Structure of bidimensional phospholipidic crystallites on formamide determined by X-ray diffraction

By using grazing incidence X-ray diffraction, we determined the structure of the solid phase of a C18C18 phosphocholine monolayer spread at the air-formamide interface. The chains are organized on an oblique lattice, and their tilt, measured from Bragg rod scans, is about 35°. No structural change is observed when the surface pressure is varied from 1 to 38 mN/m. In the solid phase, the area per molecule is 46 Å2, and the compression modulus is large. The phase stiffness is dominated by the interactions between the polar heads and the formamide, more than between the aliphatic chains.