We report on the variation of the dissociation of a Langmuir monolayer of arachidic acid at the air/water interface as a function of the subphase pH and for several cations (Cd 2+ , Ca 2+ , Mg 2+ , and Na + ) with the help of the polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) method. The infrared spectra give access to the relative concentration of acid and salt molecules and allow us to determine the influence of the subphase pH on the acid dissociation reaction for each cation. It is shown that Na + obeys the purely electrostatic Gouy-Chapman theory quite well, whereas the behavior of Mg 2+ , and even more so that of Ca 2+ and Cd 2+ , requires the introduction of some complexation constant to be understood.

Pure α-helices and antiparallel β-sheets were obtained using poly-γ-benzyl-l-glutamate (PBG) and model synthetic peptide K(LK)7, respectively. Monolayers of these polypeptides were transferred by the Langmuir−Blodgett technique on calcium fluoride plates and gold mirrors. The optical constants (index of refraction and extinction coefficient) were determined in the space coordinate system from normalized polarized transmittance and reflectance spectra. Considering the symmetry of the two secondary structures, the anisotropic optical constants were calculated in the molecular coordinate system of the α-helix and the β-sheet. From anisotropic components of the extinction coefficients, oscillator strengths of amide I and amide II modes have been calculated for the two secondary structures. These data give important information on the relative intensities of the amide I and amide II modes; moreover, for PBG the angle between the transition moment of the amide I mode and the helix axis has been estimated to be 34−38°. Finally, on the basis of these anisotropic optical constants, PM-IRRAS spectra of a single monolayer at the air/water interface were simulated for the two secondary structures.

Spectral simulations of polarized attenuated total reflection (ATR) spectra have been used to quantitatively determine the orientation of R-helical polypeptides. Transmittance and polarized ATR spectra of five monolayers of poly-γ-benzyl-L-glutamate (PBG) transferred by the Langmuir-Blodgett techniques on germanium crystal have been recorded, and the dichroic ratios of the amide I and amide II bands have been calculated. Simulations were performed using anisotropic optical constants of PBG in the molecular coordinate system and using the 4 × 4 matrix formalism of Berreman. Assuming that the R-helices are parallel to the plane of the ATR crystal, an average angle of 38°( 1°between the helix axes and the withdrawing direction has been found. Simulated spectra for various orientations of the R-helices are then given and influence of the azimuthal and tilt angles on the dichroic ratios of the amide bands has been investigated.

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