300
8 Ionizing Radiation and Life
required may destroy the object being studied, so that a copy of that object has to be
used each time a new image is needed.
8.6.7 Small-Angle X-ray Scattering
There are circumstances when we wish to get structural information at the nanoscale
for biologically interesting molecules and systems of molecules in environments
as close to their natural state as possible. By scattering X-rays of wavelengths
near 0.15 nm with glancing angles from biological nanoscale objects distributed
in solution, some structural information about the shape of these objects can be
extracted.
Detection of small angle scattering is used to capture only those X-rays which
were scattered by the surface of the objects, as the interest is getting at their size
and shape. However, unlike X-ray crystallographic studies, now the detected X-rays
come from a sum of scattered waves made by a large number of rather arbitrarily
oriented identical objects in solution. (The contribution due to scattering from water
can be subtracted.) By trying various configurations of the molecules in a theoretical
calculation of the resulting X-ray scattering intensity as a function of the scattering
angle and then comparing with experiment, the possible configurations which nature
picks can be selected. In this way, the tertiary and quaternary protein structures in
situ can be unveiled. 6
8.6.8 X-ray Spectroscopy
X-ray Spectroscopy X-ray beams or electron beams, with energies in the keV range,
and sent through materials, are preferentially taken up by electrons in the inner
(K, L shells) of heavy atoms, kicking those electrons to a higher energy level or
completely out of the atom. When another electron falls back into the ‘hole’ left
by the kicked-out electron, radiation is emitted that is characteristic of that atom.
Studying this radiation gives information about not only the presence of the atom,
but about the environment around the atom affecting the electron orbital energies.
For example, the stereochemistry of anti-cancer drugs can be studied by seeing how
the drug makes attachments to cancer-active molecules. 7
The emitted X-ray photon energies can be detected by various techniques,
including Bragg diffraction from a crystal (giving the photon wavelengths) and
6 See, for example, Jessica Lamb, Lisa Kwok, Xiangyun Qui, Kurt Andresen, Hye Yoon Park, Lois
Pollack, Reconstructing three-dimensional shape envelopes from time-resolved small-angle X-ray
scattering data, J Appl Crystallography 41, 1046–1052 (2009).
7 See Czapla-Masztafiak et al., X-Ray Spectroscopy on Biological Systems, IntechOpen (2017).
8 Ionizing Radiation and Life
required may destroy the object being studied, so that a copy of that object has to be
used each time a new image is needed.
8.6.7 Small-Angle X-ray Scattering
There are circumstances when we wish to get structural information at the nanoscale
for biologically interesting molecules and systems of molecules in environments
as close to their natural state as possible. By scattering X-rays of wavelengths
near 0.15 nm with glancing angles from biological nanoscale objects distributed
in solution, some structural information about the shape of these objects can be
extracted.
Detection of small angle scattering is used to capture only those X-rays which
were scattered by the surface of the objects, as the interest is getting at their size
and shape. However, unlike X-ray crystallographic studies, now the detected X-rays
come from a sum of scattered waves made by a large number of rather arbitrarily
oriented identical objects in solution. (The contribution due to scattering from water
can be subtracted.) By trying various configurations of the molecules in a theoretical
calculation of the resulting X-ray scattering intensity as a function of the scattering
angle and then comparing with experiment, the possible configurations which nature
picks can be selected. In this way, the tertiary and quaternary protein structures in
situ can be unveiled. 6
8.6.8 X-ray Spectroscopy
X-ray Spectroscopy X-ray beams or electron beams, with energies in the keV range,
and sent through materials, are preferentially taken up by electrons in the inner
(K, L shells) of heavy atoms, kicking those electrons to a higher energy level or
completely out of the atom. When another electron falls back into the ‘hole’ left
by the kicked-out electron, radiation is emitted that is characteristic of that atom.
Studying this radiation gives information about not only the presence of the atom,
but about the environment around the atom affecting the electron orbital energies.
For example, the stereochemistry of anti-cancer drugs can be studied by seeing how
the drug makes attachments to cancer-active molecules. 7
The emitted X-ray photon energies can be detected by various techniques,
including Bragg diffraction from a crystal (giving the photon wavelengths) and
6 See, for example, Jessica Lamb, Lisa Kwok, Xiangyun Qui, Kurt Andresen, Hye Yoon Park, Lois
Pollack, Reconstructing three-dimensional shape envelopes from time-resolved small-angle X-ray
scattering data, J Appl Crystallography 41, 1046–1052 (2009).
7 See Czapla-Masztafiak et al., X-Ray Spectroscopy on Biological Systems, IntechOpen (2017).