THE PROTEIN CRYSTALLISATION DIAGNOSTICS FACILITY: PCDF

ESA’s Protein Crystallisation Diagnostics Facility (PCDF), presently under development at DASA-DORNIER, follows on from the experience acquired with the Advanced Protein Crystallisation Facility (APCF).

This new instrument focuses on understanding the nucleation and crystallisation processes and the influence of gravity thereon. It will enable the monitoring of, and in turn control of, these processes over long periods in microgravity using advanced diagnostics instruments. The PCDF will be installed in the European Drawer Rack (EDR) in ESA’s Columbus Laboratory (COF) in the 2002-2003 time frame.

PCDF GENERAL DESCRIPTION

The PCDF consists of two units:

	a process unit, including the process chamber with the crystallisation reactors
	an electronics unit accommodating the main controls and support systems for the performance of experiments.

The electronics unit will be accommodated in a drawer. It houses the main electronics, the power unit, the central processor unit and the electronics controlling the process unit. It also contains an optical and video unit, a light scattering unit, and, optionally, a mass memory unit.

The process unit will be contained in a locker in order to enable its alternative accommodation in the Shuttle mid-deck. It includes the process chamber with four reactors. Either the batch technique or the dialysis technique is implemented in all four reactors with solution composition control capability in respectively the protein chamber and the buffer chamber. Each reactor is housed in a box with independent temperature control. Furthermore, the temperature of the process chamber itself is controlled in the range 14 to 30°C. This is achieved by two electronics boxes and a heat exchanger which are included in the process unit. Thereby, the temperature of the chamber can be controlled during launch and return in the Shuttle mid-deck.

THE REACTORS AND THEIR HOUSING

The quartz glass reactors consist of chambers containing the different solutions separated by one-way valves.  A batch reactor is made up of the protein chamber where the crystallisation takes place and two injection reservoirs. A dialysis reactor includes, in addition, a buffer chamber to which the two injection reservoirs are connected, and separated from the protein chamber by a membrane.

An extended length protein chamber has been designed to provide a long diffusion path for dialysis experiments.

Each experiment box provides for the individual temperature control of the reactor it contains. This is achieved by means of Peltier elements and temperature sensors. The reactor temperature can thus be controlled within 10°C from the chamber temperature. There is provision for temperature ramping up and down of the reactors at rates chosen by the investigators. The experiment box drives the solution injection systems as well as a stirrer. The stirrer is located in the protein chamber of a batch reactor, or in the buffer chamber of a dialysis reactor. This allows for controlled and homogeneous changes of the composition of the protein solution under investigation or of the buffer. In principle, these individual controls, of the temperature and of the concentration in a reactor, enable several cycles of crystallisation/re-dissolution to be performed.

THE DIAGNOSTICS

The diagnostics are either mounted on a central carousel in the process chamber and observe all reactors in sequence, or they are installed directly on the experiment housing.

The diagnostics provided by the PCDF include a monochrome digital video camera with wide field of view or microscope optics and a back-scatter (170°) based static and dynamic light scattering system, both mounted on a carousel to enable access to all reactors in sequence. Additionally, either another set of light scattering optics at a 90° angle coupled with the light scattering system on the carousel, or a Mach-Zehnder interferometer with phase shifting capability can be implemented at some reactors.

The design of the experiment housings and their interface with the electronics allow for possible addition of other sensors in the future, in particular for osmotic pressure and pH measurement.

The following Table summarises the reactor and diagnostics technical data.

POTENTIAL MISSION SCENARIO

The mode of operation is either automatic by pre-programmed sequences controlled by the PCDF built-in electronics, or remotely from the ground by telecommand, or locally by a crewmember via a portable computer and/or command interfaces on the PCDF front panel.

The present mission scenario assumes an initial upload of the electronics unit in the European Drawer Rack during the launch of ESA's Columbus Orbital Facility module. The process unit, fitted with the first four reactors (either batch or dialysis), is sent to the Station as a powered locker in the Shuttle mid-deck. It will remain in orbit for the duration of a mission increment, i.e. a minimum of 3 months. While the electronics unit will stay on board the Station for typically 2 to 3 years, the process unit will be flown back and forth from ground to orbit. Between consecutive up and down flights of the process unit, experiment boxes can be exchanged on the ground, close to the Shuttle launch and landing facility at NASA's Kennedy Space Center. After harvesting the crystals in a temperature controlled ground laboratory, the reactors are emptied, cleaned and refilled with fresh solutions for the next flight or exchanged for the other type of reactors (dialysis or batch).

The development of a second process unit to fly in sequence with the first one is presently being considered.

Data and digital video images are either stored on board or transmitted to the ground control station depending on the transmission capabilities available at that time. Video images, including microscope images and interferogrammes, can be stored either in the PCDF dedicated subsystem or in the EDR video system and transmitted to the ground when a link is available. Data and video are then forwarded from the receiving ground station to the investigators.