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The relationships between the molecular packing and the morphology of rubredoxin crystals, a metalloprotein, have been studied in detail through the periodic bond chains (PBC) method of Hartman and Perdok (1955). To carry out the PBC analysis the authors considered that the crystallizing particle consisted of the protein molecule plus a tightly bound layer of solvent molecules. Molecular interactions such as hydrogen bonds, van der Waals contacts, ion pairs and hydrophobic free energy have been systematically analysed. In every case, they involved direct protein-protein and protein-solvent-protein contacts. The structure of the respective growth layers (slices) and of the PBCS has been determined with the help of molecular graphics. To get a quantitative insight into crystal packing forces they have estimated, for different faces, the interaction energy of the crystallizing particle with its nearest neighbours in the respective underlying growth layers. There is an inverse correlation between this energy and the morphological importance of the faces. The role of hydrogen bonding is also discussed.

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