Torsion Angles of Epothilones Derivation of Dihedral Angles from CH–CH Dipolar–Dipolar Cross-Correlated Relaxation Rates: AC–C Torsion Involving a Quaternary Carbon Atom in Epothilone A Bound to Tubulin** Teresa Carlomagno,* Víctor M. Sµnchez, Marcel J. J. Blommers, and Christian Griesinger* Cross-correlated relaxation (CCR) rates constitute a valid alternative to coupling constants for the determination of dihedral angles. [1] For high-molecular-weight systems the measurement of scalar coupling constants poses serious problems caused by large linewidths and by differential relaxation effects. [2] Conversely, CCR measurements are feasible also for molecules of large size and have been successfully employed in the past years to determine dihedral angles in biomolecules. [3–6] We have recently reported the usage of cross-correlated relaxation rates to derive angular [*] Dr. T. Carlomagno, Prof.Dr. C. Griesinger, Dr. V.M. Sµnchez Max-Planck-Institut für Biophysikalische Chemie Am Fassberg 11, 37077 Göttingen (Germany) Fax: (+ 49) 551-201-2201 E-mail: taco@nmr.mpibpc.mpg.de cigr@nmr.mpibpc.mpg.de Dr. M. J. J. Blommers Novartis Pharma AG P.O. Box, 4002 Basel (Switzerland) [**] This work was supported by the MPG, the DFG and the Fonds der Chemischen Industrie. V.M.S. is supported by the EU through a Marie Curie fellowship. All measurements were performed at the Large Scale Facility for Biomolecular NMR in Frankfurt (HPRI-1999- CT-00014). The authors thank T. Schupp and F. Petersen of Novartis Pharma AG for the preparation of 13 C-labeled epothilone. Angewandte Chemie 2619 Angew. Chem. 2003, 115, 2619 – 2621 DOI: 10.1002/ange.200350950 # 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim