Racemic crystallography

Racemic protein crystallography is a recently developed technique of structural biology, in which crystals of a protein molecule are grown from a mixture of an ordinary chiral protein molecule and its mirror image; where ordinary protein molecules made of 'left-handed' L-amino acids can be produced in bacteria, yeast, or other cellular expression systems, the mirror image molecule requires chemical synthesis from 'right-handed' D-amino acids.

Development

Laura Zawadzke and Jeremy Berg were the first to explore the idea in 1993 using the small (45 amino acid) protein rubredoxin. An early motivation for pursuing such studies was the idea that structure determination might be easier or more robust using diffraction data from a centrosymmetric crystal, which requires growth from a racemic mixture. Aside from this, there is reason to believe that racemic crystallography could have a more profound impact, by dramatically improving the ease with which crystals of protein molecules can be obtained in the laboratory; the protein crystallization problem remains the most challenging and unpredictable obstacle in macromolecular crystallography. In 1995, Stephanie Wukovitz and Todd Yeates, while laying out an explanation for why protein molecules tend strongly to crystallize in certain preferred symmetries, predicted that proteins would crystallize with much greater ease from racemic mixtures owing to the existence of especially favored racemic crystal symmetries that can only be obtained when using a racemic protein mixture. A further prediction was made that a specific crystal symmetry known as P1(bar) would be the dominant space group observed.

The invention of native chemical ligation methods by Phil Dawson and Stephen Kent in the mid-1990s opened up prospects for chemically synthesizing larger protein molecules. Kent and co-workers have since tested racemic crystallography on a wide range of protein molecules. Current data (reviewed in ref. provide support for the idea that proteins do crystallize with relative ease from synthetic racemic mixtures (and most often in P1(bar)) as predicted.