CRYSTALLINE-STRUCTURES OF MONOLAYERS ON THE WATER-SURFACE AS DETERMINED BY GRAZING-INCIDENCE X-RAY-DIFFRACTION - N-EICOSANOYL-3-AMINOPROPIONIC ACID AND ITS MIXTURES WITH EICOSANAMIDE

The crystalline structures of monolayers of the N-eicosanoyl-3-aminopr opionic acid CH3-(CH2)18-CONH-(CH2)2-CO2H and its mixtures with eicosa namide CH3-(CH2)18-CONH2 on a water subphase were determined, at near- atomic resolution, by grazing incidence X-ray diffraction (GID), using synchrotron radiation. The N-eicosanoyl-3-aminopropionic acid molecul e must adopt a very particular conformation in order to ensure satisfa ctory packing requirements. The packing characteristics were tested by fitting the measured GID data using atomic coordinate models for vari ous conformers, complemented by lattice energy calculations. The struc ture of the mixed monolayer in 1:1 and 1:2 molar ratios is different f rom that of each pure component. We propose that the two components do not form a random solid solution but rather the N-eicosanoyl-3-aminop ropionic acid forms domains within the sea of eicosanamide molecules, stabilized by the interactions between the -(CH2)2-CO2H moietics.