X-ray diffraction of a protein crystal anchored at the air/water interface.

We report the first successful in situ x-ray diffraction experiment with a 2D protein array at the lipid/water interface and demonstrate that the order can be controlled via lateral pressure or density. A protein (streptavidin) was bound to a monolayer of biotinylated lipid at the air/water interface, and diffraction of the protein layer could be measured to many orders. Compression of the monolayer changed the diffraction pattern drastically, indicating that the protein structure can be strongly influenced by external parameters like lateral pressure or density. From the width of the peaks, we find that aggregates consisting of as few as 100 monomers contribute to the diffraction. This indicates that the structure of even low order aggregates can be studied in situ. Grazing incidence diffraction can become a strong new method to study the crystallization and the interactions between proteins free from artifacts by staining or sample preparation.