Infrared Reflection Absorption Spectroscopy of Monolayers at the Air-Water Interface

Infrared Reflection Absorption Spectroscopy of Monolayers at the Air-Water Interface 

Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, FL 33124-0431, USA 

There has been considerable interest in the characterization of the conformation and orientation of amphiphilic molecules in Langmuir monolayers in the past few decades [1-5]. Monolayers at the air-water interface are widely employed as convenient experimental paradigms that mimic many vital biological processes. The advantages realized when using monolayers as models for biological interfaces arise primarily from the ease with which experimental variables may be manipulated. These include parameters that are not readily controlled in bulk phases or in films prepared on solid substrates, such as lateral pressure, surface area, and domain size and shape [6]. 

Ellipsometry and nonlinear optics have proven their sensitivity in studying monolayer thickness and molecular orientation, but generally the anisotropic nature of the interface makes results rather difficult to interpret [20-22], In many cases, these methods were limited in their potential to supply information about biophysical issues such as head group structure, enzyme secondary structure, and the orientation of ordered regions. 

This review includes the structural information that has been carried out to date from films of single chain amphiphiles, enzymes, and proteins using both IRRAS and PM-IRRAS methods. 

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Baekmark TR, Wiesenthal T, Kuhn P, Albersdorfer A, Nuyken O, Merkel R (1999) A systematic infrared reflection-absorption spectroscopy and film balance study of the phase behavior of lipopolymer monolayers at the air-water interface. Langmuir 15 3616-3626... 

Chapter 1 summarizes methods for the stabilization of artificial lipid membranes. They include synthesis of new types of polymerizable lipids and polymerization of membranes. Creation and characterization of novel poly(lipid) membrane systems, as well as their functionalization for biotechnological applications, are also described. Chapter 2 addresses experimental studies on the design and characterization of lipopolymer-based monolayers at the air-water interface. Thermodynamic and structural data collected with X-ray and neutron reflectrometry, infrared reflection absorption spectroscopy, and sum frequency generation spectroscopy provide... 

Films are also characterized [129,132] with FT-IR spectroscopy and it is customary to use transmission and infrared reflection absorption spectroscopy (IR-RAS) to study the orientation. The films are prepared on a modified Lauda film balance and the thickness as well as conductivity depends on the mole ratio of monomers in the dispersing solvent. Varying this ratio results in the formation of films at the air-water interface ranging in thickness from 1 monolayer to multilayers. The most highly conducting films are produced with mole ratios of pyrrole to 3-octadecyl-pyrrole of 5,000 1. 

Mendelsohn, R., Braunei J.W., and Gericke, A. (1995) External infrared reflection- absorption spectroscopy of monolayer films at the air/water interface. 

Most of the above membrane-oriented studies were carried out for peptides in multilayer systems that were collapsed or transferred onto a sample cell surface. An alternative and very interesting way to study membrane systems is by IRRAS (infrared reflection absorption spectroscopy) at the air-water interface. In this way, unilamellar systems can be studied as a function of surface pressure and under the influence of various membrane proteins and peptides added. Mendelsohn et al.[136] have studied a model series of peptides, [K2(LA) ] (n = 6, 8, 10, 12), in nonaqueous (solution), multilamellar (lipid), and unilamellar (peptide-IRRAS) conditions. In the multilamellar vesicles these peptides are predominantly helical in conformation, but as peptide only monolayers on a D20 subphase the conformation is (1-sheet like, at least initially. For different lengths, the peptides show variable surface pressure sensitivity to development of some helical component. These authors further use their IR data to hypothesize the existence of the less-usual parallel (i-sheet conformation in these peptides. A critical comparison is available for different secondary structures as detected using the IRRAS data for peptides on H20 and D20 subphasesJ137 ... 

Alpers W and Hiihnerfuss H (1989) The damping of ocean waves by surface films a new look at an old problem. J Geophys Res 94 6251-6265 Benvegnu DJ and McConnell HM (1992) Line tension between liquid domains in lipid monolayers. J Phys Chem 96 6820-6824 Gericke A, Michailov AV, and Hiihnerfuss H (1993) Polarized external infrared reflection-absorption spectroscopy at the air/water interface comparison of experimental and theoretical results for different angles of incidence. Vib Spectroscop 4 335-348... 

Figure 9 Optical setup for external reflection infrared measurements of Langmuir-Blodgett monolayers on liquid water surfaces. The arrow indicates the shuttling direction of the Langmuir trough in order to switch between the sample (monolayer on water) and the reference (pure water) position. (Reprinted with permission from Flach CR, Gericke A, and Mendelsohn R (1997) Quantitative determination of molecular chain tilt angles in mono-layer films at the air/water interface infrared reflection/absorption spectroscopy of behenic acid methyl ester. Journal of Physical Chemistry S 101 58-65 American Chemical Society.)...

Fourier transform reflection-absorption infrared (RA-IR) spectroscopy is used to probe the structure and properties of sodium dodecyl sulfonate (C12S) monolayers that are self-assembled from dilute solution at an air-water interface. Recent optical models for the interpretation of signal intensity measurements are briefly reviewed. The methylene stretching peaks of C12S monolayers in the RA-IR spectra are used to determine die chain orientation, the surface concmtration and the conformational state of the alkyl chains. Conqiarisons are drawn between monolayers and C12S crystals. A phase transition is found as the concentration of C12S in the subphase below the monolayer is reduced. The effect of salt on the monolayers is presented. The infrared data is interpreted in terms of the surface tension behavior. 

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