DIRECT-METHOD STRUCTURE DETERMINATION OF THE NATIVE AZURIN II PROTEIN USING ONE-WAVELENGTH ANOMALOUS SCATTERING DATA. Q. Hao, Department of Applied Physics, De Montfort University, Leicester LE1 9BH, England; Zheng Xiao-feng & Fan Hai-fu, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China; F. E. Dodd & S. S. Hasnain, CCLRC Daresbury Laboratory, Warrington WA4 4AD, England.

The one-wavelength anomalous scattering (OAS) X-ray diffraction data of azurin II, a copper-containing protein from Alcaligenes xylosoxidans were collected at the Photon Factory at a "routine" wavelength of 0.97Å. The structure had been originally solved by the molecular replacement method (Dodd, Hasnain, Abraham, Eady & Smith (1995) Acta Cryst., D51, 1052-1064). As a technique of ab initio structure determination, the direct method (Fan, Hao, Gu, Qian & Zheng (1990). Acta Cryst. A46, 935-939) was attempted to break the phase ambiguity intrinsic to one-wavelength anomalous scattering data. The phases were then improved using the solvent-flattening method. The final electron-density map clearly shows most C( positions and many side chains and it is traceable without prior knowledge of the structure. It is concluded that the direct method is capable of phasing anomalous scattering data collected at one wavelength from moderate sized native proteins (Mw ~ 20kDa) which contain copper or atoms with a similar scattering power.