THE USE OF SNB TO DETERMINE AN ANOMALOUS SCATTERING SUBSTRUCTURE

The positions of eight Se atoms in a selenomethionyl 35 kDa protein we re determined at 2.0 and 2.5 Angstrom resolution using the direct-meth ods program SnB. Data at the selenium peak, edge and remote wavelength s were measured and processed independently. Anomalous difference E ma gnitudes at each wavelength were derived by two different procedures: renormalized diffE values were calculated according to the equation di ffE = {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 magnitu des were calculated from Delta F-2. Locally normalized E magnitudes co rresponding to \F-A\ were also derived from the combination of the dat a at all three wavelengths through the use of the MADSYS program suite . Each of the independent sets of anomalous difference E magnitudes wa s capable of producing the correct solution, as did the E data obtaine d from the F-A data. Higher success rates with SnB were observed for t he 2.0 Angstrom peak and edge diffE data.