and the location of the metal can be identified. This approach is now com-
monly used to provide phase information by incorporating selenium at
positions normally occupied by methionine by replacement of that residue
with selenomethionine.
In addition to use of anomalous dispersion for phase information,
for proteins with metal cofactors, the anomalous dispersion provides an
unambiguous identification of the metal as each metal has a different
wavelength dependence. This identification is particularly useful for the
assignment of individual atoms in complex metal clusters or new cofactors.
For example, the protein phenoxazinone synthase (Figure 15.9) was known
to be a multicopper oxidase with four copper atoms. The electron density
calculated for the protein showed the presence of a fifth metal cofactor
(Figure 15.15). The diffraction was measured at different wavelengths at
the Advanced Light Source synchrotron and the electron-density map
calculated for copper showed a peak at the same position, demonstrating
that the metal was a fifth copper atom. The role of this copper atom has
not been determined and is under investigation (Smith et al. 2006).
330 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY
Figure 15.15Electron-density maps showing for one region the positions of (a) all atoms of
the protein phenoxazinone synthase and (b) only the copper atoms. Modified from Smith
et al. (2006).
