Hydrophobic-like

An alternative candidate for a cell membrane is one composed of mostly hydrophobic molecules that would spontaneously aggregate if suspended in water. It is important that the molecule is not entirely hydrophobic, like hexane or benzene, as some part of it must be solvated if it is to aggregate with others to form a structure capable of encapsulating other species. Candidates for this type of molecule are the lipids and related compounds. The formation of protocells by lipids has been discussed previously as part of the Lipid World hypothesis. In the context of compartmentalization it is important to consider how such an aggregate could evolve. 

There are other chemical questions surrounding the emergence of life. For example, membranes are needed to confine the contents of cells and provide a stable, controlled environment for biochemical reactions. Some fairly simple organic molecules (amphipathic molecules), in which one end has an affinity for water (i.e. it is hydrophilic) but the other does not (i.e. it is hydrophobic, like oil), can naturally form membranelike structures, which, in the presence of water, form cell-like spheres (see Box 1.7). Self-replication is another vital factor, which needs to be accomplished with minimal error.The information necessary for con-... 

Over the next night, one should test only hydrophobic phases (e.g., from quite hydrophobic like Purospher and Discovery Cjg to very hydrophobic like Ascentis Cjg or SynergiMAX RP). 

The reaction rate and selectivity also largely depend on the solvents. Detailed studies using lipases have been conducted to investigate the effect of solvents on the reaction. Basically, the enhanced rate using hydrophobic solvents can be explained as follows. Lipase has a lid at the entrance of the substrate-binding site, and the lid is open when a hydrophobic triacyl glycerol comes close to the enzyme in its natural environment. Therefore, the lid is always open in a nonpolar solvent like hexane that is also hydrophobic like triacyl glycerol. 

Another important class of materials which can be successfiilly described by mesoscopic and contimiiim models are amphiphilic systems. Amphiphilic molecules consist of two distinct entities that like different enviromnents. Lipid molecules, for instance, comprise a polar head that likes an aqueous enviromnent and one or two hydrocarbon tails that are strongly hydrophobic. Since the two entities are chemically joined together they cannot separate into macroscopically large phases. If these amphiphiles are added to a binary mixture (say, water and oil) they greatly promote the dispersion of one component into the other. At low amphiphile... 

Figure B3.6.4. Illustration of tliree structured phases in a mixture of amphiphile and water, (a) Lamellar phase the hydrophilic heads shield the hydrophobic tails from the water by fonning a bilayer. The amphiphilic heads of different bilayers face each other and are separated by a thin water layer, (b) Hexagonal phase tlie amphiphiles assemble into a rod-like structure where the tails are shielded in the interior from the water and the heads are on the outside. The rods arrange on a hexagonal lattice, (c) Cubic phase amphiphilic micelles with a hydrophobic centre order on a BCC lattice.

It is of particular interest to be able to correlate solubility and partitioning with the molecular stmcture of the surfactant and solute. Likes dissolve like is a well-wom plirase that appears applicable, as we see in microemulsion fonnation where reverse micelles solubilize water and nonnal micelles solubilize hydrocarbons. Surfactant interactions, geometrical factors and solute loading produce limitations, however. There appear to be no universal models for solubilization that are readily available and that rest on molecular stmcture. Correlations of homologous solutes in various micellar solutions have been reviewed by Nagarajan [52]. Some examples of solubilization, such as for polycyclic aromatics in dodecyl sulphonate micelles, are driven by hydrophobic... 

The hydrophobic effect. Water molecules around a non-polar solute form a cage-like structure, which ices the entropy. When two non-polar groups associate, water molecules are liberated, increasing the entropy. 

Apart from the thoroughly studied aqueous Diels-Alder reaction, a limited number of other transformations have been reported to benefit considerably from the use of water. These include the aldol condensation , the benzoin condensation , the Baylis-Hillman reaction (tertiary-amine catalysed coupling of aldehydes with acrylic acid derivatives) and pericyclic reactions like the 1,3-dipolar cycloaddition and the Qaisen rearrangement (see below). These reactions have one thing in common a negative volume of activation. This observation has tempted many authors to propose hydrophobic effects as primary cause of ftie observed rate enhancements. 

Chapter 5 also demonstrates that a combination of Lewis-acid catalysis and micellar catalysis can lead to accelerations of enzyme-like magnitudes. Most likely, these accelerations are a consequence of an efficient interaction between the Lewis-acid catalyst and the dienophile, both of which have a high affinity for the Stem region of the micelle. Hence, hydrophobic interactions and Lewis-acid catalysis act cooperatively. Unfortunately, the strength of the hydrophobic interaction, as offered by the Cu(DS)2 micellar system, was not sufficient for extension of Lewis-acid catalysis to monodentate dienophiles. 

Throughout this thesis reference has been made to hydrophobic effects. Enforced hydrophobic interactions are an important contributor to the acceleration of uncatalysed and also of the Lewis-acid catalysed Diels-Alder reactions which are described in this thesis. Moreover, they are likely to be involved in the beneficial effect of water on the enantioselectivity of the Lewis-acid catalysed Diels-Alder reaction, as described in Chapter 3. Because arguments related to hydrophobic effects are spread over nearly all chapters, and ideas have developed simultaneously, we summarise our insights at the end of this thesis. 

In the case of the retro Diels-Alder reaction, the nature of the activated complex plays a key role. In the activation process of this transformation, the reaction centre undergoes changes, mainly in the electron distributions, that cause a lowering of the chemical potential of the surrounding water molecules. Most likely, the latter is a consequence of an increased interaction between the reaction centre and the water molecules. Since the enforced hydrophobic effect is entropic in origin, this implies that the orientational constraints of the water molecules in the hydrophobic hydration shell are relieved in the activation process. Hence, it almost seems as if in the activated complex, the hydrocarbon part of the reaction centre is involved in hydrogen bonding interactions. Note that the... 

The rate of the Lewis-acid catalysed Diels-Alder reaction in water has been compared to that in other solvents. The results demonstrate that the expected beneficial effect of water on the Lewis-acid catalysed reaction is indeed present. However, the water-induced acceleration of the Lewis-add catalysed reaction is not as pronounced as the corresponding effect on the uncatalysed reaction. The two effects that underlie the beneficial influence of water on the uncatalysed Diels-Alder reaction, enforced hydrophobic interactions and enhanced hydrogen bonding of water to the carbonyl moiety of 1 in the activated complex, are likely to be diminished in the Lewis-acid catalysed process. Upon coordination of the Lewis-acid catalyst to the carbonyl group of the dienophile, the catalyst takes over from the hydrogen bonds an important part of the activating influence. Also the influence of enforced hydrophobic interactions is expected to be significantly reduced in the Lewis-acid catalysed Diels-Alder reaction. Obviously, the presence of the hydrophilic Lewis-acid diminished the nonpolar character of 1 in the initial state. 

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