1. College of Bioscience and BiotechnologyYangzhou University Yangzhou Jiangsu 225009 China
2. Testing CenterYangzhou University Yangzhou Jiangsu 225009 China
3. Department of ChemistryStanford University Stanford California 94305 USA
4. Present address: Amgen Inc. 1 Amgen Center Drive Thousand Oaks CA 91320 USA