The molecular-replacement solution of an intermediate-sized helical polypeptide, antiamoebin I

The successful use of molecular-replacement methods for the solution of the intermediate-sized helical polypeptide antiamoebin I required the careful consideration of a number of parameters and exhibited some unusual characte ristics when compared with molecular-replacement solutions of globular prot eins. High-resolution data were required owing to several features, includi ng the comma-like shape of the molecule (which results in a pseudo-symmetri c structure at low resolution), the relative uniformity of the structure in the direction along the helix axis and the small differences between the t wo independent molecules in the P1 asymmetric unit. Other parameters which were important for the solution of this relatively low solvent content clos ely packed cell included the radius of integration, the use of normalized s tructure factors and especially the choice of starting model.