Molecular recognition by macrocyclic receptors having multiple hydrophobic branches in a synthetic bilayer membrane

Piezoluminescence at the air-water interface through dynamic molecular recognition driven by lateral pressure application

The steroid cyclophanes with a cyclic core consisting of a 1,6,20,25-tetraaza[6.1.6.1]paracyclophane connected to four steroid moieties (cholic acid or cholanic acid) through a flexible l-lysine spacer were spread on water as Langmuir monolayers. The pi-A isotherm of the cholic-type steroid cyclophane includes a transition to the condensed phase with a limiting area of approximately 2 nm(2). This value is close to the cross-sectional area of the steroid cyclophane with a standing-up conformation of the cholic acid moieties, strongly suggesting that the cavity converts from a two-dimensional cavity to a three-dimensional cavity upon compressing the monolayer. Surface-reflective fluorescence spectroscopy of the monolayer using an aqueous fluorescent probe (6-(p-toluidino)naphthalene-2-sulfonate (TNS)) showed an abrupt increase in the TNS fluorescence intensity at a molecular area of 2 nm(2). Efficient binding of the guest probe would occur upon the completion of the three-dimensional cavity. Repeated compression and expansion induces periodic changes in the fluorescence intensity. This indicates a piezoluminescence effect through the catch and release of the TNS guest upon dynamic cavity formation. Analyses of the binding behavior of TNS to the steroid cyclophane resulted in binding constants in the range of approximately (5-9) x 10(4) M(-1) which are similar to that observed in lipid bilayer media (K = 5.1 x 10(4) M(-1)). The fluorescence intensity within the condensed phase was significantly increased with increasing pressure, suggesting that suppression of the molecular motion of the bound TNS may retard the nonemission process. Similar monolayer experiments were carried out with the monolayer of the cholanic-type steroid cyclophane that cannot form an open conformation on water. Both the phase transition in the pi-A isotherm and the change in the fluorescence intensity were negligible, confirming that the dynamic characteristic of the cavity is indispensable for the efficient pressure-induced binding of the guest and the consequent luminescence.

Creation of asymmetric bilayer membrane on monodispersed colloidal silica particles

Novel organic-inorganic nanohybrids, each having an inorganic core covered with an asymmetric lipid-bilayer membrane, were prepared through two-step self-assembling of a Cerasome-forming organoalkoxysilane lipid, N-[N-(3-triethoxysilyl)propylsuccinamoyl]dihexadecylamine (1), as the inner layer with an appropriate bilayer-forming amphiphile, N,N-dihexadecyl-N alpha-[6-(trimethylammonio)hexanoyl]alaninamide bromide (2), sodium N,N-dihexadecyl-N alpha-(6-sulfohexanoyl)alaninamide (3), or dimyristoylphosphatidylcholine (DMPC; 4), as the outer layer on a monodispersed colloidal silica particle. The particle thus obtained was characterized by various physical measurements, such as FT-IR spectroscopy, transmission electron microscopy, differential scanning calorimetry, and zeta-potential measurements. These data strongly supported the successful formation of the asymmetric bilayer structure on the surface of the silica particle. The current method is widely applicable to various kinds of hybrids of inorganic particles with lipid membrane components.