In addition to trapping proteins in different functional states it is now
possible to monitor protein structure as it undergoes changes in time. Since
data collection requires at least 1 h at a synchrotron, the experimental
approach must be modified to observe subsecond structural changes. In
considering the Bragg law (eqn 15.4), instead of measuring the diffrac-
tion for different angles of the crystal, it is also possible to use a range
of wavelengths using the Laue method. For a fixed orientation of the
crystal, the Bragg condition for different reflections will be satisfied
simultaneously for different diffraction peaks for different combinations
of wavelengths. In this case, a full data-set can be measured, ideally with
only one orientation of the crystal, and the exposure time can be as short
as a few milliseconds.
Research direction: nitrogenase
As discussed in Chapter 3, nitrogen fixation is a fundamental biological
process that is part of the overall nitrogen cycle for the Earth. The
formation of ammonia from nitrogen and hydrogen gas is, overall,
336 PART 2 QUANTUM MECHANICS AND SPECTROSCOPY
Figure 15.21The structure of myoglobin with CO (shown as a small
rod) bound to the heme (dark blue) and after dissociation (light purple).
Notice that one histidine shifts in response to binding while the other
does not appreciably move.
