Protein crystal structure solution by fast incorporation of negatively andpositively charged anomalous scatterers

The preparation of derivatives by the traditional methods of soaking is one of the most time-consuming steps in protein crystal structure solution by X-ray diffraction techniques. The `quick cryosoaking' procedure for derivat ization with halides (monovalent anions) offers the possibility of signific antly speeding up this process [Dauter et al. (2000), Acta Cryst. D56, 232- 237]. In the present work, an extension of this technique is proposed and t he use of two different classes of compounds (monovalent and polyvalent cat ions) that can be successfully utilized in the quick cryosoaking procedure for the derivatization and phasing of protein crystals is described. This a pproach has been tested on hen egg-white lysozyme and has been successfully used to solve the structure of a novel trypsin inhibitor. The possibility of using cations in the fast cryosoaking procedure gives additional flexibi lity in the process of derivatization and increases the chances of success in phase determination. This method can be applied to high-throughput cryst allographic projects.