CHIRAL MOLECULAR RECOGNITION IN MONOLAYERS OF DIASTEREOMERIC N-ACYLAMINO ACID METHYL-ESTERS AT THE AIR-WATER-INTERFACE

This article continues our study of the effects of headgroup geometry and temperature on chiral recognition in the force-area isotherms, thermodynamics of spreading, and surface shear viscosities of monolayers and mixed monolayers of long-chain amino acid ester surfactants. N-Stearoyl-and lauroyl-derivatives of the methyl esters of cysteine, cystine, and threonine are compared to the previously-reported serine derivative. The structural points at issue are as follows: (a) the effects of replacing the hydroxyl group of serine with the thiol group of cysteine; (b) the effect of joining two cysteine groups through their sulfur atoms to produce the two-chain cystine surfactant; and (c) the effect of attaching a methyl group to the carbon bearing the hydroxyl group in stearoylserine methyl ester (SSME) to produce the bulkier stearoylthreonine methyl ester (STME). Comparison is first made for the melting point versus enantiomer composition of the crystals for each compound. In all four cases a racemate is formed. Next, the corresponding effects of enantiomeric composition versus the appropriate surface properties are presented and behavior similar to the melting point curves is seen, implying stereoselective behavior when the monolayers are in equilibrium with their crystals or quasicrystalline condensed surface phases. Diastereomeric effects were small, since meso-dilauroylcystine dimethyl ester (DLCDME) showed properties which were nearly identical to its D and L enantiomers, and the allo form of STME was similar to its enantiomers. All four compounds showed distinctly different force-area curves for their enantiomers versus their racemic mixtures, but the shapes of the curves and phase behavior (between liquid-expanded and liquid-condensed films) depended heavily on temperature. All force-area curves show hysteresis effects in the difference between the compression and expansion regions, indicating, as we have shown before, that relaxation of compressed monolayer states is slow and that the films are in metastable states. Phase behavior is an erratic function of headgroup and temperature. Also, there is no general pattern of whether racemates or enantiomers are most expanded. No crystals of quality sufficient for X-ray analysis could be grown, so rigorous interpretation of properties and behavior in terms of structure cannot be made. However, clear differences between the behavior of stearoylcystine methyl ester (SCME) and SSME can be interpreted in terms of hydrogen bonding of the serine hydroxyl group to the water subphase. Furthermore, comparison of force-area curves for a series of diastereomeric mixtures of L-STME and L-allo-STME with D- and L-SSME suggests that the stereochemistry at the carbon between the ester and amide functions is primarily responsible for the stereoselectivity in the packing of STME films. Films of SCME were too condensed to allow a surface viscosity study, but those of DLCDME and STME exhibited Newtonian flow with essentially no stereoselectivity in their flow properties.