials are assembled by a growth process. In a fluid-like thermodynamic phase, such as a simple liquid, the molecules are free to diffuse throughout the system with a finite probability that every molecule can explore every possible position in the system. 

Many soft systems are formed slightly differently, as a growing cluster in which particles aggregate to form a larger-scale structure. One example of this process is the flocculation of colloidal particles. Particles in a solution tend to stick to each other (we will learn more about why this can happen in Chapter 5), so dispersed particles in a liquid are attracted to each other, forming tenuous clusters. As these clusters grow, it becomes impossible for newly added particles to penetrate to the center of the cluster, and they can only stick on the exterior therefore, a loose internal structure is developed and an interesting soft material is created. 

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Polymeric nanomaterials need not be based on nanoscale supports. Micelles are spherical assemblies that differ from star polymers in the absence of a covalently linked core. Typically, this morphology arises from polymeric strands possessing hydrophobic and hydrophilic regions, which will self-aggregate and assemble based on similar polarities. In polar solvents, the... 

Fig. 5. Protein folding. The unfolded polypeptide chain coUapses and assembles to form simple stmctural motifs such as -sheets and a-hehces by nucleation-condensation mechanisms involving the formation of hydrogen bonds and van der Waal s interactions. Small proteins (eg, chymotrypsin inhibitor 2) attain their final (tertiary) stmcture in this way. Larger proteins and multiple protein assembhes aggregate by recognition and docking of multiple domains (eg, -barrels, a-helix bundles), often displaying positive cooperativity. Many noncovalent interactions, including hydrogen bonding, van der Waal s and electrostatic interactions, and the hydrophobic effect are exploited to create the final, compact protein assembly. Further stmctural...

FIGURE 24.3 (a) A duct at the junction of the pancreas and duodenum secretes pancreatic juice into the duodenum, the first portion of the small intestine, (b) Hydrolysis of triacylglycerols by pancreatic and intestinal lipases. Pancreatic lipases cleave fatty acids at the C-1 and C-3 positions. Resulting monoacylglycerols with fatty acids at C-2 are hydrolyzed by intestinal lipases. Fatty acids and monoacylglycerols are absorbed through the intestinal wall and assembled into lipoprotein aggregates termed chylomicrons (discussed in Chapter 25). 

A few questions come to mind. What is the structure of the aggregate and why are 2 equiv of each reagent essential Is it due to the acidic proton (N-H) in 41 Before going into the detail of the mechanism, let us assemble more circumstantial evidence on this reaction. 

Chithra P, Varghese R, Divya KP, Ajayaghosh A (2008) Solvent-induced aggregation and cation-controlled self-assembly of tripodal squaraine dyes optical, chiroptical and morphological properties. Chem Asian J 3 1365-1373... 

John, D.C.A., Grant, M.E. and Bulleid, N.J. (1993) Cell-free synthesis and assembly of prolyl 4-hydroxylase - the role of the beta-subunit (pdi) in preventing misfolding and aggregation of the alpha-subunit. EMBOJoumal 2,1587-1595. 

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