X-RAY CRYSTALLOGRAPHY 83
density functional theory. The data were fi t with a Fe · · · Cu distance of ∼ 3.72 Å.
Very little change in bond distances or angles were found between the solid
vs. solution EXAFS results.^8 The results are discussed in Section 7.8.4 (see also
Figure 7.44 ).
3.3 X - RAY CRYSTALLOGRAPHY
3.3.1 Introduction,
X - ray crystallographic molecular structures of proteins have been available
since the 1960s and 1970s when pioneering work by Kendrew^9 and Perutz^10
produced X - ray diffraction structures of myoglobin and hemoglobin. These
oxygen - carrying metalloproteins are discussed in Chapter 7. Since that time
the introduction of sophisticated computer hardware and software has made
the solution of protein structure in the solid state using X - ray crystallography
more accurate and less time - consuming. The fi eld continues to evolve as hard-
ware and instrument design improvements are implemented and as crystal-
lographers discover more powerful software algorithms for solving structures
after the necessary data have been collected. At the time of this writing, 380+
X - ray crystallographic data sets were deposited in the Research Collaboratory
for Structural Bioinformatics ’ Protein Data Bank (RCSB - PDB at http://www.
rcsb.org/pdb/ ) for hemoglobin and hemoglobin as well as 245+ data sets for
myoglobin and myoglobin mutant species. Nuclear magnetic resonance protein
structure determination in solution provides a complementary structural tech-
nique that does not require the production of single crystals necessary for X -
ray diffraction studies. However, at this time, NMR solution structures are
limited to smaller proteins of molecular weights less than 30,000. In contrast,
X - ray crystallography can produce structures of proteins of up to 1 × 10^6
molecular weight. Recombinant DNA technology has aided the X - ray crystal-
lographic study of proteins by allowing large amounts of a protein of interest
to be produced through expression of its cloned gene in a microorganism.
Site - directed mutagenesis of a selected protein ’ s gene has allowed researchers
to study three - dimensional structural changes brought about by amino acid
replacement in the protein ’ s primary amino acid sequence. These techniques
are discussed in Sections 2.3.4 and 2.3.5. Much of the discussion in this section
on X - ray crystallography has been taken from a recent text written by author
and crystallographer Jan Drenth.^11 Readers are referred to the Department of
Crystallography site at W ü rzburg University ( http://www.mineralogie.uni -
wuerzburg.de/crystal/teaching/teaching.html ) for tutorial on X - ray diffraction
methodology. The site includes interactive tutorials describing basic examples,
reciprocal space, the crystallographic phase problem, and diffuse scattering
and defect structures. Tutorials on convolution theorem, modifi cation of a
structure, solving a simple structure, anomalous scattering, and powder diffrac-
tion are also found on this site.
