X-ray diffraction by phospholipid monolayers on single-crystal silicon substrates

Monolayers of dipalmitoyl phosphatidylcholine have been transferred from an air-water interface to single-crystal silicon wafers previously alkylated with octadecyltrichlorosilane. By using synchrotron radiation it has been found possible to observe diffraction by single-crystal regions of these monolayers with negligible background scattering from the solid substrate. In one series of experiments, diffraction signals at Bragg spacings of 0.4247 ± 0.0002 nm and 0.4253 ± 0.0002 nm were observed. The supported phospholipid monolayer crystals show remarkably high in-plane order: the positional coherence length is at least 500 nm and the orientational order is better than 0.01°. Preliminary temperature scans were carried out. The data reveal the existence of a phase transition at about 65°C.