Application of the P_s-function method to macromolecular structure determination

The P_s function derived from anomalous-dispersion data [Okaya, Saito, & Pepinsky (1955). Phys. Rev. 98, 1857-1858] has been tested with observed data for an Hg derivative of a small protein, avian pancreatic polypeptide [Glover, Moss, Tickle, Pitts, Haneef, Wood & Blundell (1985). Adv. Biophys. 20, 1-12]. The P_s map was superimposed on the four Hg sites via a sum function and negative densities were eliminated from the resultant map. This map, with appropriate density inserted at Hg sites, closely resembles a map calculated with true phases; the two maps have a correlation coefficient of 0-67. For 2109 reflexions the unweighted mean phase error is 39.9° but with |F_oF_c| weighting this reduces to 29.5°.