Phasing power at the K absorption edge of organic arsenic

Single/multiple-wavelength anomalous dispersion (SAD/MAD) experiments were performed on a crystal of an organic arsenic derivative of hen egg-white lysozyme. A para-arsanilate compound used as a crystallizing reagent was incorporated into the ordered solvent region of the lysozyme molecule. Diffraction data were collected to high resolution (≤2.0 Å) at three wavelengths around the K edge (1.04 Å) of arsenic at beamline BM30A, ESRF synchrotron. Anomalous Patterson maps clearly showed the main arsanilate site to be between three symmetry-related lysozyme molecules, at a location previously occupied by a para-toluenesulfonate anion. MAD phases at 2 Å derived using the program SHARP led to an electron-density map of sufficient quality to start manual building of the protein model. Amplitudes from a second crystal measured to a resolution of 1.8 Å at the peak wavelength revealed two additional heavy-atom sites, which reinforced the anomalous subset model and therefore dramatically improved the phasing power of the arsenic derivative. The subsequent solvent-flattened map was of such high accuracy that the program ARP/wARP was able to build a nearly complete model automatically. This work emphasizes the great potential of arsenic for de novo structure determination using anomalous dispersion methods.