The use of SnB to determine an anomalous scattering substructure

The positions of eight Se atoms in a selenomethionyl 35 kDa protein were determined at 2.0 and 2.5 Angstrom resolution using the direct-methods program SnB. Data at the selenium peak, edge and remote wavelengths were measured and processed independently. Anomalous difference E magnitudes at each wavelength were derived by two different procedures: renormalized diffE values were calculated according to the equation diffE = {Sigma[(f + f')(2) + f "(2)]\\E+\ - \E-\\}(1/2)/2q(Sigma f "(2))(1/2), where q is a least-squares fitted renormalization function of sin theta/lambda such that [diffE(2)] = 1.0; and difference E magnitudes were calculated from Delta F-2. Locally normalized E magnitudes corresponding to \F-A\ were also derived from the combination of the data at all three wavelengths through the use of the MADSYS program suite. Each of the independent sets of anomalous difference E magnitudes was capable of producing the correct solution, as did the E data obtained from the F-A data. Higher success rates with SnB were observed for the 2.0 Angstrom peak and edge diffE data.