1. Howard Hughes Medical Institute and Department of Biochemistry, Brandeis University, Waltham, MA 02454, USA.
2. Laboratory of Organic Chemistry, ETH Zürich, 8093 Zürich, Switzerland.
3. Department of Chemical Engineering, Institute of Applied Life Sciences, University of Massachusetts, Amherst, MA 01003, USA.
4. Stanford Synchrotron Radiation Lightsource, Menlo Park, CA 94025, USA.