Synchrotron X-ray study of the structure of a Langmuir monolayer and the attached solute molecular layer

We have determined the packing arrangement of a floating monolayer of palmitoyl-(R)-lysine at the air-water interface by grazing incidence X-ray diffraction and reflection measurements. These techniques utilize the unique properties of synchrotron radiation: high intensity within a small natural collimation. In the grazing angle diffraction experiment two peaks were detected in the “two-dimensional powder” pattern from a monolayer of palmitoyl-(R)-lysine. Their widths indicated coherence lengths of 500 Å. The positions and intensities of these peaks allowed us to choose between various models and to determine the monolayer structure. The packing arrangement of the α-amino acid headgroups in the model proved to be very similar to that found in the crystal structures of the α form of glycine and several hydrophobic α-amino acids, thus providing a simple explanation for the oriented crystallization of α-glycine at monolayer-solution interfaces. The tilt of the molecule calculated from the model is consistent with the results from reflectivity measurements and X-ray powder diffraction data of the crystalline powder material. Reflectivity measurements indicate that at surface pressures as high as 30 mN m$^{−1}$ the monolayer covers only about 90% of the surface. Reflectivity measurements of the monolayer over water and over solutions of 2% (S)-glutamine showed significant differences, indicating binding of the solute molecules to the monolayer