The guest-induced oscillation of a monolayer composed of polypeptide containing beta-cyclodextrin at the terminal

We prepared a rod-like amphiphile with a molecular recognition end group, a lpha-helical and hydrophobic poly(gamma-methyl L-glutamate) (PMG) containin g hydrophilic beta-cyclodextrin (CyD) as an active end group (PMG-CyD), and formed its monolayer at the n-hexane/water interface. The interfacial pres sure (pi)-area (A) isotherms of the monolayer showed that alpha-helix rod o f PMG-CyD could be vertically oriented at the oil/water interface, facing t he hydrophilic terminal CyD group to the water phase, by increasing the int erfacial concentration of the polypeptide. Under the condition 2-p-toludiny l-naphthalene-6-sulfonate (TNS), an intimate guest molecule for the CyD in water was introduced into the water phase beneath the monolayer. Within a m inute the monolayer began to oscillate which could be monitored by the rhyt hmic response of the interfacial pressure of the monolayer. The oscillation continued over ten minutes and then terminated. The mode of the oscillatio n was found to change with time, i.e., the initial stage showing a periodic sharp reduction in the interfacial pressure (period I), the second stage h aving sharp increase in the pi value (period II), and the last stage of irr egular oscillations (period III). The Fourier analysis of each period also supported the three stages during the oscillatory process. It was also foun d that when the alpha-helix rod of PMG-CyD lay down in the monolayer, the g uest TNS did not induce any changes in the interfacial tension. This nonlin ear rhythmic interfacial phenomenon was explained in terms of the periodic movement of the PMG-CyD monolayer resulting from the binding and releasing of the guest TNS across the oil/water interface. (C) 1999 American Institut e of Physics. [S1054-1500(99)00402-4].