The use of selenomethionine and anomalous dispersion has become the most widely used way of solving the phase problem for de novo protein structure determination. In this paper, MAD data collected from oxidized and reduced selenomethionine-containing protein are described, and it is shown that oxidized selenomethionine has a very strong phasing power and can be efficiently used if the oxidation is uniform. The comparison was performed on threonine synthase crystals. For example, the phasing power of the oxidized data is doubled for the dispersive signal and is 20% stronger for the anomalous signal at the peak wavelength. The strength of the anomalous signal can be used to improve the signal when a protein contains few methionines or for single anomalous dispersion. The oxidation of some selenomethionines shows in the electron-density map through the presence of water molecules within hydrogen-bonding distance of the putative O atom.
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