X-ray crystallography is only one of several disciplines requiring protein crystals as essential materials. Some of us X-ray centric structure biologists may actually be surprised that there's a use for protein crystals for anything else besides X-ray diffraction. Here's my run-down:
1. Neutron diffraction
You're right - this is a lame one, protein structure determination with neutrons instead of X-rays. To diffract a neutron beam you need protein crystals. Massive ones. Think milli meters, not micro meters. But you'll see hydrogen atoms and their 'gymnastics'. Here's Dean Myles arguing for a bright future of neutron diffraction.
2. Solid State Nuclear Magnetic Resonance
Protein crystals sizes under 1 um? No problem. The good news with ssNMR: nanocrystalline material is fine.
Martin RW, Zilm KW.
Preparation of protein nanocrystals and their characterization by solid state NMR.
J Magn Reson. 2003 Nov;165(1):162-74.
3. Protein Purification
Crystallization is a way to purify and enrich your target protein, remember?
Seriously, I predict that we'll be seeing a renaissance of protein crystallization as a purification method for bulk material processing.
Russell A. Judge, Michael R. Johns, Edward T. White
Protein purification by bulk crystallization: The recovery of ovalbumin
Biotechnology and Bioengineering, Volume 48 Issue 4, Pages 316 - 323 (2004)
Why? Columns are expensive, crystallization is cheap.
4. Protein Formulations
Many small molecule drugs are delivered as carefully formulated crystalline powders. The same can be done for protein therapeutics. See:
Alexey L. Margolin et al.
Stabilized protein crystals, formulations comprising them and methods of making them
US Patent 7,351,798 B2
This is a patent claiming dried protein crystals formulation as a means to provide slow release for biomedical delivery applications such as vaccines and modern biologics protein drugs.
And this may be an interesting primer if you're interested in the use of protein crystals for protein formulation design.
Anna J., Merkle, H.P.
Diamonds in the Rough: Protein Crystals from a Formulation Perspective (great title!)
Journal Pharmaceutical Research
Volume 18, Number 11 (2001)
Plenty of job opportunities out there for protein crystallizers!
Peter