1. Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk 790-784, Korea, School of Chemical and Biological Engineering, Seoul National University, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-744, Korea, and Biotechnology Department, Central Research Institute of Shinpoong Pharmaceutical Company, 434-4 Moknae-dong, Danwon-gu, Ansan, Kyunggi 425-100, Korea