IUCr journals news
Two-wavelength MAD phasing: in search of the optimal choice of wavelength
Gonzalez et al. [Acta Cryst. D55 (1999), 1449-1458] reported in their recent article that interpretable electron-density maps, similar in quality to those calculated with data collected at three wavelengths, could be obtained using only two wavelengths if the wavelengths were chosen so as to give a large contrast in the dispersive component of the scattering factor. Four different crystals, which contain iron, gold, iridium, or selenium atoms, were used in their study.
The multiwavelength anomalous dispersion (MAD) method exploits the structure-factor variation with wavelengths around the absorption edges of heavy atoms within the protein crystals. The variation consists of the real part (or the dispersive component, f') and the imaginary part (or the anomalous component, f''). The importance of using wavelengths that can provide the largest f' was explained as follows: “Firstly, the refinement of the anomalous scatterer positions and occupancies is highly dependent on the dispersive differences measured from centric reflections. Secondly, a MAD data collection is usually performed in a way which partially cancels out systematic errors in the structure factors at different wavelengths, leading to dispersive differences which are less affected by errors in the data.”
Two approaches have been used in calculating phase information from a MAD experiment. One approach uses an analytical method to solve the phase problem and calls for three or more wavelengths to optimize the results. The other approach treats MAD as a traditional isomorphous replacement phasing. The importance of selecting a large difference in the real part of the scattering factor in the two-wavelength experiment is interesting, as it resembles what is known from the single isomorphous replacement and anomalous scattering (SIRAS) experiment, i.e., phasing power increases with increased differences in the real part of the structure factors, which is achievable by the incorporation of heavier heavy atoms.
U. of Georgia, Athens, GA, USA