GROWTH-MECHANISM AND MORPHOLOGY OF TETRAGONAL LYSOZYME CRYSTALS

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 i s by employing the periodic bond chain (PBC) theory of Hartman & Perdo k [Hartman & Perdok (1955). Acta Cryst. 8, 49-52, 521-524, 525-529]. H owever, complexities resulting from the packing arrangements and the n umber 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 carri ed out, coupled with an approach incorporating the molecular orientati ons of the crystal structure. The analysis revealed the existence of a helical tetramer building block of the entire crystal structure, cent ered around the 4(3) crystallographic axes, resulting in double-layere d slices and PBC's throughout. The analysis also indicated that the cr ystallizing 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} t o be essentially rough S faces. The predicted morphology and growth me chanisms were found to explain numerous experimental observations from electron and atomic force microscopy, etching studies, lysozyme aggre gation studies and measurements of growth kinetics.