Surface chemistry and spectroscopic studies of the peptidolipids and proteins Langmuir monolayer

There was a specific interaction between paraoxon and the peptidolipid, C18H35O (Stearoyl)-Phe-Trp-Ser-His-Glu. The aromatic residue groups were involved in the recognition between paraoxon and the peptidolipid, because the fluorescence of Trp in the peptidolipid at 351 nm was quenched by paraoxon in the subphase. When paraoxon was dissolved in the subphase, the surface potential-area (DeltaV-A) isotherm for the peptidolipid A displayed an unusual shape. This was interpreted on the basis of Infrared Reflection-Absorption Spectroscopy (IRRAS) results as being due to the reorientation of the benzene ring of paraoxon. The secondary structure of organophosphorus acid anhydrolase (OPAA) Langmuir monolayer in the absence and presence of diisopropylfluorophosphate (DFP) in the subphase was also studied by the IRRAS and Polarization Modulated-IRRAS (PM-IRRAS). 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 OPAA Langmuir monolayer when DFP was dissolved in the subphase. Using the Langmuir monolayer technique, the surface properties of aequorin were studied. The results showed that aequorin formed a stable Langmuir monolayer and the surface pressure-area isotherms were dependent on both pH and ionic strength. At a pH higher or lower than 7.6, the limiting molecular area decreased. The addition of calcium chloride to the Tris/HCI buffer subphase (pH 7.6) caused an increase of the limiting molecular area of the aequorin Langmuir monolayer. The fluorescence spectra of a Langmuir-Blodgett (LB) monolayer of aequorin in the presence of calcium chloride indicated that the aequorin transformed to the apoaequorin. The Langmuir monolayer of aequorin and apoaequorin was studied by IRRAS and PM-IRRAS techniques. The different behavior of aequorin and apoaequorin at the air-water interface was explained by the fact that aequorin formed dimers at air-water interface but apoaequorin was monomer. It was more difficult for a dimer to be tilted by the compression of the Langmuir monolayer.