Figure 9 The second derivative in the isomorphous replacement (a) or the second-wavelength data in MAD (b) indicates which alternative solution is correct. In MAD it is, however, necessary that the contribution has to be different at the two wavelengths; otherwise, the selection cannot be achieved, even if values are different. Therefore, in the MAD approach the values indicate the two possible values of the protein phase and the dispersive differences in provide an indication which of the two alternatives is correct.

© International Union of Crystallography 2002