Supersaturation patterns in counter-diffusion crystallisation methods followed by Mach-Zehnder interferometry

We present experimental observation of the spatio-temporal pattern of super saturation in counter-diffusion methods. These complex patterns were record ed by dynamical interferometric analysis using a Mach-Zehnder configuration . Tetragonal hen egg white lysozyme crystals were grown inside APCF (advanc ed protein crystallisation facility) reactors. Salt and protein diffusion p rofiles were obtained independently by performing duplicated experiments wi th and without protein in the protein chamber; salt gradients were observed directly while protein concentration profiles are computed from the differ ences in refractive index between the two experiments. As expected from com puter simulations, the time evolution of supersaturation shows a maximum ab out 45 h after activation (although this value can change as a function of the starting conditions and the geometry of the reactor). Nucleation takes place before this maximum supersaturation is reached. This explains the tre nd of the growth rate versus time curves for experiments performed within A PCF reactors (both on ground and in space) and in capillaries by the gel ac upuncture technique. By using very low concentration agarose gel in the pro tein chamber, sedimentation and buoyancy effects are eliminated so that the effects of gravity on fluid dynamics and hence on the spatio-temporal evol ution of supersaturation can be assessed. These results confirm experimenta lly the predicted behaviour of counter-diffusion systems and support their use in growing large high-quality protein single crystals. (C) 1999 Elsevie r Science B.V. All rights reserved.