A rapid method for quantifying heavy atom derivatives for multiple isomorphous replacement in protein crystallography

A rapid and simple X-ray fluorescence-based method is reported for characterizing heavy atom derivatives of proteins for protein crystallography using multiple isomorphous replacement (MIR). MIR is a widely used technique for solving protein crystallographic structures which requires that a `heavy atom' be incorporated into the protein to provide a strong signal in the diffraction pattern. Current methods for determining the effectiveness of these protein-heavy atom reactions are not always successful. In contrast, X-ray fluorescence quickly determines the presence of heavy atom modifications of proteins and the stoichiometry of these modifications.