Higher-ordered aggregates

Balguerie, A., Dos Reis, S., Coulary-Salin, B., Chaignepain, S., Sabourin, M., Schmitter, J. M., and Saupe, S.J. (2004). The sequences appended to the amyloid core region of the HET-s prion protein determine higher-order aggregate organization in vivo. J. Cell Sci. 117, 2599-2610. 

In this example the solvent - a fluorinated alcohol - forms higher order aggregates and activates for the epoxidation of electron rich olefins. HFIP accelerates this oxidation reaction up to 100,000-fold (relative to that in 1,4-dioxane as solvent). Which hydrogen bond network involving olefin, and fluorinated alcohol gives rise to such spectacular accelerations ... 

Fig. 17. Fluorous patches direct the pairing of protein segments in lipid micelles. The hydrophobic peptides partition into lipid micelles, forming a-helices. Then, the superhydrophobic hexafluoroleucine residues seek each other, causing self-association into dimers and higher order aggregates. Fluorine is light, while the backbone of the a-helices is dark. From Ref. [81], with permission.

Table 26. Formation of higher-order aggregates of polymer complexes composed of synthetic polymers...

Fig. 3.15. Analytical size-exclusion chromatography distinguishes between dimeric and monomeric chorismate mutases [97]. Wild-type MjCM (top trace) was found to be a mixture of dimer and higher-order aggregates. Most selected variants (e.g. traces A, B and C) were dimers or mixtures of dimers and monomers. Only oneof26 variants tested (mMjCM, bottom trace) eluted as a monomer. It had the six amino acid insert shown. Analytical ultracentrifugation confirmed that this protein is monomeric in solution.

NMR techniques.116 Finally, the possible aggregation of ion pairs to form species such as ion quadruples, hextuples, and higher-order aggregates has also been investigated.101,105,121-125 The main conclusion seems to be that, at the concentrations typically used in olefin polymerizations, catalyst ion pairs are unlikely to be present as higher aggregates.105,123,124 These aspects have been summarized in pertinent reviews.41,116... 

Solution behavior of ionomers can be divided into two types, primarily depending on the polarity of the solvent [46,47], One is polyelectrolyte behavior due to the dissociation of counterions in polar solvents (e.g., DMF), and another is association behavior due to the formation of ion pairs and even higher order aggregates in less polar solvents (e.g., THF). Table 2 shows the solvents frequently used for the study of ionomer solutions, as well as their dielectric constants. As the dielectric constant decreases, the degree of counterion binding and also ion pair formation changes (increases) gradually, and so does the solution behavior. In this chapter, only the polyelectrolyte behavior of ionomers in a polar solvent is described. Some brief... 

Divalent anionic impurities with filled valence-electron shells such as the chalcogenides are potential effective-mass traps for holes, but these ions have a propensity to cluster. The resultant dimers and higher order aggregates... 

---