Polarization-Modulated FT-IR Spectroscopy Studies of Acetylcholinesterase Secondary Structure at the Air−Water Interface

Abstract

The vibrational spectrum of acetylcholinesterase (AChE) at the air−water interface in its free form and bound to either its substrate, acetylthiocholine, or organophosphorus (OP) inhibitor has been studied by polarization modulation infrared reflection absorption spectroscopy (PMIRRAS). The shape and position of the amide I band was used to gauge the surface orientation of α-helices and β-sheets. The measured secondary structure content indicated that the enzyme did not unfold for the surface pressures used (0−30 mN/m). At low surface pressures, a strong amide I band indicated that the average tilt axis of the helices was aligned parallel to the air−water interface. Upon further compression, the α-helix component was significantly reduced, because the tilt axis of the helix relative to the water surface achieved a perpendicular orientation. PMIRRAS was also used to investigate the effect of phospholipids on molecular organization and orientation of AChE at the air−water interface. The enzyme was found to ...