1. Key Laboratory of Chemical Biology (Ministry of Education), School of Pharmaceutical Sciences, Shandong University, Jinan, Shandong 250012, P. R. China
2. State Key Laboratory of Microbial Technology, Shandong University, Qingdao, Shandong 266237, P. R. China
3. Department of Chemistry, University of Nebraska Lincoln, Lincoln, Nebraska 68588, United States