The secondary structure of the organophosphorus acid anhydrolase (OPAA) Langmuir monolayer in the absence and presence of diisopropylfluorophosphate (DFP) in the subphase was studied by infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated IRRAS (PM-IRRAS). The results of both the IRRAS and the PM-IRRAS indicated that the a-helix and the β-sheet conformations in OPAA were parallel to the air-water interface at a surface pressure of 0 mN·m-1 in the absence of DFP in the subphase. When the surface pressure increased, the a-helix and the β-sheet conformations became tilted. When DFP was added to the subphase at a concentration of 1.1 × 10-5 M, the α-helix conformation of OPAA was still parallel to the air-water interface, whereas the β-sheet conformation was perpendicular at 0 mN·;m-1. The orientations of both the a-helix and the β-sheet conformations did not change with the increase of surface pressure. The shape of OPAA molecules is supposed to be elliptic, and the long axis of OPAA was parallel to the air-water interface in the absence of DFP in the subphase, whereas the long axis became perpendicular in the presence of DFP. This result explains the decrease of the limiting molecular area of the OPAA Langmuir monolayer when DFP was dissolved in the subphase.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry