Simple geometry for diffusion limited protein crystal growth: harnessing gravity to suppress convection

A new upside-down geometry is proposed to achieve the beneficial effects of microgravity crystal growth by making use of buoyant forces instead of compensating for them. We show by growth experiments on sodium chlorate and lysozyme that crystal growth in an upside-down geometry leads to the formation of a buoyancy assisted depletion zone where convection is suppressed. The effects on growth rate and morphology that are observed are all indicative of diffusion limited growth, just as would happen in the absence of gravity. The optical quality of the lysozyme crystals clearly improved, but no effects of the growth method on X-ray diffraction quality could be observed. The simplicity of the growth geometry offers the possibility to perform large scale protein crystal growth experiments under microgravity-like conditions, without the requirement of compensating for gravity.