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 determination of the conformational state of a periodic artificial protein adsorbed at the air−water interface. The protein, which consists of 36 copies of an octapeptide sequence [(AG)3EG] 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 contrast, the adsorbed chain undergoes a transition from a disordered (coil-like) state to a β-sheet conformation as the pH of the subphase is reduced. The conformational transition is offset by ca. 4 pH units from the titration curve of the polymer in aqueous solution, suggesting that the ionization behavior of the polymer is substantially perturbed by interfacial effects. In situ determination of the conformation of the adsorbed chain allows observation of a coil-to-sheet transition that is not observed in bulk solution.