Step velocity in tetragonal lysozyme growth as a function of impurity concentration and mass transport conditions

Abstract

The step velocity on {1 1 0} faces of tetragonal lysozyme crystals has been measured in situ by high-resolution phase contrast microscopy. The movement of the steps shows a distinct anisotropy with respect to the growth velocity and the interaction with impurities. Under high-purity conditions (using lysozyme of 99.99% purity), the step velocity v step in the 〈1 1 0〉 direction exceeds the one in the 〈0 0 1〉 direction by a factor of 6. Spiral hillocks as well as 2D islands show an elongated, lens shaped morphology with pronounced tips in the 〈1 1 0〉 direction. The ratio of v step〈1 1 0〉/ v step〈0 0 1〉 is reduced to ≈2–4 when commercial grade lysozyme (purity: 98.5%) is used. The morphology of the hillocks/islands changes from the lens-shaped one to a rounded one. The step velocity into the fast growing 〈1 1 0〉 direction is significantly more affected by impurities than the growth into the 〈0 0 1〉 direction. The step velocity of monolayer islands (5.6 nm in height), of multilayer islands and of growth spirals is similar within the accuracy of the measurements. On the topside, the crystal is exposed to protein solution of ≈0.5 mm width. This results in convective mass transport conditions. On the backside of the crystal, there is a small gap of a few hundred nanometers. Within this gap, the crystal is growing under diffusive transport conditions. Changing the focus position from the topside of the crystal to the backside, the growth can be observed on the same crystal under convective as well as under diffusive conditions. Using commercial grade lysozyme, the growth velocity is higher under diffusive conditions than under convectively dominated ones. The morphology of the islands grown from a commercial grade solution under diffusive mass transport conditions is similar to the island morphology obtained otherwise only from high-purity solutions. Using high-purity solution, the step velocity and the island morphology does not differ as a function of the mass transport conditions. These results prove the extension of the depletion zone of the impurities and the reduced transport of impurities toward the growing interface under diffusive transport conditions.