Incorporation of impurities into macromolecular crystals

The chemical, mechanical and diffraction properties of crystals grown from solution, as well as their growth kinetics and morphological development, depend very much on the types and concentrations of impurities present in their mother liquor. The situation appears vastly more complicated in the case of macromolecular crystals because of the complex nature of the molecules and the biochemical milieu from which they are derived. An attempt is made here to catalog and characterize these various impurities. One class of impurities, large foreign particles (such as dust), microcrystals, misoriented three-dimensional nuclei, and large molecular clusters has been investigated in detail using atomic force microscopy. With this technique we have directly visualized the incorporation of such larger impurities and have delineated some of their more striking consequences. In particular we have found that in some cases such incorporation is accompanied by visible defect formation or dislocations. In other cases of small three-dimensional nuclei, coalescence proceeds in a smooth manner, with alignment and knitting together of the respective lattices. A calculation of the overall defect density in canavalin crystals shows the number of defects to be many orders of magnitude greater than found for most conventional crystals.