Hydrophobicity Interface

The water structure at the water/surfactant interface depends on the nature of the surfactant head group, whereas the hydrophobic interface plays only a secondary role [91-93],... 

In 1958 Sarda and Desnuelle [79] discovered the lipase activation at the interfaces. They observed that porcine pancreatic lipase in aqueous solution was activated some 10-fold at hydrophobic interfaces which were created by poorly water-soluble substrates. An artificial interface created in the presence of organic solvent can also increase the activity of the lipase. This interfacial activation was hypothesized to be due to a dehydration of the ester substrate at the interface [80], or enzyme conformational change resulting from the adsorption of the lipase onto a hydrophobic interface [42,81,82]. 

A novel approach for ion sensing is based on the use of potential-sensitive or polarity-sensitive dyes (PSDs) and was presented first106 in 1987. PSDs are charge dyes and typically located at the interface between a lipophilic sensor phase and a hydrophilic sample phase. The transport of an ion into the lipophilic sensor layer causes the PSD to be displaced from the hydrophilic/hydrophobic interface into the interior of the respective phase (or vice versa), thereby undergoing a significant change in its fluorescence properties107 110. 

Several variations in chemical constitution, which lead to a depression of the Krafft-Point (for example, branching of the hydrophobic part of the molecule), frequently result in diminished hydrophobicity of the molecule. At constant molecular weight, the critical micelle concentration (Cj.) is shifted with increased branching to higher concentrations, the surface activity diminishes, the tendency to adsorb at hydrophobic interfaces decreases, etc. (j, 14, 15). Therefore, the nature of the oxyethylene groups in aTkyl ether sulfates is of major importance. 

Schnarr, N. A. Kennan, A. J. Specific control of peptide assembly with combined hydrophilic and hydrophobic interfaces. J. Am. Chem. Soc. 2003,125, 667-671. 

Hadley, E. B. Geltman, S. H. An antiparallel a-helical coiled-coil model system for rapid assessment of side-chain recognition at the hydrophobic interface. J. Am. Chem. Soc. 2006,128,16444-16445. 

Figure 14.4 Primary sequence and molecular model of a coiled coil dimer and self-assembled polymer with the hydrophobic interface highhghted. The final bundle fiber structure is shown at the bottom. Reprinted from Wagner et al. (2005). Copyright 2005 National Academy of Sciences.

Figure 8.1 Schematic representation of structure of the flexible disordered caseins at a planar hydrophobic interface (a) asi-casein (b) p-casein. The solid bars denote hydrophobic regions of the molecules they do not imply rigidity. Reproduced from Home (1998) with permission.

These micellar cubic mesophases require large surface curvature and low charge density. Their formation is thus favored by the use of surfactant molecules with large polar head group, and acidic conditions under which the charge density at the silicate/surfactant is always limited. The fact that this phase can be prepared with CTAB when PTES is present, suggests the existence of specific interactions between the phenyl groups and the polar head of the surfactant molecules. It was indeed reported that benzene molecules are preferably located at the hydrophilic-hydrophobic interface [29]. 

The interfacial adsorption theory proposes that the orientation of odorant molecules is dependenl upon their behavior at the hydrophilic-hydrophobic interface, taking into account interaction with the mucus and adjacent olfactory membrane. 

What distinguishes coiled coils from multiple-helix bundles is that coiled coils contain a narrow hydrophobic face on the surface of each a-helix from the 3-4 hydrophobic repeat, whereas in multiple-helix bundles the hydrophobic interface can contain a wider hydro-phobic surface involving hydrophobes additional to the 3-4 repeat (Scheme 1 for a review, see retf22 ). The wider hydrophobic surface will change the hydrophobic packing and relative orientation of the a-helical chains with respect to each other in bundles as opposed to coiled coils.[17 ... 

Scheme 2 shows a cross-sectional representation of a two-stranded a-helical coiled coil of 35 residues per polypeptide chain. The design of this idealized coiled coil incorporated the factors that maximize coiled-coil stability, for example, the hydrophobicity and packing effects in the hydrophobic core, intrachain electrostatic attractions, helical propensity contribution s[2324 from residues outside of the hydrophobic interface, and interchain electrostatic interactions.125 ... 

As long ago as 1984, Lau et al.1311 demonstrated that stable coiled coils of four heptads in length could be formed. Subsequent work set out to examine the factors that would maximize stability of the coiled coil in the shortest possible chain length, for example, hydro-phobicity and packing effects in the hydrophobic core, intrachain electrostatic interactions, and the helical propensity contribution of residues outside the hydrophobic interface.123,24 30 321... 

Fig. 7. Schematic illustration of free energy assignments for three different types of interactions. The examples illustrate the situation existing for folded, unfolded, and one partially folded intermediate. For hydrophobic interfaces, partially folded intermediates always include an extra term Ag corresponding to the solvent exposure of protein regions that have not undergone unfolding. The unfolded state lacks this uncompensated exposure term. For a bonded or liganded interface, cooperative behavior is created when the unfolding of either domain results in the disruption of the bonded interface or the dissociation of the ligand molecule. [Reprinted from Freire el al. (1991)].

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