1. National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States
2. Department
of Biochemistry and Molecular Biology, The Pennsylvania State University, Hershey, Pennsylvania 17033, United States
3. Laboratory
of Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, United States
4. National
Center for Magnetic Resonance in Wuhan, Wuhan Institute of Physics
and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China