Chiral monolayers

The following sections describe examples of the application of these approaches to the study of chiral molecular recognition in thin film systems, using the battery of techniques outlined above. 

Our first investigations of the stereospecific aggregation of molecules in a monolayer involved the use of a novel chiral surfactant, AT-(a-methylbenzyl)stearamide, spread on aqueous acid subphases (Arnett and Thompson, 1981 Arnett et al, 1982). This surfactant was chosen for study because of the potential for strong hydrogen bonding between enantiomers, which should in theory yield closely packed aggregates in a film system. 

In the bulk crystalline phases, large differences exist in the properties of the racemic mixture and the pure enantiomers. X-ray powder diffraction patterns showed that the racemic mixture was a true racemate, and the melting transition points and heats of fusion of the racemate were markedly different from those of the pure enantiomers [which were identical (Arnett and Thompson, 1981)]. 

Enantiomeric recognition was clearly displayed in films spread from solution and films in equilibrium with their crystals, and was sharply dependent on the acidity of the subphase. Protonation of the amide group appeared to be necessary for spreading to stable monolayers. For example, the crystals of the racemate deposited on a 10n H2S04 solution at 25°C spread quickly to yield a film with an ESP of 7.7 dyn cm 1, while the single enantiomers spread only to a surface pressure of 3.9 dyn cm-1 (Table 1). Similar effects are observed at 15 and 35°C. The effect of stereochemistry on equilibrium spreading is even more pronounced at lower subphase acidities. On 6n sulfuric acid, the racemate spread to an equilibrium surface pressure of 4.9 dyn cm-1, while the enantiomeric systems spread to less than 1 dyn cm-1. 

When spread from dilute hexane solution, acid-dependent enantiomeric discrimination was observed in the 11/A compression isotherms of the monolayer at 25°C (Fig. 12). It is interesting to note that at higher subphase acidities, both racemic and enantiomeric film systems become more highly expanded, and the surface pressures where enantiomeric discrimination commences occur at high (85-90 A2/molecule) average molecular areas. This may be taken as direct evidence of headgroup ionization effects. The surface 

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From the perspective of physical organic chemistry, the study of chiral monolayers is a classic example of how the purposeful interaction of two fields that were separated by their traditional identifications with organic and physical chemistry can help to illuminate both areas. 

The implications for films cast from mixtures of enantiomers is that diagrams similar to those obtained for phase changes (i.e., melting point, etc.) versus composition for the bulk surfactant may be obtained if a film property is plotted as a function of composition. In the case of enantiomeric mixtures, these monolayer properties should be symmetric about the racemic mixture, and may help to determine whether the associations in the racemic film are homochiral, heterochiral, or ideal. Monolayers cast from non-enantiomeric chiral surfactant mixtures normally will not exhibit this feature. In addition, a systematic study of binary films cast from a mixture of chiral and achiral surfactants may help to determine the limits for chiral discrimination in monolayers doped with an achiral diluent. However, to our knowledge, there has never been any other systematic investigation of the thermodynamic, rheological and mixing properties of chiral monolayers than those reported below from this laboratory. 

Although the standard II / A isotherms of chiral monolayers can disclose much information on stereospecific packing arrangements, they do not allow... 

The instability of these chiral monolayers may be a reflection of the relative stabilities of their bulk crystalline forms. When deposited on a clean water surface at 25°C, neither the racemic nor enantiomeric crystals of the tryptophan, tyrosine, or alanine methyl ester surfactants generate a detectable surface pressure, indicating that the most energetically favorable situation for the interfacial/crystal system is one in which the internal energy of the bulk crystal is lower than that of the film at the air-water interface. Only the racemic form of JV-stearoylserine methyl ester has a detectable equilibrium spreading pressure (2.6 0.3dyncm 1). Conversely, neither of its enantiomeric forms will spread spontaneously from the crystal at this temperature. 

Mixed chiral monolayers special systems differentiated only by symmetry... 

The methyl esters of stearoylalanine [1] and stearoylserine [2] were considered as quasi-racemate candidates because of their slight structural differences. No quasi-racemate behavior was observed, however, in their force-area isotherms although clear diastereomeric discrimination was seen for this combination (Verbiar, 1983). We have seen no indication of quasi-racemate behavior for any other mixed chiral monolayers. 

Lundquist and the Stenhagens concentrated their efforts on the physical aspects of monolayer chemistry and did not elaborate then-work much in the direction of structural variation of the surfactant molecules. Their results show clearly, however, that the response of chiral monolayers to changes in surface pressure and temperature is sharply dependent on both the molecular structure of the surfactant and the optical purity of the sample. The Stenhagens were keenly aware of the possible application of the monolayer technique to stereochemical and other structural problems (72) however, they failed to exploit the full potential suggested by their initial results and, instead, pursued the field of mass spectrometry, to which they made substantial contributions. 

An interesting possibility raised by this experiment is that of spontaneous resolution in chiral monolayers. If this were a reasonably common phenomenon, it would give yet another possible answer to the perennial question of the chiral environment for the primordial stereospecific condensation reaction that produced the first chiral biopolymers. As our knowledge of chiral monolayers develops, we should have a better perspective on the likelihood of a racemic film spontaneously unmixing to produce patches of enantiomeric film at lower surface energy. The relevance of such a result to the origin of terrestrial life problem will have to remain eternally speculative and untestable. 

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