It was just over a century ago that William Lawrence Bragg and his father, William Henry Bragg, kick-started the science of X-ray crystallography in a talk at the Cambridge Philosophical Society on November 11, 1912. Since then, thousands upon thousands of structures have been solved, from table salt and diamond to RNA polymerase and the ribosome.
Back in the Braggs’ time, crystals were analyzed using laboratory X-ray tubes, which are relatively weak sources of continuous, noncoherent light—that is, light that travels in all directions, like lamplight. In the 1970s, researchers started using synchrotron particle accelerators, which can shoot partially focused (i.e., relatively coherent), highly intense X-ray radiation at a crystal. According to Jianwei Miao, a professor of physics and astronomy at the University of California, Los Angeles, a synchrotron can produce at least nine orders of magnitude more photons per second than a lab X-ray source, producing higher-quality diffraction data from smaller crystals.
Since 2009, a select number of researchers have had another option. The Linac [linear accelerator] Coherent Light Source (LCLS) at Stanford University and the SACLA (SPring-8 Angstrom Compact Free Electron Laser) in Japan are the world’s first “hard” X-ray free-electron lasers (XFELs), capable of producing light some billion times more brilliant than that from a synchrotron, and colliding it with tiny crystals in pulses just femtoseconds long. That’s like taking all the sunlight that hits the Earth and focusing it into one square millimeter, explains Janos Hajdu, a professor of molecular biophysics at Uppsala University in Sweden. Under such intense irradiation, the sample is destroyed almost instantaneously—but not before it diffracts, producing a weak but detectable signal.
As X-ray intensity increased, the crystal size needed to solve a structure has decreased, from about 1 mm or more for an X-ray tube, to 100–200 μm for a synchrotron, to as small as 200 nm for an X-ray laser, says Sébastien Boutet, a staff scientist at LCLS. That’s a boon for structural biologists, because growing large crystals has been a perennial source of torment. That synchrotrons and XFELs can use smaller crystals, which are easier and faster to grow, has improved matters—but the ultimate goal is to get rid of crystals altogether: to image individual molecules, molecular complexes, or viruses. That would represent a huge advance for scientists; not all proteins crystallize, and those that do sometimes adopt conformations that are not representative of their native forms in vivo.
Researchers have not reached that crystal-free point yet, at least not with individual proteins, but they have made significant leaps.