BioPHYSICAL chemistry

slowly by vapor exchange with a large
reservoir solution that is initially poised
at a higher salt concentration. Since the
container is a closed system, the protein
drop will concentrate slowly with time and
hopefully produce crystals once the pro-
tein concentration exceeds the solubility.
Protein precipitate has no particular
shape but protein crystals have well-
defined facets and edges despite their
large solvent content (Figure 15.9). The
overall morphology of the crystal will
usually reflect the space group of the
crystal. Proteins will often crystallize in
more than one space group as the con-
tacts that stabilize the crystal involve
hydrogen bonds, and salt bridges can
be formed by different combinations of
surface resides.
Some proteins are crystallized easily
whereas others require much effort be-
cause the proper conditions, such as pH,
temperature, salt concentration, and pro-
tein concentration, can be found only by
trial and error, which requires consider-
able time, effort, and protein. Sometimes
the crystallization is restricted by limita-
tions in the stability of a protein. All of
these problems are tremendously multi-
plied in proteomics, which has the goal
of determining the structure of every
protein in a given organism. Despite
these difficulties, efforts are underway in
a number of laboratories to achieve this
goal by developing expression systems
that can yield large quantities of each
gene product and large-scale robotic
control of crystallization.

Diffraction from crystals

The diffraction pattern from a com-
plex molecule cannot be solved directly
from Bragg’s law, but requires a more

CHAPTER 15 X-RAY DIFFRACTION AND EXAFS 323

Salt reservoir

Protein drop

Several days

Several days

Figure 15.8Crystallization using batch- and

vapor-diffusion methods. The protein drop in the

vapor-diffusion method is usually very small, at

about 1μl, to minimize the amount of protein.

Figure 15.9Crystals of the protein phenoxazinone

synthase. See Smith et al. (2004).