The majority of macromolecular structures solutions over the last thirty years have been by the
isomorphous replacement method typically using complex ions of the heavy atoms Pt, Au, Hg, Pb and U
(see Blundell & Johnson, Chap. 8 [18]). By far the most successful of these
heavy atoms has been mercury because of its high affinity for binding to
sulphur groups, for example through mercuration of cysteinyl 
groups not involved in disulphide
bridges, to give isomorphous derivatives with only a very small number of highly occupied heavy atom
sites making solution of the heavy atom partial structure by the difference Patterson method,
possible if not straightforward. Single wavelength anomalous scattering experiments have played an
additional role in the isomorphous replacement method by providing extra phase information through
the violation of Friedel's law and in some cases has done away with the need for a second
isomorphous derivative.
Multiple wavelength Anomalous Diffraction methods up until now have mainly been used for the
solution of metallo-enzymes (see for example [40], [42] and [43]), and
seleno-methionyl proteins (see [50], [52] and [120]). Metallo-enzymes
typically contain heavy atoms from the 1st transition series of the periodic table and their
neighbours. The K-edges of most of these metals fall within the X-ray energy range accessible by
synchrotron radiation but are at low energies 
. In most of the cases studied K-edges have
displayed only small deviations from theory [25] in the 
values near
the absorption edge. This fact
means that little advantage is to be gained by performing an optimised anomalous scattering
measurement at the absorption maximum of the heavy metal concerned. In the case of proteins which
have been engineered to contain a number of selenium atoms the K-edge lies at a usefully high energy
(
) where radiation damage becomes less severe due to the reduced effect of
absorption. In the best case the white line at the K absorption edge of Se can increase the
value from 
to 
however this has been shown to be highly
sensitive to the orientation of the selenium ligands with respect to the X-ray polarisation
vector [52] resulting in shifts in the height and position of the absorption maximum.
A third style of experiment has also been carried out where calcium atoms present in some proteins
have been substituted by heavier atoms from the lanthanide series. Two such cases involved
substituting Ho [115] and Tb [62] atoms where observed white line
maxima of 
and 
respectively were reported. In fact the metals in the lanthanide
series in general are particularly good candidates for white line
experiments [94] [107] at the 
and 
edges at X-ray energies
ranging from 
. In each of these cases experiments were performed at energies
corresponding to the maximum, 
minimum and at a third energy chosen
so as to obtain contrast in the value of .