Two open-flow cryostats for macromolecular crystallography

Two open-flow cryostats for macromolecular crystallographic studies, which make it possible to freeze macromolecular crystals very rapidly and maintain them for long periods of time at reduced temperatures, have been constructed. Both systems avoid the use of opaque shrouds and hence permit monitoring of the optical properties of protein crystals at all temperatures. The first uses a cold-gas stream derived from boil-off of liquid nitrogen, with a surrounding warm-gas stream, and covers a temperature range of 80 K to room temperatures. The second uses a cold-gas stream derived from boil-off of either liquid helium or liquid nitrogen to cover a temperature range of 10 to 70 K or above 80 K, respectively. Special care has been taken to make it possible to switch back and forth between the two cryogenic liquids in routine operation. The temperature stability of the cold-gas stream is ±0.5 K, which is achieved by controlling both the flow rate of the gas stream and the power of a heater within the cold-gas stream prior to its exit from the nozzle. The liquid helium consumption is less than 21 h^-1 at a working temperature of 40 K.