1. Department of Biological SciencesPurdue UniversityWest LafayetteIndiana47907USA
2. Department of Molecular Biophysics and BiochemistryYale UniversityNew HavenConnecticut06611USA
3. Department of Molecular BiologyKarolinska InstituteCenter for Structural Biochemistry, NOVUMS‐14157HuddingeSweden
4. Division of Chemistry and Chemical Engineering, 147‐75CHCalifornia Institute of TechnologyPasadenaCalifornia91125USA