Anomalous signal of solvent bromides used for phasing of lysozyme

The anomalous signal of bromide ions, present in the crystal structure of t etragonal hen egg-white lysozyme through the substitution of NaCl by NaBr i n the crystallization medium, was used for phasing of X-ray data collected to 1.7 Angstrom resolution with a wavelength near the absorption edge of br omine. Phasing of a single wavelength data set, based purely on anomalous d elta f " contribution, led to easily interpretable electron density, equiva lent to the complete multiwavelength anonalous dispersion phasing based on four-wavelength data. The classic small-structure direct methods program SH ELXS run against all anomalous differences gave a successful solution of si x highest peaks corresponding to six bromide ions in the structure with dat a limited up to a resolution of 3.5 Angstrom. Interpretable maps were obtai ned at a resolution up to 3.0 Angstrom using programs MLPHARE and DM. Bromi de ions occupy well ordered positions at the protein surface. Phasing based on the single wavelength signal of anomalous scatterers introduced into th e ordered solvent shell can be proposed as a tool for solving structures of well diffracting crystals. (C) 1999 Academic Press.