Extension of the diffraction resolution of crystals

The diffraction resolution of crystals of the guanine nucleotide-exchange factor complex, EF-Tu-Ts, has been extended from 5.0 to 2.5 Å by lowering the solvent content in the crystals as well as the temperature of data collection. The common form of EF-Tu-Ts crystal belongs to space group P2₁2₁2₁ with a = 81.1, b = 109.9, c = 207.5 Å and has a solvent content of 61%. The crystals diffract to a resolution of 5.0 Å at 293 K and 4.0 Å at 273 K. When cryoprotective agents are slowly diffused into the crystals, the cell constants shrink to a = 74.4, b = 109.9, c = 198.7 Å and the solvent content falls to 55%. After the cryoprotective agent has been added, the crystals diffract to 2.7 Å resolution at 293 or 273 K and 2.5 Å at 250 K. X-ray diffraction data, collected before and after the transformation of individual EF-Tu-Ts crystals, demonstrate that a large percentage of the improvement in diffraction resolution is due solely to the addition of cryoprotective agents. The transfer procedures for the successful introduction of cryoprotective agents into EF-Tu-Ts crystals as well as the general applicability to other crystal systems will be discussed.