A 7 μm mini-beam improves diffraction data from small or imperfect crystals of macromolecules

A simple apparatus for achieving beam sizes in the range 5-10 mu m on a synchrotron beamline was implemented in combination with a small 125 x 25 mm focus. The resulting beam had sufficient flux for crystallographic data collection from samples smaller than 10 x 10 x 10 mm. Sample data were collected representing three different scenarios: (i) a complete 2.0 angstrom data set from a single strongly diffracting microcrystal, (ii) a complete and redundant 1.94 angstrom data set obtained by merging data from six microcrystals and (iii) a complete 2.24 angstrom data set from a needle-shaped crystal with less than 12 x 10 mu m cross-section and average diffracting power. The resulting data were of high quality, leading to well refined structures with good electron-density maps. The signal-to-noise ratios for data collected from small crystals with the mini-beam were significantly higher than for equivalent data collected from the same crystal with a 125 x 25 mu m beam. Relative to this large beam, use of the mini-beam also resulted in lower refined crystal mosaicities. The mini-beam proved to be advantageous for inhomogeneous large crystals, where better ordered regions could be selected by the smaller beam.