Cryogenic (<20 K) helium cooling mitigates radiation damage to protein crystals

In experiments conducted at the Bio-CARS beamline 14-BM-C (APS, Argonne National Laboratory, USA), Streptomyces rubiginosus D-xylose isomerase (EC 5.3.1.5) crystals were used to test the effect of cryogen temperature on radiation damage. Crystals cooled using a helium cryostat at an 8 K set temperature consistently showed less decay in the signal-to-noise ratio, < I/sigma(I)>, and in average intensity, < I >, compared with those cooled with a nitrogen cryostat set to 100 K. Multiple crystals grown using ammonium sulfate as precipitant were used at each cryostat set temperature and comparisons were made for crystals of similar size and diffraction resolution. Maximum resolution for the crystals was 1.1 - 1.3 angstrom, with He at <= 20 K extending the lifetime of the high-resolution data by > 25% compared with crystals cooled with N-2 at 100 K.