Abstract
Surface potential and Infrared Reflection−absorption spectroscopy (IRRAS) measurements have been used to investigate the interaction between a peptidolipid (stearoyl−Phe−Trp−Ser−His−Glu) Langmuir monolayer and paraoxon aqueous solution as subphase. The molecular recognition in this system involved π−π interactions between the nitrobenzene group of paraoxon and aromatic groups in the peptidolipid. Of particular importance was the dependence of the surface potential on the concentration of paraoxon. Effects were practically negligible for 1.0 × 10-5 and 1.0 × 10-6 M paraoxon concentrations, but the Langmuir monolayer surface potential dropped due to the interaction with paraoxon at a concentration equal or higher than 1.0 × 10-4 M. At 1.5 × 10-3 M, the surface potential−area (ΔV−A) isotherm for the peptidolipid displayed an unusual shape, with an almost constant, near-zero surface potential during the monolayer compression. This was interpreted on the basis of IRRAS results as being due to reorientation of the benzene ring of paraoxon, which changed from parallel to the air−water interface in the absence of a monolayer to a tilted orientation upon interacting with the peptidolipid monolayer.
