The 7 crystal systems
Tiny mineral operations transcribe handsomely regular crystals
There are 230 space groups (ways of packing molecules into a regular lattice), but since proteins are chiral — with a “handedness” — that number drops to just 65.
Each of these can be assigned to one of the 7 crystal systems, above. The system is determined by the shape and dimensions of what’s known as the unit cell, the lattice’s fundamental repeating unit.
Crystallisation requires the ordered formation of large (>0.1mm dimensions), stable crystals with sufficient long-range order to diffract X-rays. Structures from X-ray diffraction are only as good as the crystals they are obtained from.
To form a crystal, protein molecules assemble into a periodic lattice from super-saturated solutions (starting with pure protein and adding precipitants to perturb protein-solvent interactions). The equilibrium in solution shifts towards protein-protein association, and at some point nucleation sites form — a critical first step.
This stage is followed by expansion, and cessation as the crystal’s size reaches its limit. Crystals are produced in labs by vapour diffusion — the standard method, good for small volumes and commonly in what’s known as a ‘hanging drop' — and less commonly by equilibrium dialysis — for low and high ionic strength solutions, equilibrating with a solution of precipitant which seeps across a semipermeable membrane inducing formation of a crystal.
There’s currently some very intriguing work being done looking into the processes of nucleation and crystal growth on so-called “labs-on-a-chip”: microfluidic devices which allow highly controlled and precisely observed high-throughput screening of protein crystals.
This is a particularly interesting use of the equilibrium dialysis method that seems to hold some definite advantages over the easy setup of vapour diffusion — particularly as refining the parameters for crystal growth is crucial for getting the best quality structures possible.
The microfluidic toolbox to manipulate liquids in networks of microchannels with 1–100 μm length scales. Such networks mimic classical experiments performed in a laboratory, but with an unequalled control of the transport phenomena. In the specific context of crystallization, these fluidic tools essentially permit the manipulation of aqueous solutions around room temperature. The range of application was originally quite limited but constant progress in the microfluidic technology now yields original features that permits the researcher to:
1. Perform high-throughput data acquisition using crystallization assays down to 1 nL;
2. Design specific kinetic routes using the excellent control of the mass and heat transfers due to the reduction of the length scales and on-chip integration of sensors and actuators;
3. Bring new experimental conditions to investigate crystallization, with no turbulence, no or little gravity effect, confinement, and large surface/volume ratio.
Additionally, the small volumes V of microfluidics are of special interest for nucleation. The mean nucleation time (∝ 1/V) may exceed the growth kinetics of the crystals and only one nucleation event is therefore statistically observed: this mononuclear mechanism is essential to estimate nucleation kinetics and investigate polymorphism.
♦ Leng and Salmon (2009) Microfluidic crystallization. Lab Chip, 9: 24‒34.
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