Fluorescence microscopy study of Langmuir monolayers of racemic and enantiomeric N-stearoyltyrosine.

Monolayers of N-stearoyltyrosine have been investigated by fluorescence microscopy and surface pressure measurements. The racemic films are more condensed than the enantiomeric films, indicating heterochiral discrimination. The surface pressure at the transition to the condensed phase on compression decreases monotonically between XD = 0 and XD = 0.5 at 33 degrees C. The condensed-phase domains of the monolayer of the pure L enantiomer appear as anisotropic shapes under a fluorescence microscope. The condensed-phase domains of the racemic monolayer appear more compact at lower temperatures, but their morphology becomes progressively more anisotropic as the temperature increases. The difference between the surface pressure isotherm of the enantiomeric L and the racemic film are smaller and homochiral for the N-stearoyltyrosine methyl ester on a pH 7 buffered subphase than for the N-stearoyltyrosine on a pH 2 HCl acidified subphase at 22 degrees C, indicating the importance of hydrogen bonding in accounting for the origin of the chiral discrimination.