Influence of Stereochemistry on the Monolayer Characteristics of -alkanoyl-Substituted Threonine and Serine Amphiphiles at the Air-Water Interface.

Thermodynamic and structural properties of the -alkanoyl-substituted α-amino acids threonine and serine, differing only by one CH group in the head group, are determined and compared. Detailed characterization of the influence of stereochemistry proves that all enantiomers form an oblique monolayer lattice structure whereas the corresponding racemates build orthorhombic lattice structures due to dominating heterochiral interactions, except -C16-dl-serine-ME as first example of dominating homochiral interactions in a racemic mixture of -alkanoyl-substituted α-amino acids. In all cases, the liquid expanded-liquid condensed (LE/LC) transition pressure of the racemic mixtures is above that of the corresponding enantiomers. Phase diagrams are proposed. Using the program Hardpack to predict tilt angles and cross-sectional area of the alkyl chains shows reasonable agreement with the experimental grazing incidence X-ray diffraction (GIXD) data.