A PH DEPENDENT COIL-TO-SHEET TRANSITION IN A PERIODIC ARTIFICIAL PROTEIN ADSORBED AT THE AIR-WATER-INTERFACE

External infrared reflectance spectroscopy allows in situ determinatio n of the conformational state of a periodic artificial protein adsorbe d at the air-water interface. The protein, which consists of 36 copies of an octapeptide sequence [(AG)(3)EG] constituted from alanine (A), glycine (G), and glutamic acid (E), adopts a random coil conformation in aqueous solution over a broad range of pH (5 < pH < 14). In contras t, the adsorbed chain undergoes a transition from a disordered (coil-l ike) state to a beta-sheet conformation as the pH of the subphase is r educed. The conformational transition is offset by ca. 4 pH units from the titration curve of the polymer in aqueous solution, suggesting th at the ionization behavior of the polymer is substantially perturbed b y interfacial effects. In situ determination of the conformation of th e adsorbed chain allows observation of a coil-to-sheet transition that is not observed in bulk solution.