Chiral amphiphiles, aggregates

Laufersweiler MJ, Rohde JM, Chaumette J-L, Sarazin D, Parquette JR. Synthesis, aggregation, and chiroptical properties of chiral, amphiphilic dendrimers. J Org Chem 2001 66 6440-6452. 

An extensive amphiphilic character of a chiral dopant is believed to favor an effective induction of phase chirality. Amphiphilic dopants exhibit an increased orientational order within their host aggregates, as compared to merely hydrophilic or hydrophobic molecules. The observed phenomena as well as the hypothesis lit qualitatively to the model of an intra-micellar double-twist [23], see Section 14.3. 

Figure 10.11. (a) A schematic of the intermolecular interaction of Sq-1 in microcrystals (b) schematic representation of an extended aggregate formed by amphiphilic squaraine Sq-23 (solid enclosure denotes the chiral unit aggregate) (c) the stacking arrangement of Sq-25 in the solid state. 

The PGSE methodology has also been applied to study the dependence of enan-tioselectivity on the distribution of the chiral Rh-hydrogenation catalyst 137 between an aqueous and micellar phase. The observed increase in enantioselectivity when amphiphiles are added to the water is associated with an aggregation of the catalyst to the micelles [309]. 

In another study, Ragunathan and Bhattacharya described the formation of gels and fibrous assemblies from L-serine-based amphiphiles in chloroform [74], In this instance also, chiral purity was essential for the observed gelation. The microscopic examinations of these fibers reveal the existence of braided fibers and helices in these aggregates. 

It is apparent that the solubilization of amphiphilic molecules with chiral head groups produces aggregates such as braided fibers, helices, ribbons, rods, or tubules. When these organizations are sufficiently extended, they start to get entangled and develop the ability to entrap solvent molecules in the resulting reticulum. Many such instances eventually lead to the formation of gels. This phe-... 

In contrast to typical chiral thermotropic constituent molecules, protein molecules have a huge number of chiral centres, and the twist between assembled proteins is typically much larger (-1/10 of a revolution). We can expect to find a range of similar liquid crystalline phases in protein aggregates, although with significantly smaller lattice parameters (compared with typical protein dimensions). Indeed, the aggregation processes of (e.g. structural) proteins may be driven by their chirality as much as by their molecular shape (and amphiphilicity). This is discussed in more detail in Chapter 6. 

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