Bolaform amphiphiles

Beside the conventional surfactants with one polar head group and one nonpolar tail, dimeric and oligomeric surfactants have attracted considerable interest in academia and industry [521], Dimeric surfactants, also called Gemini surfactants, are made up of two amphiphilic moieties connected closely to the head group by a spacer group (Fig. 12.1). In bolaform surfactants the connection is in the middle of the alkyl chain or close to the end, so that they can be considered as two polar head groups connected by a long hydrophobic chain. 

Since these arborols (111) are comprised of two hydrophilic groups connected by a hydrophobic linkage, they fit the simple definition of a bolaamphiphile a term derived from bolaform amphiphile, originally introduced in 1951 by Fuoss and EdlesonJ128 In 1984, when Fuhrhop and Mathieu 129 reported the synthesis and self-assembly of several bolaamphiphiles, these two-directional surfactant-like macromolecules represented a simple entrance to the bolaamphiphile field this subject has recently been highlighted1130 and reviewed.1131 ... 

Bhattacharya et al. have also shown [45] that it is possible to generate vesicles via ion-pairing of two cationic amphiphiles with one bis-anionic bolaform amphiphile (see below). To explore whether subtle structural differences in the starting ion-pairs are reflected in the properties of the supramolecular assemblies, Bhattacharya et al. synthesized four hybrid bolaphile/amphiphile... 

A new family of crown-ether-based bolaamphiphiles, 21, that aggregate into a previously unknown type of bolaamphisome was prepared by Gokel and coworkers [50]. Evidence was presented on vesicle formation from the aqueous suspension of such bolaform amphiphiles. Several reports describing the synthesis of various macrocyclic models related to archael compounds have also appeared in the literature [51]. Fyles et al. prepared a novel series of asymmetric bis-macrocyclic bolaphiles, 22, and evaluated their transport activities in vesicles and planar bilayer membranes [52]. 

Roberts et al.40 used bolaform phosphatidylcholine as a probe of water soluble phospholipase catalysis. These bolaphiles (Figure 14) contain two phosphatidylcholines, as the ionic head groups permitting the evaluation of the proposal that two phosphatidylcholines are required for phospholipase activity. Phospholipase activity was measured using micelles formed from these bolaphiles and phosphatidylcholine containing amphiphiles. Increased membrane stability of these bolaform... 

Fuhrhop, J.-H., Krull, M., Schulz, A., Mobius, D. (1990). Bolaform amphiphiles with a rigid hydrophibic bixin core in surface monolayers and lipid membranes, Lang-muir, 6 497. 

Bolaform sugar amphiphiles 68 exhibit columnar mesophases [154]. The aggregated structures of 68 in aqueous media depend on the stereochemical factors of the spacers. Layered structures are formed in crystalline states by self-assembly of glucopyranose-based bolaamphiphiles due to the cooperation of the hydrogen-bonding interactions of hydroxyl groups and amide linkages [155]. 

With the advent of polymersomes and vesicles of other nonconventionar (i.e., not phospholipid-like) amphiphiles, numerous examples of monolayer vesicles in water have been reported. Typically, monolayer vesicles are prepared from small molecules with a hydrophobic core and two hydrophilic head groups (bolaform amphiphiles, see below) or from triblock-copolymers with two hydrophilic terminal blocks. The molecule must have a cylindrical or rectangular shape, so that it can arrange into a monolayer. 

Small amphiphiles need not necessarily have a phospholipid-like structure with two tails and one head group. For example, bolaform (or bipolar) amphiphiles are amphiphilic molecules that contain two head groups separated by an extended hydrophobic chain. Bolaform amphiphiles form monolayer vesicles in which each amphiphile extends across the monolayer membrane, exposing both head groups to water and sheltering the hydrophobic chain from water. However, also these types of... 

Figure 15 Possible mechanism for adsorption (a) of bolaform cationic amphiphiles in mixed systems and (b) of single flexible bolaform cationic amphiphiles systems on glass. (Reprinted from J. Coll. Int. Sci. 337, Yun Yan, Ting Lu, Jianbin Huang Recent advances in the mixed systems of bolaamphiphiles and oppositely charged conventional snrfactants, 1, 2009 with permission from Elsevier.)...

Saccharide-based gemini surfactants, like bolaform amphiphiles, exhibit diverse properties depending on the structure of the hydrophilic and... 

Figure 1 Possible molecular architectures of monomeric amphiphiles classical amphi-philes (a) bolaform, bipolar amphiphiles (b) multipolar (c) and amphotropic (d) amphiphiles. The circles represent the ionic head-groups and the hydrophobic chains are depicted by the wavy lines. (Adapted from Ref. 1.)...

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