Aggregation and Self-association

The self-association of 9-anthraldehyde and 9-phenanthralde-hyde, dibenzyl phenylboronate and t-butylphenol have been studied by n.m.r. techniques. Molecular-aggregation phenomena have been studied with metallochlorophylls (e.g., zinc phaeophytin-a) and with the dye acridine orange (90) in dilute aqueous solution. 

In the latter case, the pairs of flat molecules are arranged with Cg and Cj of one molecule arranged over the terminal rings of the second. 

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Fig. 4.25 Schematic representation of PAA-induced aggregation and self-association of [Au(CN)2] ...

Theories or computer simulations used to calculate the potential of mean force W(r) are typically based on numerous simplifying assumptions and approximations (de Kruif, 1999 Bratko et al., 2002 Prausnitz, 2003 de Kruif and Tuinier, 2005 Home et al., 2007 Jonsson et al., 2007). Therefore they can provide only a qualitative or, at best, semi-quantitative description of the potential of mean force. Such calculations are nevertheless useful because they can serve as a guide for trends in the factors determining the interactions of both biopolymers and colloidal particles. Thus, an increase in the absolute value of the calculated negative depth of W(r) may be attributed to a predominant type of molecular feature favouring aggregation or self-association. To assist with such a theoretical analysis, expressions for some of the mean force potentials will be presented here in the discussion of specific kinds of interactions occurring between pairs of colloidal particles covered by biopolymers in food colloids. 

A careful analysis of the temperature dependence and self-association patterns of dodecylammoniumpropionate (DAP) in benzene and cyclohexane have been conducted by Adams, Fendler et al.137. The investigation represents in part a considerable extension of Kreutzer s130 description of open and closed aggregation. Several association models were tested. The best fit of the experimental data was obtained with the sequential, indefinite self-association model the observed temperature dependence corresponded to this result, i.e., decreasing degree of association with an increase of the temperature, where at constant temperature the degree of self-association in cyclohexane was larger than in benzene. 

Specific chemical interactions can be considered by assuming appropriate combinations of solvation and self-association reactions. If, for example, the true species are the diluent and TBP monomers Zai and Zbi, the TBP dimer zs, and various combinations of aggregates or complexes (e.g., AB, Zab, AB2 ZAB2, and so forth), then for the jth solvation or self-association reaction in a binary mixture of diluent (A) and TBP (B) ... 

The nnfolding of lysozyme structure was reported to be induced by PEF (35 kV cm and 300 ps) this was accompanied by the cleavage of disulfide bonds and self association aggregation when the applied PEF dosage was higher than the critical level (Zhao et al., 2007). 

Toluene, THF and acetone are described as basic solvents. The residual solvent molecules can therefore only weakly interact with the acrylate groups. Interactions between chains are then favored and self-associations between PMMA chains appear. The rotation of the aery late groups becomes more difficult compared to the bulk PMMA, probably due to self-aggregation of some PMMA chains (even if the studied polymer is not stereoregular). 

A. Pan, B. Naskar, G. K. S. Prameela, B. V. N. Kumar, A. B. Mandal, S. C. Bhattacharya and S. P. Moulik, Amphiphile behavior in mixed solvent media I Self-aggregation and ion association of sodium dodecylsulfate in 1,4-diox-ane- water and methanol-water media. Langmuir 28,13830-13843 (2012). N. Patra and P. Krai, Controlled self-assembly of filled micelles on nanotubes. J.Am. Chem. Soc. 133,6146-6149 (2011). 

The conclusion of all these thermodynamic studies is the existence of thiazole-solvent and thiazole-thiazole associations. The most probable mode of association is of the n-rr type from the lone pair of the nitrogen of one molecule to the various other atoms of the other. These associations are confirmed by the results of viscosimetnc studies on thiazole and binary mixtures of thiazole and CCU or QHij. In the case of CCU, there is association of two thiazole molecules with one solvent molecule, whereas cyclohexane seems to destroy some thiazole self-associations (aggregates) existing in the pure liquid (312-314). The same conclusions are drawn from the study of the self-diffusion of thiazole (labeled with C) in thiazole-cyclohexane solutions (114). 

Insulin is composed of two peptide chains covalently linked by disulfide bonds (Figures 5.17 and 6.35). This monomer of insulin is the active form that binds to receptors in target cells. However, in solution, insulin spontaneously forms dimers, which themselves aggregate to form hexamers. The surface of the insulin molecule that self-associates to form hexamers is also the surface that binds to insulin receptors in target cells. Thus, hexamers of insulin are inactive. 

Coacervation occurs in tropoelastin solutions and is a precursor event in the assembly of elastin nanofibrils [42]. This phenomenon is thought to be mainly due to the interaction between hydro-phobic domains of tropoelastin. In scanning electron microscopy (SEM) picmres, nanofibril stmc-tures are visible in coacervate solutions of elastin-based peptides [37,43]. Indeed, Wright et al. [44] describe the self-association characteristics of multidomain proteins containing near-identical peptide repeat motifs. They suggest that this form of self-assembly occurs via specific intermolecular association, based on the repetition of identical or near-identical amino acid sequences. This specificity is consistent with the principle that ordered molecular assembhes are usually more stable than disordered ones, and with the idea that native-like interactions may be generally more favorable than nonnative ones in protein aggregates. 

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