Hydrogen-Bond-Induced Chiral Discrimination in Monolayers of Bipolar Methyl Dihydroxyoctadecanoates,,

Abstract

The insertion and systematical displacement of two vicinal hydroxyl groups along the hydrophobic alkyl chain of methyl octadecanoates generates bipolar amphiphiles. These molecules offer the possibility to investigate the influence of the position and the stereochemistry of the hydroxyl groups on monolayer properties and intermolecular interactions. Monolayers of enantioenriched and racemic vicinally dihydroxylated methyl octadecanoates (DHOs) were investigated using fluorescence microscopy, fluorescence spectroscopy, and grazing incidence X-ray diffraction (GIXD). The monolayers of racemic threo-2,3-DHO, erythro-9,10-DHO, and 17,18-DHO show heterochiral discrimination in fluorescence microscopy investigations due to a preferential interaction between the enantiomers with the opposite configuration. GIXD measurements exhibit rectangular unit cells for the racemic films, while the enantiomers form oblique and therefore chiral structures. Erythro-9,10-DHO exhibits an extraordinarily large lattice spacing due to the disturbing effect of the hydroxyl groups. The formation of rectangular lattices by racemic films strongly suggests that chiral phases such as in the case of the enantiomers are prevented by the dimerization due to stereospecific hydrogen bonds between the two antipodal enantiomers. Accordingly, the individual enantioenriched amphiphiles are able to distinguish between the same and antipodal enantiomers, and thus they are able to recognize chemical structures at a molecular level. Fluorescence spectroscopy at the air−water interface revealed whether the hydroxyl groups or the methyl ester of the α,ω-bipolar amphiphile 17,18-DHO is directed to the water surface in the condensed state at subphase temperatures of 278 and 293 K, respectively.