1. Key Laboratory of Magnetic Resonance in Biological Systems, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics National Center for Magnetic Resonance in Wuhan Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences Wuhan 430071 P. R. China
2. Graduate University of Chinese Academy of Sciences Beijing 100029 P.R. China
3. Department of Chemistry Department of Biochemistry and Biophysics University of North Carolina-Chapel Hill Chapel Hill NC 27599-3290 USA