Chiral discrimination in the monolayer packing of N-(.alpha.-methylbenzyl)stearamides on aqueous acid subphases: thermodynamic behavior

Abstract

The study of monolayers at the air-liquid interface offers unusual opportunities to investigate the effects of stereochemical factors in organized systems. As a first step toward developing this field, we have examined some surface properties of 7V-(a-methylbenzyl)stearamide, comparing the behavior of monolayer films of its racemic modification with its two enantiomeric forms and also with the achiral position isomer TV-phenethylstearamide. X-ray powder diffraction patterns showed that the racemic material was, in fact, a true racemate. Also, the transition points and heat of fusion of the racemate were markedly different from those of the enantiomers, as shown by differential scanning calorimetry. Although the stearamides did not spread as monolayers on water as a subphase, aqueous sulfuric acid media above 6 N evoked good surfactant behavior as manifested by reduction of the subphase surface tension. Chiral discrimination was clearly apparent in the monolayer behavior and was sharply acid dependent. Thus, the racemic A-(a-methylbenzyl)stearamide began to show surface activity on 6 N H2S04 while the enantiomers required 8 N acid to give the same response. Equilibrium spreading pressures (ESP's) for the films in equilibrium with the pure crystals were measured on a Langmuir film balance as a function of subphase acidity and temperature. Thermodynamic properties for the surface films were evaluated from these ESP values and also from static surface tension measurements made at different temperatures. By all criteria, the molecules of the optically pure samples are more strongly and completely associated, both in the crystalline and in the monolayer phase, than are those of the racemate. We propose that the acid dependence of chiral discrimination between enantiomeric and racemic films is caused by protonation of the amide head group by the aqueous acid subphase. This produces a driving force for expansion of the film which is opposed by the aggregation forces in the neutral amide molecules. These forces are stronger for the enantiomers than for the racemate.As a test of this model, a series of liquid-liquid distribution experiments was performed for the transfer of A-(a-methylbenzyl)butyramide from hexane to the series of aqueous acid solutions used for monolayer subphases. A close correlation was found between the acid dependence of the free energy of spreading for the stearamide and the free energy of transfer of the butyramide. The pXBH+ of the butyramide was calculated to be -1.5 ± 0.5 from the distribution experiments and -1.5 ± 0.2 from an independent series of indicator measurements. The free energy correlation suggests that the pXBH+ of the protonated stearamide at the air-acid interface is close to that of the butyramide in homogeneous solution. The force-area behavior and dynamic properties of the monolayers will be reported subsequently. This paper is limited to a description of the thermodynamic properties under bona fide reversible conditions.

Homochiral Recognition among Organic Molecules on Copper(110)

“…14 It has been predicted that 2D chiral resolution on a surface should occur more easily than in three-dimensional crystals. 15 Due to confinement in the plane, certain symmetry elements, like the center of inversion or the glide plane parallel to the surface, are precluded, and enhanced chiral interactions are therefore expected. 16 However, the balance between the molecule-substrate interaction and the lateral intermolecular interactions determines the nature of self-assembly 17 and thus the outcome of lateral resolution.…”

Ester hydrolysis in aqueous sulphuric acid. Effects of solvent ionizing power and nucleophilicity separated from the effects of protonation of substrate

“…There are often severe complications from medium effects,' despite preliminary indications that medium effects could be ignored.lgb It has also been reported that samples underwent considerable heating during the 1-2 h required for data collection. 20 We observed a gradual and substantial increase in frequency differences between signals for a-and P-carbon atoms for ethyl mesylate (Table 7), which we cannot relate to extent of protonation.…”

Chiral Recognition in Biomembrane Models: What is behind a ‘Simple Model’

“…4 In the investigation of monolayers of N-(1-phenylethyl)stearamide by equilibrium spreading pressure and static surface pressure, it was found that the molecules of the homochiral samples were more strongly associated than those of the racemates. 5 Differences in the intermolecular packing of homochiral compared with racemic amphiphiles yielded the spontaneous segregation of enantiomers from racemic mixtures in monolayers of N-dodecylmannonamide and of some hydrophobic amino acids; this was nicely observed by Brewster angle microscopy 6 and grazing incidence X-ray diffraction experiments, respectively. 7 Note that it was proposed that the spontaneous segregation of enantiomers from a racemic mixture in monolayer aggregates might have played an important role in the transformation from racemic chemistry to chiral biology.…”

“…A number of investigations have focused on this facet of biological membranes and have pointed out, through different models, some consequences of lipid chirality. [4][5][6][7][8][9][10][11][12][13][14][15][16] It was found that the chiral information embedded in the molecular structure of monomers affects the organization of amphiphile aggregates. In fact, the transition temperature (T c ) of homochiral vesicles of 2-methyldodecanoic acid was found to be lower compared with the T c of the racemic vesicles; therefore, at 10 °C, racemic vesicles destabilized causing phase separation, whereas homochiral vesicles remained stable.…”