The use of magnetic forces to suppress convection in the crystallization process

Abstract

Over the past decades a significant amount of effort has been invested in the endeavor to grow large macromolecular crystals under microgravity conditions, for use in x-ray crystallographic studies of molecular structure. A major limitation on crystal growth is the effect of convection. On earth, a convective cell is established as the fluid surrounding the crystal is depleted of protein material. This cell transports fresh materiil to the solid surface, keeping the growth rate too high to form high quality crystals. In space, convection is absent, and the growth rate is slower, controlled by diffusion. We describe a technique to suppress the convective effect on earth in a solution with one precipitating component, using magnetic forces. With existing technology, magnetic levitation is achievable for materiils with an absolute mass susceptibility > 0.35x10-” cc/gm, which includes many common materials. The application of a magnetic force at the molecular level is one of the few ways to truly cancel gravitational forces throughout the precipitating fluid while crystals are forming, as opposed to supporting it with surface acting forces such as buoyancy. INTRODUCTION Over the past decade a significant amount of effort has been invested in an endeavor to grow various relatively large macromolecular crystals under microgravity conditions for use in x-ray crystallographic studies of their molecular structure. This work has borne fruit in a number of interesting cases, from bovine and human insulin to canavalin and satellite tobacco mosaic virus, and will continue to be actively pursued during the Space Station era. It is considered one of the most valuable applications of the microgravity environment to the problems of humanity. Since microgravity time is so limited, and will remain so for years to come, it would seem valuable if new methods could be found to grow a significant fraction of the crystals of interest on the ground. Copyright