Envelope skeletonization as a means to determine monomer masks and non-crystallographic symmetry relationships: application in the solution of the structure of fibrinogen fragment D

An algorithm is described which utilizes the solvent mask generated by the solvent-flattening technique to calculate a monomer molecular envelope. In the case where non-crystallographic symmetry (NCS) is present in the crysta l and self-rotation angles are known from a self-rotation function, the res ultant monomer envelopes can be used to search for the translation componen t of the NCS element by a three-dimensional search in real space. In the ab sence of self-rotation angles, the monomer envelope may be used to derive t he NCS operators by reciprocal-space techniques, Thus, an automatic procedu re for averaging directly from the solvent-flattening stage can be implemen ted. The procedure was instrumental in the structure solution of fibrinogen fragment D, which is presented as an example.