Special features of monolayer characteristics of N-alkanoyl substituted threonine amphiphiles.

The monolayers of N-alkanoyl substituted threonine amphiphiles, similar to those of other N-alkanoyl-substituted amino acid amphiphiles, point to substantial differences in the main characteristics compared to usual amphiphilic monolayers. π-A measurements of the enantiomeric and racemic forms of N-alkanoyl-substituted threonine monolayers with C16 and C18 chain lengths reveal that, independent of the alkyl chain length, all compression curves are located above the corresponding decompression curves. A theoretical model developed for the kinetics of two-dimensional condensation of Langmuir monolayers can describe this behavior concluding the attachment of monomers to large aggregates. The linear fit of the entropy changes versus temperature (ΔS = f(T)) at the LE/LC phase transition and extrapolation to zero ΔS specifies the critical temperature Tc, above which the monolayer cannot be compressed into the condensed state. The relatively small ΔTc difference between the enantiomeric and the racemic forms is consistent with the increased strength of van der Waals interactions between the longer alkyl chains reducing the influence of chirality on the thermodynamic parameters. The BAM experiments reveal clearly the absence of inner anisotropy as a specific feature of the domain topology of N-palmitoyl-threonine monolayers. Furthermore, the growth kinetics of the racemic N-palmitoyl-dl-threonine domains reveals a transition from homochiral discrimination and chiral separation within the domain to a state with heterochiral preference. GIXD studies show that at all pressures the enantiomers exhibit three Bragg peaks indicating an oblique lattice structure, whereas the racemates show only two Bragg peaks indicating a NNN tilted orthorhombic structure. Characteristic for the structure of all condensed monolayer phases is the large tilt angle of ∼49°, nearly independent of the lateral pressure. The transition from the oblique lattice structures, as detected for enantiomeric monolayers, to orthorhombic structures of racemic monolayers is clear evidence that the dominant heterochiral interaction in the racemic mixtures leads to the formation of a compound with congruent transition pressure having with ∼20.0 Å2 an essentially smaller alkyl chain cross-sectional area than the enantiomers with ∼20.7 Å2.