^*Correspondence to Kalyaneswar Mandal, Department of Biochenistry and Molecular Biology, The University of Chicago, Chicago, IL 60637

Funded by:
 Office of Science (BER), U.S. Department of Energy; Grant Number: DE-FG02-07ER64501
 National Institutes of Health; Grant Number: R01 GM075993
 Office of Science (BER), U.S. Department of Energy; Grant Number: W-31-109-Eng-38

We describe the use of racemic crystallography to determine the X-ray structure of the natural product plectasin, a potent antimicrobial protein recently isolated from fungus. The protein enantiomers L-plectasin and D-plectasin were prepared by total chemical synthesis; interestingly, L-plectasin showed the expected antimicrobial activity, while D-plectasin was devoid of such activity. The mirror image proteins were then used for racemic crystallization. Synchrotron X-ray diffraction data were collected to atomic resolution from a racemic plectasin crystal; the racemate crystallized in the achiral centrosymmetric space group P with one L-plectasin molecule and one D-plectasin molecule forming the unit cell. Dimer-like intermolecular interactions between the protein enantiomers were observed, which may account for the observed extremely low solvent content (13%-15%) and more highly ordered nature of the racemic crystals. The structure of the plectasin molecule was well defined for all 40 amino acids and was generally similar to the previously determined NMR structure, suggesting minimal impact of the crystal packing on the plectasin conformation.