2 OPEN-FLOW CRYOSTATS FOR MACROMOLECULAR CRYSTALLOGRAPHY

Two open-flow cryostats for macromolecular crystallographic studies, w hich make it possible to freeze macromolecular crystals very rapidly a nd maintain them for long periods of time at reduced temperatures, hav e been constructed. Both systems avoid the use of opaque shrouds and h ence permit monitoring of the optical properties of protein crystals a t all temperatures. The first uses a cold-gas stream derived from boil -off of liquid nitrogen, with a surrounding warm-gas stream, and cover s 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 liqui d nitrogen to cover a temperature range of 10 to 70 K or above 80 K, r espectively. 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 t he power of a heater within the cold-gas stream prior to its exit from the nozzle. The liquid-helium consumption is less than 2 1 h-1 at a w orking temperature of 40 K.