To resolve the molecular structure of a protein, good quality protein crystals are a necessity. The best structure determinations are done by means of X-ray diffraction and as a rule, the better the crystal, the better the molecular structure determination. However, growing good protein crystals is not very easy and is often the quality limiting step in structure determination. Growing crystals in microgravity, thus in space, is a method that is used to solve this problem. In collaboration between the HFML, the Solid State Chemistry department and the Biochemistry department of the Radboud University Nijmegen, we have shown that it is not necessary to go to space at all. Microgravity can also be created much easier and much cheaper on earth, by using strong magnetic fields.

microgravity-crystal-growth1 microgravity-crystal-growth2
Figure 1: (a) During crystal growth, the solution near the crystal gets depleted of solute (light blue), because of which it will rise and forms a convection plume.

(b) In microgravity, without convection, the depleted zone around the crystal expands due to diffusion. Also, the convection plume has disappeared

Protein crystals grown in microgravity, for instance on board the International Space Station ISS or the Space Shuttle; often have better crystal quality than crystals grown on earth. During growth, the solution near the crystal gets depleted of protein molecules, resulting in a lower density. Under the influence of gravity, according to the principle of Archimedes, the solution with lower density rises, resulting in convective flows around the crystal, as can be seen in figure 1a. Because convection is a fast means of mass transport, the crystal growth rate is high, and defects in the crystal do not have time to heal. Also, impurities are easily transported to the crystal, where they become incorporated (Figure 1b). Al these factors have a negative effect on crystal quality.

When crystals are grown in an environment where gravity is absent, the crystal quality will be better, because there is no convection. Without convection, all molecules have to be transported by much slower diffusion. A large depletion zone will form, the growth rate will decrease dramatically and larger impurities will be less incorporated. Many protein crystallization experiments have been performed on board the ISS and the space shuttle. These experiments however are extremely expensive and difficult to control.

Figure 2: The floating frog in a magnet of the HFML.

A promising alternative is the use of strong magnetic fields. In 1997, scientists from HFML have shown that, on first sight, non-magnetic materials can levitate in high magnetic fields. Famous is the floating frog (Figure 2). This is possible because all materials, including frogs and protein solutions, are very weakly magnetic, that is diamagnetic. With very strong magnetic fields it is possible to impose a magnetic force on such materials. And if the force has the same magnitude as the gravitational force, but is in opposite direction, the solution will levitate. Crystals were grown under levitation conditions, but a detailed analysis shows that levitation is not the proper condition to suppress convection during growth. The gravitational and magnetic forces acting on the solution depend on the concentration, and because concentrations vary during growth, this has to be accounted for. We have used this knowledge for the first time to suppress convection during crystal growth, in a gradient magnetic field. Using a special schlieren microscope, we have shown that the convection plume disappears and the depletion zone expands, just like expected for microgravity crystal growth (figure 3). This has been done for a Nickelsulfate-hexahydrate crystal and solution as a model system for protein crystal growth. In this way we have succeeded to create a condition of weightlessness which is much cheaper, much more accessible and much more stable than on board a space station. It is even possible to create a “negative” gravity, in which the convection plume flows downwards instead of upwards.

Research is ongoing, and soon the first protein crystals will be grown under weightlessness conditions. Not in space, but at the HFML.

Figure 3: False color Schlieren microscope images of a growing Nickelsulphate crystal. The intensity in a Schlieren image is proportional to the concentration gradient in the horizontal direction (from left to right). Red corresponds to a negative concentration gradient, blue to a positive, according to the color bar on the right. (a) Normal gravity. A convection plume and a thin depletion zone is visible. (b) Microgravity. Convection is suppressed, the plume has disappeared and the depletion zone is expanding. (c) Negative gravity. It is also possible to flip the direction of convection, with the plume oriented downwards. The blue stripe on the bottom right is the mounting wire of the crystal. The scale bar on the bottom left corresponds to 0.5mm.

This work was published in:

P. W. G. Poodt, M. C. R. Heijna, K. Tsukamoto, W. J. de Grip, P. C. M. Christianen, J. C. Maan, W. J. P. van Enckevort and E. Vlieg,
Suppression of convection using gradient magnetic fields during crystal growth of NiSO4·6H2O,
Appl. Phys. Lett. 87, 214105 (2005)