>Unless you are interested in finding curious objects, what would you do with >protein quasicrystal? The practices of macromolecular crystallography is about >determining 3-dimensional structure of objects being crystallized. Protein >quasicrystal are really unlikely to diffract to high enough resolution, and >even ignoring all other practical aspects, like writing programs to solve such >a structure, chances of building an atomic model are really slim.
Right, if crystallography is seen as purely a tool for biology I agree. As for curious objects, I think almost all profound breakthroughs come from unadulterated curiosity and not desire for some practical end. Not sure why a priori this should be so, but just consider your favorite scientific breakthrough and whether the scientist set out to make the discovery or not. Some are, but most are not, I think. Maybe aperiodic protein crystals have some important function in biology somewhere, or have unforeseen materials science properties, analogous to silk or something. >> This is easy to test by analyzing diffraction patterns of individual >> crystals. > In practice, the dominant contribution to angular broadening of > diffraction peaks is angular disorder of microdomains, particularly in > cryo-cooled crystals. > However, exceptions do happen, but these rare situations need to be > handled on case by case basis. >The interpretation of the data presented in this article is that variation in >unit cell between microcrystals induce their spatial misalignment. The data do >not show variation of unit cell within individual microscrystalline domains. >Tetragonal lysozyme can adopt quite a few variations of the crystal lattice >during cryocooling. Depending on the conditions used, resulting mosaicity can >vary from 0.1 degree (even for 1mm size crystal) to over 1. degree. Consequently, measured structure factors from a group of tetragonal lysozyme crystal can be quite reproducible, or not. As a test crystal, it should be handled with care. 1 degree mosaicity is not an impediment to high quality measurements. However, high mosaicity tends to correlate with presence of phase transitions during cryo-cooling. If such transition happen during cryo-cooling, crystals of the same protein, even from the same drop, may vary quite a lot in terms of structure factors. Additionally, even similar values of unit cell parameters are not guarantee of isomorphism between crystals. So I think you are saying that tetragonal lysozyme is an atypical case, and that normally the main contributor to the fitted parameter "mosaicity" is the phenomenon of microdomains shifted slightly in orientation. Maybe we can get the author to repeat the study for the other usual-suspect protein crystals to find out the truth, but the score currently seems to be 1-0 in favor of cell parameter shifts versus microcrystal orientation... JPK
