Rigidity in synkinetic molecular monolayers of functional lipids containing Angstrom gaps

Bolaamphiphiles ("bola") are compounds with a hydrophobic core and hydrophilic head groups at both ends. The head groups as well as the hydrophobic core may be functional in nonsymmetric arrangements. One end group may, for example, be bound to a solid subphase, the other may dissolve and react with solutes in the bulk water phase. The hydrophobic core may contain reactive C=C double bonds. If a monolayer membrane is formed from the bolaamphiphile, only the outer head groups and the double bonds at the edges of the membrane can react with polar reagents in the bulk water phase, provided the membrane is rigid enough. Rigidity may be achieved either by rigid hydrophobic skeletons (carotenes, steroids, porphyrins) or by two hydrogen bond chains between secondary amide groups within an oligomethylene chain. Rigid monolayers have so far been used to prepare water-free, dry vesicle membranes, to protect solid surfaces from aggressive reagents by an ultrathin lipid layer and, most important, to construct functionalized gaps in membranes on solid surfaces. The latter are thought to contain water molecules, which do not exchange with the bulk water ("hydrophobic water") and may bind water-soluble molecules.