Patterson and molecular-replacement techniques

Abstract

The Patterson function is a phaseless Fourier synthesis of diffraction intensities that corresponds to an autocorrelation of electron densities in a crystal cell. Prominent non-origin peaks in a Patterson function represent interatomic vectors; deconvolution of this information can be used to locate heavy atoms, known structures or structural fragments. Interpretation of Patterson maps lies at the foundation of most macromolecular structure elucidation processes, both in determining the position of heavy atoms and/or anomalous scatterers for the isomorphous replacement and anomalous scattering methods, and in determining the orientation and position of a homologous protein model for the molecular-replacement method. The molecular-replacement method also encompasses the use of noncrystallographic redundancy within a crystal or between crystal forms to phase and refine crystal structures. Applications of Patterson and molecular-replacement techniques to structure determination including Patterson interpretation methods, rotation functions, translation functions, symmetry averaging, density modification and phase extension are discussed. This chapter is also available as HTML from the International Tables Online site hosted by the IUCr.