On the application of anomalous scattering in oligonucleotide crystallography

Simulated anomalous-scattering differences, at wavelengths between 1.5 and 5.5 A, were used with MULTAN to locate P atoms in an oligonucleotide hexamer. The success of the method depended heavily on the level of errors in the data. With error-free data most or all P atoms were located at all wavelengths. With noisy data, the best results were obtained by refining the phases associated with the largest values of |DeltaF|/sigma(|DeltaF|) rather than with the largest values of |DeltaF|. In this case a few of the P-atom positions could be located, with the best results occurring at wavelengths between 3.0 and 4.0 A. Further improvements were gained by reducing the values of the thermal parameters of the P atoms. MULTAN figures of merit had limited success in indicating the best phase sets, but a small improvement was gained by modifying the procedure for selecting those reflections used in the calculation of PSIZERO.