Selenium-based MAD phasing: setting the sites on larger structures

Over the past few years the method of macromolecular structure determination known as multiwavelength anomalous diffraction (MAD) has been transformed from an esoteric technique, used in a few favorable cases, into a mature and generally applicable approach to solving the crystallographic phase problem[1]. Since 1993 there has been a dramatic increase in the number of structures solved by MAD[2] and one of the major contributing factors has been the production of selenomethionyl (SeMet) proteins. In most cases, methionine auxotrophic expression systems have provided an elegant mechanism for ensuring the incorporation of anomalous scattering atoms[3] and this technique seems to be broadly applicable to many macromolecular systems[4]. Additionally, the selenium K absorption edge (at 12.658 keV) is at an ideal energy for most synchrotron radiation sources. As a result, selenium has become the most popular anomalous scattering atom for MAD phasing and has been used in approximately two-thirds of all reported applications[2].