Two-wavelength MAD phasing

Two-wavelength MAD phasing
A. González, J.-D. Pédelacq1, M. Solà2, F. X. Gomis-Rüth 2, M. Coll2 , J.-P. Samama1 , S.
Benini
European Molecular Biology Laboratory (EMBL), Hamburg Outstation, Notkestrasse 85, 22603
Hamburg, Germany.
1 Groupe de Cristallographie Biologique, Institut de Pharmacologie et de Biologie Structurale du
CNRS, 205 route de Narbonne, 31077 Toulouse, France.
2Centre d’Investigació i Desenvolupament, CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain.
Project Number:508
EMBL Hamburg beam line(s) X31 and BW7A
The Multiwavelength Anomalous Dispersion (MAD) method is being increasingly used to determine protein crystal structures. In theory, data collection at two wavelengths is enough to determine
MAD phases, but most often three or even more wavelengths are used.
In this experiment we analyzed the results of the phasing procedure using only two wavelengths for
the phase determination of several proteins containing different types of anomalous scatterers. In
these cases, this approach leads to interpretable maps, not substantially inferior in quality to those
obtained with data from three wavelengths, provided that the wavelengths are carefully chosen so
as to give a big contrast in the real part of the anomalous scattering factor f’.
Figure 1: Detail of the 2.1 Å MAD map of the cytochrome c553 calculated with phases from a
two-wavelength data set, showing an -helix and the refined model.