Crystallography is the most common experimental method for obtaining
a detailed picture of a protein or protein complex. Such experiments involve
interpretation of the diffraction of X-rays from many identical molecules
in an ordered array commonly referred to as a crystal. This experimental
technique provides information on the positions of individual atoms
within a biological complex. Having a detailed structure of a macromolecule
aids greatly in understanding its function. Determining a protein’s structure
by X-ray crystallography consists of growing crystals of the purified pro-
tein, measuring the directions and intensities of X-ray beams diffracted
from the crystals, and using computers to transform the X-ray measure-
ments. This method produces an image of the crystal’s contents. This image
must be interpreted, which involves computer graphics to display the elec-
tron density of atoms in the molecule and the construction of a consistent
molecular model.
X-rays are used in diffraction experiments
because they have a wavelength of about 1 Å,
which matches the atomic bond. The basic
principles of diffraction are the same for X-rays
as for visible waves, which have much larger
wavelengths. Waves are described by their
amplitude, A, wavelength, λ, frequency, ν, and
velocity, 9 (Figure 9.1). When two waves come
together in space they can add together
constructively or destructively (Figure 15.1).
When the position of two peaks matches they
will add up constructively, but when a peak
is added to a trough the two waves cancel.
Diffraction is the property of many waves
coming together. As a simpleexample, consider
15 X-ray diffraction and extended X-ray absorption fine structure
15 X-ray diffraction and extended X-ray absorption fine structure
(a) (b)Figure 15.1Waves can add together either
(a) constructively or (b) destructively.