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The tetragonal form of hen egg-white lysozyme is the most investigated protein crystal for growth studies, but the relationship between its surface morphology and internal structure is still not well understood. One method of determining this relationship for inorganic crystals is by employing the periodic bond chain (PBC) theory of Hartman & Perdok [Hartman & Perdok (1955). Acta Cryst. 8, 49-52, 521-524, 525-529]. However, complexities resulting from the packing arrangements and the number of intermolecular bonds in protein crystals have resulted in the use of only simplified versions of this theory so far. In this study a more complete PBC analysis of tetragonal lysozyme crystals was carried out, coupled with an approach incorporating the molecular orientations of the crystal structure. The analysis revealed the existence of a helical tetramer building block of the entire crystal structure, centered around the 43 crystallographic axes, resulting in double-layered slices and PBC's throughout. The analysis also indicated that the crystallizing units for the faces are at least as large as this tetramer, with the experimental evidence suggesting that it is a tetramer unit for the {101} faces and an octamer unit for the {110} faces. The {110} faces were shown to be molecularly smooth F faces, while the {101} to be essentially rough S faces. The predicted morphology and growth mechanisms were found to explain numerous experimental observations from electron and atomic force microscopy, etching studies, lysozyme aggregation studies and measurements of growth kinetics.
