Droplet streams for serial crystallography of proteins

Serial diffraction of proteins requires an injection method to deliver analyte molecules-preferably uncharged, fully hydrated, spatially oriented, and with high flux-into a focused probe beam of electrons or X-rays that is only a few tens of microns in diameter. This work examines conventional Rayleigh sources and electrospray-assisted Rayleigh sources as to their suitability for this task. A comparison is made and conclusions drawn on the basis of time-resolved optical images of the droplet streams produced by these sources. Straight-line periodic streams of monodisperse droplets were generated with both sources, achieving droplet diameters of 4 and 1 micrometer, respectively, for the conventional and electrospray-assisted versions. Shrinkage of droplets by evaporation is discussed and quantified. It is shown experimentally that proteins pass undamaged through a conventional Rayleigh droplet source.