Interfacial energetics surface activity

Surfactants. Some compounds, like short-chain fatty acids, are amphiphilic or amphipathic that is, they have one part that has an affinity for the nonpolar media (the nonpolar hydrocarbon chain), and one part that has an affinity for polar media, that is, water (the polar group). The most energetically favorable orientation for these molecules is at surfaces or interfaces so that each part of the molecule can reside in the fluid for which it has the greatest affinity (Figure 4). These molecules that form oriented monolayers at interfaces show surface activity and are termed surfactants. As there will be a balance between adsorption and desorption (due to thermal motions), the interfacial condition requires some time to establish. Because of this time requirement, surface activity should be considered a dynamic phenomenon. This condition can be seen by measuring surface tension versus time for a freshly formed surface. 

In the TSA and substrate-based inhibitor complexes, the sn-1 and 5m-2 hydrocarbon chains lie in a hydrophobic channel that extends from the active site to the surface of the enzyme. If the lengths of the hydrocarbon moieties were increased from the 7 to 8 carbons present in the model compounds to that of natural substrates (commonly 14 to 18 carbons long), they would protrude from the enzyme surface. It is probably safe to assume that these hydrophobic tails remain imbedded in the membrane during interfacial catalysis in order to minimize the energetic costs of substrate transfer. Thus the adsorption surface chosen by Scott et al. (1990a), which envelops the hydrophobic channel, is the most likely surface to contact the bulk phase lipid directly. 

---