Lower dimer impurity incorporation may result in higher perfection of HEWLcrystals grown in microgravity - A case study

Crystals of tetragonal hen egg white lysozyme (HEWL) grown on a series of s pace missions and their terrestrial counterparts were analyzed by gel elect rophoresis and X-ray diffraction. The crystals were produced by vapor-diffu sion and dialysis methods. The microgravity and terrestrial grown HEWL crys tals were found to have effective partitioning coefficients (K-eff) for an oxidatively formed covalent dimer impurity (MW 28 K) of 2 and 9, respective ly, i.e. the latter contain 4.5 times more dimers. The microgravity grown c rystals allowed the collection of 24% more useful reflections and improved the resolution from 1.6 to 1.35 Angstrom. Other improvements were also note d including lower isotropic B-factors of 16.9, versus 23.8 Angstrom(2) for their terrestrial counterparts. High-resolution laser interferometry was ap plied quantitatively to evaluate the influence of dimer impurity on growth kinetics. It is shown that the growth of the (1 0 1) face from solution int o which 1% dimers were introduced decelerates with increasing solution flow rate and the growth stops at a flow rate of about 0.2 mm/s. This effect oc curs faster than in ultrapure solutions. The covalently bound dimers essent ially increase the amplitudes of the striation-inducing growth rate fluctua tions. The effect is ascribed to the enhanced transport of growth inhibitin g HEWL dimer to the interface. Theoretical analysis shows that a stagnant s olution around a growing crystal is strongly depleted with respect to impur ity by about 60% for the measured growth parameters as compared to the solu tion bulk. Thus, a crystal in microgravity grows from essentially purer sol ution than the ones in the presence of convection flows. Therefore, it trap s less stress inducing impurity and should be more perfect. For crystal/imp urity systems where K-eff is small enough microgravity should have an oppos ite effect. (C) 1999 Elsevier Science B.V. All rights reserved.