The results of protein crystallization experiments performed in microgravity have been controversial. Of the more than 8000 experiments flown, only a percentage have shown that microgravity improves the X-ray diffractive quality of protein crystals. Such improvement is of significance in structural biology as the maximum extent of the X-ray diffraction pattern from a protein crystal, called resolution limits, generally limits the information available from a protein structure. A convincing case linking the improvement in diffractive ability with protein crystal growth in microgravity accepted by the scientific community of structural biologists remains to be made.

The premise that reduced supersaturation at the crystal interface improves protein crystal quality and enhances resolution limits remains to be demonstrated for macromolecules. A reduced supersaturation zone, also referred to as depletion zone, always exists in the vicinity of the growing crystal interface because this interface consumes the crystallizing molecules. Its depth is controlled by the crystal growth kinetics, specific to each protein. Supersaturation in the mother liquor stabilize protein aggregates while the lower supersaturation at the crystal face favours their dissociation into the elementary growth units. The zone not only reduces the number of protein aggregates incorporated into the crystal but also acts as a sort of mass filter by diminishing incorporation of impurities such as denatured protein during growth as these tend to diffuse more slowly than native protein.

On earth, gravity disturbs this zone due to sedimentation and convection effects and is the motivation for growing protein crystals in space. A priori selection for space experiments of proteins that exhibit significant zone depth upon microgravity crystallization was not made in the past. As protein crystallization conditions do not yield reduced supersaturation zones when rate of surface attachment is slower than protein transport to crystal face, protein crystallization experiments were flown whose kinetics in all likelihood would not have been sensitive to exposure to a microgravity environment.

Our overall objective is to position microgravity protein crystallization as a useful alternative in tackling difficult to solve protein structures. Difficult crystal growth problems represent realistic opportunities for microgravity application as all other approaches have been exhausted at this point and a large database of crystal growth experiments can be drawn upon to evaluate the feasibility of the microgravity approach.

We have developed a novel microgravity technique to remotely initiate protein crystal growth in space that showed a beneficial effect by microgravity on protein crystal growth. We have also clarified the role of impurities on protein crystal growth in low gravity and have examined gravity-induced disturbances during crystal growth. We are currently investigating the role of depletion zones in protein crystal growth as an explanation for the beneficial effect of microgravity and are setting up a protein crystal growth platform to analyze by protein concentration differences using Michelson interferometry kinetics of protein crystal growth. Preliminary results indicate that the protein depletion zone also exists on earth whenever crystallization occurs on a submillimeter scale.