In monolayers at the air-water interface

Chirality and molecular recognition in monolayers at the air-water interface, 28,45 CIDNP and its applications, 10, 53 Conduction, electrical, in organic solids, 16,159... 

Molecular recognition, chirality and, in monolayers at the air-water interface, 28, 45 Molecular structure and energy, calculation of, by force-field methods, 13,1... 

Chirality and Molecular Recognition in Monolayers at the Air-Water Interface... 

Since its first important flowering in the hands of purely physical chemists, interest in monolayers at the air-water interface has waxed and waned with a frequency of roughly 25 years. The first resurrection of interest came from biochemistry, primarily during the 1955-65 decade as phospholipid monolayers were studied as models for the cell membrane (see, for example, Chapman, 1968). This is still a very productive field of biophysical research. 

Unlike electron and scanning tunneling microscopy, the use of fluorescent dyes in monolayers at the air-water interface allows the use of contrast imaging to view the monolayer in situ during compression and expansion of the film. Under ideal circumstances, one may observe the changes in monolayer phase and the formation of specific aggregate domains as the film is compressed. This technique has been used to visualize phase changes in monolayers of chiral phospholipids (McConnell et al, 1984, 1986 Weis and McConnell, 1984 Keller et al., 1986 McConnell and Moy, 1988) and achiral fatty acids (Moore et al., 1986). 

This chapter discusses stereospecific intermolecular interactions in monolayers at the air-water interface, where surface-active molecules (surfactants) are partially oriented with respect to each other by the cooperative combination of interionic, hydrophobic, and hydrogenbonding forces. We believe that these reports should be of particular interest in relation to stereospecificity in assemblies such as micelles, vesicles, and bilayer membranes, where their significance has been largely ignored. 

Photochemical processes in monolayers at the air-water interface can be controlled externally by variation of the various parameters like matrix composition, subphase composition, temperature and surface pressure. When the product of the reactions has a different area per molecule, the surface pressure may change at constant monolayer area. An interfacial shock wave has been generated in this way. This technique permits the investigation of the kinetics of reorganization processes and the transmission of mechanical signals in monolayers. 

A small but significant influence on the packit in monolayers at the air/water interface was found when dextran sulfate 24, which has a much larger distance between the ionic sites, is used with azobenzene amphiphile 21 (n = 10). A large effect was found for complexes of an azobenzeme fatty acid 22 (n = 3) complexed with polylallylamine) 26 however. In this case, the area per amphiphile increased from 0.28 nm to 0.39 nm. This increase... 

Chirality and molecular recognition in monolayers at the air-water interface,... 

On the subject of spontaneous generation of chirality, it is of interest to know that spontaneous formation of chiral aggregates from nonchiral monomers is known to occur, e.g. the assembly of tetra-alkyl benzimidocyanins 3 as monitored by CD (circular dichroism). Formation of chiral crystals from achiral monomers is also reported, e.g. by photodimerization in the solid state. " In a recent example, chiral crystals of acridine 4 and diphenylacetic acid 5 give excess of the (.S)-product 6 upon a photodecarboxylating condensation reaction. Symmetry breaking is also known to occur for supramolecular complexes of achiral components e.g. glu-tarimide 7 and the diaminopyridine 8, and, as will be discussed below, in monolayers at the air-water interface. ... 

Early studies on monolayers of chiral molecules like 2-hydroxyalkanes, amphiphilic amino acids, 2-methylhexacosanoic acid esters, and hydroxy-hexadecanoic acid and its esters have been reviewed. The interesting question about monolayers of chiral molecules is whether the parameters which can be determined and the phase transitions are different for pure enantiomers and racemates. For components of biomembranes like phosphatidylcholines 10 this appears not to be the case," but for synthetic compounds like iV-(a-methylbenzyl-stearamide) 11 specific interactions between the molecules of the enantiomers are observed (Chart 2). ° In recent years, advanced techniques have been developed to probe the order in monolayers at the air-water interface, including surface X-ray diffraction, and microscopic techniques, viz. fluorescence microscopy, and Brewster angle microscopy (BAM). The X-ray diffraction technique has been used to identify homochiral and heterochiral two-dimensional domains in mono-layers of racemic amphiphilic amino acids on subphases containing glycine. Fluorescence microscopy requires the introduction in the monolayer of a small... 

Brooks CF, Fuller GG, Frank CW, Roberston CR (1999) An interfacial stress rheometer to study rheological transitions in monolayers at the air-water interface. Langmuir 15 2450-2459... 

Thermodynamic parameters for the mixing of dimyristoyl lecithin (DML) and dioleoyl lecithin (DOL) with cholesterol (CHOL) in monolayers at the air-water interface were obtained by using equilibrium surface vapor pressures irv, a method first proposed by Adam and Jessop. Typically, irv was measured where the condensed film is in equilibrium with surface vapor (V < 0.1 0.001 dyne/cm) at 24.5°C this exceeded the transition temperature of gel liquid crystal for both DOL and DML. Surface solutions of DOL-CHOL and DML-CHOL are completely miscible over the entire range of mole fractions at these low surface pressures, but positive deviations from ideal solution behavior were observed. Activity coefficients of the components in the condensed surface solutions were greater than 1. The results indicate that at some elevated surface pressure, phase separation may occur. In studies of equilibrium spreading pressures with saturated aqueous solutions of DML, DOL, and CHOL only the phospholipid is present in the surface film. Thus at intermediate surface pressures, under equilibrium conditions (40 > tt > 0.1 dyne/cm), surface phase separation must occur. 

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