Langmuir-Blodgett nanotemplates for protein crystallography

T he new generation of synchrotrons and microfocused beamlines has enabled great progress in X-ray protein crystallography, resulting in new 3D atomic structures for proteins of high interest to the pharmaceutical industry and life sciences. It is, however, often still challenging to produce protein crystals of sufficient size and quality (order, intensity of diffraction, radiation stability). In this protocol, we provide instructions for performing the LangmuirBlodgett (LB) nanotemplate method, a crystallization approach that can be used for any protein (including membrane proteins). We describe how to produce highly ordered 2D LB protein monolayers at the airwater interface and deposit them on glass slides. LB-film formation can be observed by surface-pressure measurements and Brewster angle microscopy (BAM), although its quality can be characterized by atomic force microscopy (AFM) and nanogravimetry. Such films are then used as a 2D template for triggering 3D protein crystal formation by hanging-drop vapor diffusion. The procedure for forming the 2D template takes a few minutes. Structural information about the protein reorganization in the LB film during the crystallization process on the nano level can be obtained using an in situ submicron GISASAXS (grazing-incidence small-angle X-ray scattering) method. MicroGISASAXS spectra, measured directly at the interface of the LB films and protein solution in real time, as described in this protocol, can be interpreted in terms of the buildup of layers, islands, or holes. In our experience, the obtained LB crystals take 110 d to prepare and they are more ordered and radiation stable as compared with those produced using other crystallization methods.