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 Collaborative Innovation Center of Chemistry for Life Sciences, Wuhan Institute of Physics and Mathematics Chinese Academy of Sciences Wuhan 430071 P. R. China
2. Department of Chemistry Department of Biochemistry and Biophysics University of North Carolina, Chapel Hill Chapel Hill, NC 27599-3290 USA