IRRAS applications

Other IRRAS applications to peptides and proteins. In addition to the pulmonary surfactant system, a variety of other applications employing IRRAS to study peptide and protein conformation and orientation have appeared. The secondary structure conversion of the amyloid (prion)-protein in the normal form into the abnormal form is the main cause of several human and animal diseases, such as Alzheimer s disease [68]. The secondary structure of the first 40 residues of the amyloid protein was detected by circular dichroism (CD) in aqueous solution and with IRRAS at the interface. A stable /1-sheet-enriched state of the amyloid is formed at the air-water interface, in contrast to the initial bulk solution containing high a-helix/random coil and low /l-sheet parts. The change in the pH going from bulk (alkaline pH) to the interface (neutral or slightly acidic pH) can have effects on the conformation at the interface. Another alternative might be the intrinsic hydrophobicity of the air-water interface, which is a hydrophobic-hydrophilic system with air as the hydrophobic part. 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

A great deal of success was attendant on the early application of PM-IRRAS to the gas/solid interface. Golden et ai (1981) reported the development of instrumentation, using conventional dispersive optics, able to record detailed infrared reflection-absorption spectra from molecules adsorbed on single-crystal Pt without any interference from the gas-phase species. 

Two examples of the application of SERS and potential-difference IRRAS methods to the identification of adsorbed intermediates and reaction mechanism elucidation are also described, involving the catalytic electrooxidation of carbon monoxide and small organic molecules on transition-metal surfaces. 

Electrochemists have used IR spectroscopy for many years to probe electrodeelectrolyte interfaces 107). The most popular technique is IR reflection absorption spectroscopy (IRRAS) 108). A schematic comparison of the principle of ATR and IRRAS experiments is shown in Fig. 37. One advantage of the ATR over the IRRAS technique for catalytic applications concerns diffusion. In IRRAS experiments, the IR beam passes through a thin liquid film between a window and the sample. This... 

The applications of PM-IRRAS also include fatty acids, phospholipids, and protein conformations. Desbat and co-workers reported 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 (Cd2+, Ca2 +, Mg2 +, and Na+) with the help of the PM-IRRAS method [92]. Fig. 14 shows the PM-IRRAS spectra of Langmuir monolayer of deuterated arachidic acid in the presence of CdCb as a function of the subphase pH. At low subphase pH (pH = 3.5), the spectrum only presents absorption bands related to the acid form, i.e., the C = O stretching vibration (v(C = O)) and the OH bending (<5(0-H)) located at 1720 and 1270 cm respectively. The frequency position of the v(C = O) is characteristic of a hydrogen-bonded carbonyl group. As the subphase pH is increased, the arachidic acid is progressively deprotonated to... 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

IRRAS has become an important tool for studying Langmuir monolayers and LB films. Much work has been done, in particular on the acyl chain conformational order in monolayers of single chain amphiphiles and phospholipids as a function of surface pressure and on the occurrence of phase transitions . Examples of IRRAS studies on LB films can be found Reviews of the applications of IRRAS are given by Dluhy et al. and Mendelsohn et al. ). 

Reflection Absorption (RAIR or IRRAS). A significant development in infrared spectroscopy during the 1960s has been reported in journals not commonly read by either polymer chemists or analytical chemists. It has developed in the fields of electrochemistry and catalysis and is called reflection-absorption spectroscopy. Papers by Greenler (115-118). Yates (119). and Hansen (120. 121) describe the theory and some experimental data, and Boerio and Gosselin report on its applications to polymers (122). 

Figure 17.2.5 Block diagram for an IRRAS instrument with an FT spectrometer. The photoelastic modulator (PEM) is a crystal, such as ZnSe, whose refractive index can be changed by application of a strain by a piezoelectric transducer, thus modulating the radiation between s- and p-polarization. [Reprinted from J. K. Foley, C. Korzeniewski, J. L. Daschbach, and S. Pons, Electroanal Chem., 14, 309 (1986), by courtesy of Marcel Dekker, Inc.]...

The exceptionally low sensitivity of PM IRRAS to atmospheric CO2 and H2O vapor prompted interest in the application of this technique to the study of insoluble surfactant monolayers at the air/water interface in a Langmuir trough [27, 71-78]. These studies significantly advanced the PM IRRAS technique. 

Recently, Zawis2a et al. [84-86] and Bin et al. [87, 88] demonstrated that quantitative PM IRRAS has a very important application in biomimetic research. This technique provides unique information concerning potential-induced changes in the orientation and conformation of molecules in a model biological membrane supported at an electrode surface. This point is illustrated by the ap-phcation of PM IRRAS to study the stracture of a bilayer of DMPC formed at the Au(lll) electrode surface by fusion of unilamellar vesicles [87]. 

Spectroscopic reflectance methods are UV/vis reflectance spectroscopy and infrared reflection absorption spectroscopy (IRRAS) with several variations. For the application of these methods a mirror-Uke electrode surface is needed. This can be avoided if the scattered... 

PM-IRRAS data have been reported using both dispersive [86, 87] and FTIR spectrometers [88-90]. One of the earhest applications in electrochemistry was reported by Russell and coworkers [87], who employed the technique to substantiate the theory that the bipolar band observed by Beden and coworkers [71] in their EMIRS experiment on the electro-oxidation of methanol was due to Pt—G=0 shifting its frequency as the potential was altered. The absolute spectra obtained by the authors clearly showed the monopolar Pt—C=0 feature increasing in frequency as the potential of the Pt electrode was increased (Fig. 5). 

The IRRAS method can be used to obtain information about ultrathin films not only at metals but also on semiconductor and dielectric (including liquid) substrates. This class of problem is applicable to many areas, including thin-fihn optics, electronic and electroluminescent devices [27] (Chapter 5), sensors and transducers [28], flotation technology [29] (Section 7.4.4), and biomedical problems [30, 31]. Although the sensitivity is much lower than when metallic substrates are used, the waiving of the metal selection rule allows both s- and /7-polarized spectra to be measured and thus a more thorough investigation of molecular orientation within the layer. 

Figure 3.21 allows one to deduce the applicability of the thin-fihn approximation to IRRAS of an anisotropic inorganic layer at the ZnSe and Ge substrates. The deviation from the results of the exact formulas is greatest (up to 50% for a 200-nm-thick film) for the vlo band and the Ge substrate. 

When calculating band intensities in IR spectra of layers measured by either transmission or IRRAS, the models most often nsed to describe the reflection, refraction, and absorption assume an abrupt change in the optical properties at the layer-substrate interface. An exception is microscopic models that take into account the existence of a transition layer, but the relationships derived based on these model are too cumbersome to find wide application in the analysis of optical layers [41, 74, 75]. 

Theoretical and experimental IRRAS results of studying MO in monolayers at the AW interface have been reviewed by Mendelsohn et al. [67] and Horn [532]. Horn [532] has also discussed the IRRAS experimental data for SAMs, LB films, and small molecules on metals. The application of ATR to MO measurements has been considered by Mirabella [559] from a theoretical and practical point of view. Although polarized transmission spectroscopy at inclined angles of incidence is as sensitive as the ATR and IRRAS methods (Section 2.1), the former method has been used in very few MO studies [560-563]. Normal-incidence transmission measurements combined with metallic IRRAS has been exploited for the analysis of MO, first by Greenler [564] and then by others [247, 551, 565-568]. Yarwood et al. have proposed to use ATR in combination with metallic IRRAS [569] or the normal-incidence transmission method [570]. Ishino and Ishida [571] combined normal-incidence transmission and MO ATR. Below, these problems are discussed, mainly for long-chain molecules. 

Curve 3 in Fig. 4.35 shows that the IRRAS spectra of an epoxy resin film were only distinguishable from a sampling area of 40 x 40 fim. Despite the strong dependence of the reflectance from metals on the angle of incidence in the 60-85° range (Fig. 1.11), spectra measured by /r-IRRAS can be subjected to quantitative analysis [223]. Applications of /r-IRRAS to investigating inhomogeneous ultrathin films can be found in Refs. [200, 201, 206, 207, 225],... 

SNIFTIRS is able to provide detection limits for in situ IRRAS of 10 -10 " AE/E with a spectral resolution of 8-16 cm" and several hours of data collection [361]. In general, application of this technique is restricted to reversible electrochemical systems. However, flow cell tactics enable one to utilize this method even when examining irreversible Faradaic processes if the... 

Application of such ultrasensitive methods of FTIR spectroscopy as multiple transmission and IRRAS allows one to obtain the absorption spectra of ultrathin (several monolayers) layers of silicon dioxide [20, 22, 23, 32-37] which reveal a number of new absorption bands of the Si—O bond [9, 13, 37]. Questions... 

IRRAS studies of adsorption on metal surfaces began by Greenler [94, 95] and Low and McManus [96] in the 1960s. Since that time, IRRAS has become a relatively routine method that is used in numerous laboratories for investigating such phenomena as catalysis, corrosion inhibitors, self-assembly, and lubrication. Application of IRRAS to the study of adsorbed gases has been discussed in Refs. [25, 26, 97-121]. A summary can be found in Refs. [122-126]. 

IRRAS is the leading spectroscopy for characterizing the structure and properties of monolayers adsorbed at the air-water interface (Section 4.8). Research activity in this area [332-334] is largely motivated by the potential applications of transferred Langmuir (L) monolayers in molecular electronics and nonlinear optics and by fundamental interest in the organization and dynamics of quasi... 

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