Investigating Langmuir films at the air-water interface using a planar array infrared reflection-absorption spectrograph

In this work, a new planar array infrared reflection-absorption spectrograph (PA-IRRAS) was developed to investigate a broad range of Langmuir films at the air-water interface. This instrument is capable of recording sample and reference spectra simultaneously with an optical setup that is the same as that of a single-beam instrument but splits the incident infrared beam into two sections on a plane mirror (H) or a water trough. With this design, the instrument could accommodate large infrared accessories, such as a water trough. In addition, water bands were subtracted to obtain a high quality spectrum for a poly(lactic acid) (PLA) Langmuir film on the water subphase with a resolution of about 8 cm-1 in 10.8 sec.;With this instrument, two types of monolayer systems were studied; polymeric and lipid Langmuir films at the air-water interface. For the polymeric monolayer system, PA-IRRAS was used as a probe to follow the real-time conformational changes associated with intermolecular interactions of the polymer chains during the compression of the monolayers. It was found that the mixture of poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA) (D/L) formed a stereocomplex when the mixed solution developed the two-dimensional monolayer at the air-water interface. The stereocomplexation occurred before film compression, indicating that there is no direct correlation between film compression and stereocomplexation.;For the lipid monolayer system, PA-IRRAS was also used as a probe to investigate the origin of the disruption of a lipid monolayer upon protein adsorption at the air-water interface. Analysis of the time-resolved PA-IRRAS spectra revealed that Cu(II) ion-chelated DSIDA lipid monolayer (Cu 2+-DSIDA) was readily disrupted by myoglobin adsorption as demonstrated by a blue shift of 1.7 cm-1 and a lower intensity in the vas(CH2) stretch mode of the lipid monolayer over a period of five hours. To find the origin of the disruption of the lipid monolayer, a postulated model, employing a DSIDA monolayer-deposited ZnSe window, was investigated. An FT-IR spectroscopic study demonstrated that the Cu(II) ion formed stronger chelation with an iminodiacetatic acid (IDA) lipid head group than that formed with a Zn(II) ion. In addition, no distinct difference was observed in the secondary structures of myoglobin as myoglobin was adsorbed to Cu2+-DSIDA over a period of five hours. Dynamic light scattering (DLS) data revealed that, by the addition of Cu(II) or Zn(II) ion, lysozyme was rapidly aggregated and readily precipitated. However, the hydrodynamic volume of myoglobin was not responsive to the addition of Zn(II) ion. When Cu(II) ion was added, aggregation of myoglobin was sustained without precipitation over a period of five hours. Therefore, these results strongly suggest that the disruption of Cu2+-DSIDA lipid monolayer upon myoglobin adsorption is due to myoglobin aggregation, mediated by the chelated Cu(II) ion, rather than a conformational change in adsorbed myoglobin.;Besides the above monolayer systems, PA-IRRAS is used for the rapid detection of a low concentration of aqueous species. The previous designs for a PA-IR spectrograph were not applicable to detect a low concentration of aqueous species due to the contribution from a stray light and high relative humidity (in the vicinity of 25-40%). To overcome this problem, newly designed PA-IRRAS optical setup was purged with dry nitrogen gas to keep the relative humidity at approximately 15%. In addition, baffles constructed from corrugated cardboard were placed throughout the optical setup to prevent any stray light from reaching the detector. The PA-IRRAS results obtained from poly(N-isopropylacrylamide) (PNIPAM) revealed that solutions down to a concentration of 0.005% w/w could be successfully studied. These results are quite remarkable, given the acquisition time of only 10 seconds and the direct overlap of the Amide I band of PNIPAM and the H-O-H stretch of H2O. (Abstract shortened by UMI.)...