Chemistry Reference
In-Depth Information
FIGURE 6.8
The di-nuclear ferroxidase centre Bacillus brevis Dps dodecamer. (a) Electron densities at one of the di-nuclear ferroxidase
centres at the dimer interface. The (2F
o
F
c
) map is coloured in light blue and contoured at the 1.4
s
level. The two iron ions have the highest
density values in the map, which are 12.7
s
and 7.2
s
. The existence of two water molecules at the di-iron site is revealed by the superimposed
(F
o
F
c
) map, which is coloured in red and contoured at the 4.0
s
level. The iron ions and water molecules are shown as spheres, coloured in
magenta and light orange, respectively. (b) A drawing of the coordination of the iron ions at the di-nuclear centre. The coordination is indicated
by dotted lines and distances (in
˚
). (From
Ren et al., 2003
. Copyright 2003 with permission from Elsevier.)
and their experimentally inaccessible phase constants. The phase problem was first resolved by J. Monteath
Robertson
6
for the (then) complex organic compound phthalocyanin, by isomorpous replacement in which
comparison of the molecule with either H, Ni or Cu at its centre allowed him to find phase constants and absolute
structures. This approach, multiple isomorphous replacement (MIR), was extended to protein crystals by the
preparation of derivatives in which a heavy atom is bound specifically and uniformly to molecules within the
crystal. MIR techniques further require that the heavy atom derivatisation does not introduce additional changes in
the molecular structure or change the crystallographic parameters. MIR has now been replaced by multiple
anomalous scattering
7
(MAD). MAD exploits the potential of using more than one wavelength along with the
6. Gardiner professor of chemistry in the University of Glasow, whose lectures I attended as an undergraduate in the early 1960s.
7. Anomalous scattering occurs when the frequency of the X-rays used falls near the absorption edge of one or more atoms in the sample, eg.,
transition metals, often found in metalloproteins, and other atoms such as selenium.
