1. Contribution from the Department of Molecular Biology and The Skaggs Institute of Chemical Biology, The Scripps Research Institute, MB 33, 10550 North Torrey Pines Road, La Jolla, California 92037, National Centre for Biomolecular Research, Masaryk University, Kotlarska 2, 611 37 Brno, Czech Republic, and Bruker Biospin GmbH, Rheinstetten, Germany