Lattice and thermodynamic characteristics of N-stearoyl-allo-threonine monolayers

The effect of the second chiral center of diastereomeric N-alkanoyl-allo-threonine on the main monolayer characteristics has been investigated. The characteristic features of the enantiomeric and racemic forms of N-stearoyl-allo-threonine monolayers are studied on a thermodynamic basis and molecular scale. The π-A curves of the enantiomeric and racemic allo-forms show similar features to those of N-stearoyl-threonine. The compression curves are always located above the corresponding decompression curves and the decompression curves can be used as equilibrium isotherms for both the enantiomeric and racemic N-stearoyl-allo-threonine. The absolute T₀-values (disappearance of the LE/LC-transition) are 4-5 K larger compared with the corresponding N-stearoyl-threonines, but the ΔT₀ between the enantiomeric (d) and the racemic (dl) forms is only slightly larger than that of N-stearoyl-threonine. The difference in the critical temperatures T_c, above which the monolayer cannot be compressed into the condensed state, between the enantiomeric and the racemic forms, is quite small (ΔT_c = 0.8 K) and is smaller compared to that of the corresponding threonines (ΔT_c = 1.8 K). This is consistent with the dominance of the van der Waals interactions between the alkyl chains reducing the influence of chirality on the thermodynamic parameters. GIXD studies of N-stearoyl-allo-threonine monolayers provide information about the lattice structure of condensed monolayer phases on the Angstrom scale and stipulate the homochiral or heterochiral preference in the condensed phases. Comparable to N-stearoyl-threonine, the enantiomers exhibit an oblique lattice structure, whereas the racemates form a NNN tilted orthorhombic structure demonstrating the dominance of heterochiral interactions in the racemates independent of the diasteomeric structure change of the polar head group. The A₀ values are characteristic for rotator phases. The smaller A₀ value obtained for the racemic monolayers indicates their tighter packing caused by heterochiral interactions. The program Hardpack was used to predict the geometric parameters of possible 2-dimensional packings. For comparison with the experimental GIXD data, the two-dimensional lattice parameters and characteristic features of the enantiomeric and racemic diastereomeric stearoyl-threonine monolayers were calculated and are in reasonable agreement with the experimental GIXD data.