The secondary structure of the organophosphorus acid anhydrolase (OPAA) Langmuir monolayer in the a
bsenceand presence of diisopropylfluorophosphate (DFP) in the su
bphase was studied
by infrared reflection-a
bsorption spectroscopy (IRRAS) and polarization-modulated IRRAS (PM-IRRAS). The results of
both theIRRAS and the PM-IRRAS indicated that the
-helix and the
beta2.gif" BORDER=0 ALIGN="middle">-sheet conformations in OPAA were parallelto the air-water interface at a surface pressure of 0 mN·m
-1 in the a
bsence of DFP in the su
bphase. Whenthe surface pressure increased, the
-helix and the
beta2.gif" BORDER=0 ALIGN="middle">-sheet conformations
became tilted. When DFP wasadded to the su
bphase at a concentration of 1.1 × 10
-5 M, the
-helix conformation of OPAA was stillparallel to the air-water interface, whereas the
beta2.gif" BORDER=0 ALIGN="middle">-sheet conformation was perpendicular at 0 mN·m
-1. Theorientations of
both the
-helix and the
beta2.gif" BORDER=0 ALIGN="middle">-sheet conformations did not change with the increase of surfacepressure. The shape of OPAA molecules is supposed to
be elliptic, and the long axis of OPAA was parallelto the air-water interface in the a
bsence of DFP in the su
bphase, whereas the long axis
became perpendicularin the presence of DFP. This result explains the decrease of the limiting molecular area of the OPAA Langmuirmonolayer when DFP was dissolved in the su
bphase.