Daniel Blaudez

Investigations at the air/water interface using polarization modulation IR spectroscopy

The ability of polarization modulation IR reflection absorption spectroscopy (PM–IRRAS) to study the air/water interface is presented. A brief description of the set-up and of the experimental procedure is given. Theoretical simulations accounting for the uniaxial nature of the spread monolayer lead to optimum experimental conditions (71 ° for the angle of incidence) and to a specific surface selection rule. Application to the study of cadmium arachidate, dimyristolyl phosphatidylcholine (DMPC) and polypeptidic Langmuir films illustrates the potential uses of this method.

Polarization modulation FTIR spectroscopy at the air-water interface

Abstract Mid-infrared spectra of monolayers spread at the air-water interface have been obtained, completely devoid of strong water vapor absorptions, using polarization modulation infrared reflexion absorption spectroscopy (PM-IRRAS). On normalized difference (covered vs. uncovered water) PM-IRRAS spectra, the monolayer absorption bands appear upwards or downwards depending on the orientation of their transition moment with respect to the water surface. Vibrational modes of the water subphase contribute to these difference spectra as broad dips. Study of a monolayer of cadmium arachidate has allowed observation of the vibrational modes of the polar heads and provides some evidence of their symmetrical anchoring at the water surface. Under surface compression, a monolayer of deuterated arachidic acid undergoes a molecular reorganization leading to a better ordering of the deuterated chains.

Characterization of Langmuir-Blodgett monolayers using polarization modulated FTIR spectroscopy

Abstract Structural and conformational analysis of cadmium arachidate monolayers deposited on a gold substrate has been performed on the basis of the well-defined spectra obtained by polarization modulated infrared reflection spectroscopy. This quantitative analysis gives accurate values for the tilt and twist orientations of the arachidate chains relative to the metallic substrate in agreement with a high organization of the LB molecules that decreases as the film goes to a single layer and gauche conformations appear.

The behavior of membrane proteins in monolayers at the gas-water interface : comparison between photosystem II, rhodopsin and bacteriorhodopsin

Abstract It has been postulated, without supporting evidence, for decades that proteins are denatured once spread in monolayers at the gas–water interface. In the present study, the effect of different experimental conditions on the structure of three membrane proteins has been investigated by polarization modulated infrared reflection absorption spectroscopy in situ in monolayers at the gas–water interface. We have found that photosystem II core complex (PS II CC) is less sensitive to denaturation than rhodopsin. In fact, denaturation of rhodopsin could only be prevented when spreading was performed at 4°C. In contrast, bacteriorhodopsin was found to remain native when monolayer spreading was performed in conditions that were found to denature both PS II CC and rhodopsin. This behavior may be explained by the two-dimensional crystalline structure of bacteriorhodopsin. In conclusion, conditions can be found where the native structure of membrane proteins is maintained after their spreading in monolayers at the gas–water interface.

Organization in pure and alternate deuterated cadmium arachidate monolayers on solid substrates and at the air/water interface studied by conventional and differential Fourier transform infrared spectroscopies

The molecular organization in monolayers of deuterated cadmium arachidate deposited on solid metallic and dielectric substrates or spread at the air/water interface is investigated using different techniques of Fourier transform infrared spectroscopy. For metals, the polarization modulated reflectivity shows that the molecules have a high molecular order and are oriented quasi perpendicularly with respect to the surface. The in‐plane organization is determined by complementary transmission experiments on CaF2 plates. The CD2 bending vibration in mono‐ and multilayer Langmuir–Blodgett stacks reveals the hexagonal packing of a single monolayer irrespective of the hydrophilic or hydrophobic character of the substrate. Probing the stack organization using selective monolayer deuteration in hydrogenated Langmuir–Blodgett films shows that the first deposited monolayer of an odd‐numbered stack keeps its hexagonal structure while additional bilayers adopt an orthorhombic packing induced by the strong polar head/h...

Infrared and Raman spectroscopies of monolayers at the air–water interface

Abstract Infrared and Raman spectroscopies are now currently used to obtain molecular information (orientation, conformation, organization) on monolayers at the air–water interface. In the past year, several original studies were performed on peptides and proteins and their interaction with phospholipidic monolayers.

eSpectroscopic and Structural Properties of Valine Gramicidin A in Monolayers at the Air-Water Interface

Monomolecular films of valine gramicidin A (VGA) were investigated in situ at the air-water interface by x-ray reflectivity and x-ray grazing incidence diffraction as well as polarization modulation infrared reflection absorption spectros- copy (PM-IRRAS). These techniques were combined to obtain information on the secondary structure and the orientation of VGA and to characterize the shoulder observed in its -A isotherm. The thickness of the film was obtained by x-ray reflectivity, and the secondary structure of VGA was monitored using the frequency position of the amide I band. The PM-IRRAS spectra were compared with the simulated ones to identify the conformation adopted by VGA in monolayer. At large molecular area, VGA shows a disordered secondary structure, whereas at smaller molecular areas, VGA adopts an anti-parallel double-strand intertwined 5.6 helical conformation with 30° orientation with respect to the normal with a thickness of 25 A. The interface between bulk water and the VGA monolayer was investigated by x-ray reflectivity as well as by comparing the experimental and the simulated PM-IRRAS spectra on D2O and H2O, which suggested the presence of oriented water molecules between the bulk and the monolayer.

Raman spectroscopy of LB films and monolayers at the air-water interface

Unenhanced Raman spectroscopy has so far been very rarely applied for studying monolayer organization although this method should in principle bring very rich molecular information. Moreover it is probably one of the most convenient technique for in-situ experiments, especially at the air-liquid interface. Recent technical developments (N2-cooled CCD detector, notch holographic filter, confocal microscopy, etc.) have been implemented in our laboratory in order to design an ultrasensitive Raman spectrometer able to study a single monolayer deposited on a solid substrate or spread at the air-water interface. Test Raman spectra concerning mainly hydrogenated and deuterated cadmium arachidate multi- and monolayers deposited on a CaF2 plate and a monolayer spread at the air-water interface are presented. Finally all these results are gathered and discussed in terms of molecular organization and hydrocarbon chain packing in the monolayer.

Anisotropy in Langmuir–Blodgett films studied by generalized spectroscopic ellipsometry

Abstract Langmuir–Blodgett films obtained by successive deposition of a known number of fatty-acid monolayers are generally assumed to be uniaxial with in-plane isotropy. In order to test the validity of this assumption, we measure the generalized ellipsometric response of such quasi-bidimensional molecular systems as a function of their azimuthal orientation θ relative to the incident plane. These measurements which are related to the non-diagonal terms of the Jones reflection matrix provide information on the orientation of the principal axes and the principal indices of the dielectric tensor of the film. Using a multilayer anisotropic model based on the Berreman formalism, regression of the experimental data gives the following results. The studied LB films are biaxial with one of the principal axis tilted at 1.5° with respect to the normal of the film and the in-plane birefringence varies from 2×10−2 to 1×10−2 over the 0.24–0.6 μm spectral domain. This in-plane anisotropy is connected to the withdrawing direction of the substrate during the LB film deposition.

PM-IRRAS at liquid interfaces

Publisher Summary It is well known that the analysis of ultrathin films deposited on a metallic substrate by infrared reflection absorption spectroscopy (IRRAS) is readily realizable, thanks to the surface electric field enhancement. Surface electric field anisotropy gives a selection rule that allows easy determination of the orientation of the molecular groups. On the other hand, when the substrate is dielectric, surface field enhancement does not exist anymore and the surface electric field has components both on the surface plane and perpendicular to this plane. These features result in much weaker and less readable IRRAS spectra. Obtaining good IRRAS spectra of spread monolayers is possible but requires long acquisition duration and accurate control of vapor pressure during sample and reference (uncovered water) spectrum recordings. To overcome these problems, a differential IR reflectivity technique has been developed known as PM-IRRAS. PM-IRRAS combines Fourier transform mid-IR reflection spectroscopy with fast polarization modulation of the incident beam and with two-channel electronic and mathematical processing of the detected signal. Optimization of the PM-IRRAS detection of the surface absorptions of an ultrathin film deposited on a dielectric surface is nontrivial. The main disadvantage of PM-IRRAS with respect to classical reflection absorption spectroscopy is the complexity of both the optics and electronics of the setup. The advantage of PM-IRRAS is the relative simplicity and the efficiency of its surface selection rule.