What if large crystals are not available?
The standard X-ray protein crystallography experiment requires a single protein crystal specimen that is large enough to collect a “complete” data set, that is, to collect all the available diffraction peaks to a given resolution.
But what if large crystals are not available? A team of scientists at MacCHESS and the University of Toronto is pushing what is possible for small protein crystals at storage ring sources.
While structural biologists have expanded their purview to increasingly large and complex biological systems, the necessity for reliable, atomic resolution structural data for those systems has not changed. However, it is simply not possible to grow sufficiently large crystals for many systems. The necessity of large crystals in protein crystallography stems primarily from two factors. First, all other things being equal, microcrystals diffract more weakly than large ones, because the crystal volume, and thus number of protein molecules diffracting the X-rays, is lessened. Second, and more insidiously, protein microcrystals succumb more quickly to radiation damage – a loss of diffraction intensity resulting from X-ray induced, stochastic ionization and bond cleavage. These factors result in apparently contradictory solutions: increase the beam intensity to induce more diffraction, but at the expense of crystal lifetime; or lower the beam intensity, but collect weak data.
Image caption: The sample chip loaded and placed on the piezo stage.