2014 was designated as the International Year of Crystallography by the United Nations, as part of its programme of using anniversaries to highlight topics and issues that it sees as important. There have been many international years on science-based topics, some broader than others: 2005, for example, was the International Year of Physics. And in this new year the focus moves on to another physical science topic. 2015 is the International Year of Light: or more precisely that "... of Light and Light-based Technologies".
And optics, the science of light, is very closely related to crystallography. Both light and X-rays are parts of the electro-magnetic spectrum and as such are both particles and waves; the differences in their frequencies and wavelengths lead to their very different properties. The first X-ray diffraction experiments were conducted mainly to find out whether the hypothesis that X-rays were electromagnetic waves with a wavelength of about 1 Angstrom was correct. The utility of this technique as a probe for molecular structure was initially a side effect of the similarity between this wavelength and the length of a covalent bond. (So-called "copper K-alpha" radiation, which is often used in crystallography, has a wavelength of 1.54Å, which is exactly equivalent to the length of a single C-C bond.) And the basic equation that defines X-ray diffraction - Bragg's Law, or nλ = 2d sin θ - applies to all electromagnetic radiation, not just to X-rays.
Advances in optical technology have been responsible for some important advances in crystallography. The discovery, in the mid- twentieth century, that charged particles emit electromagnetic radiation in the form of X-rays when accelerated radially was initially viewed as a problem of energy loss. This "lost" radiation, which is orders of magnitude more intense than the X-rays used in the first diffractometers, was soon exploited in synchrotron radiation sources and most non-routine protein structures are now solved using synchrotron radiation. The UK's synchrotron source, a doughnut-shaped structure half a kilometre across located at Harwell in Oxfordshire, is even known officially as "Diamond Light Source". And recent advances in optics are gradually reducing the size of intense X-ray sources so that it will become possible to do complex structural biology that now needs a synchrotron using a lab-based machine.
The next big technical advance to influence X-ray crystallography is the free electron laser, which can generate pulses of X-rays that are much more intense than those produced by even the largest synchrotrons. This is a beam of electrons that is accelerated to almost the speed of light through a side-to-side magnetic field to produce pulses of extremely intense electromagnetic radiation. These pulses are exceptionally short, each lasting only a few femtoseconds (1 fs = 10-15s). This is a feature of the technology that leads to two advantages: the crystals remain intact, even though they are exposed to extremely intense radiation, and it becomes possible to take "snapshot" structures very close together in time study the dynamic behaviour of molecules. Free electron laser radiation also enables precise structures to be obtained from much smaller crystals, which makes large complexes and membrane-bound proteins more tractable to crystallography. And it may one day be possible, using this technique, to determine a structure from the smallest "crystal" of all - a single molecule - and thus liberate protein crystallography from one of its most important bottlenecks: the need to grow protein crystals.
Acknowledgement:This post owes much to this one on the "Crystallography 365" blog (author unknown)
