Single Particles

Abstract The single-particle approach to solving macromolecular structure (see Frank, 1996, 2006) is based on the premise that the specimen is in the form of many isolated particles that are randomly oriented, collectively presenting a large range of views. It is further assumed, at least initially, that through application of biochemical methods of isolation and purfication, the specimen is homogeneous; that is, only macromolecules of one kind are present. Finally, if the specimen is prepared in such a way that anisotropic compression is avoided, then all the molecules on the electron microscopic grid can be assumed to have identical structure, and hence the geometrical relationships among the molecules in three-dimensional (3-D) space can be modeled as a set of rigid body movements. However, even if this assumption of structural identity is violated, due to some native structural variability, we can still use the single-particle approach for solving the structure at low resolution, and treat the structural variability as a high-resolution perturbation, a topic to be discussed later on.