1. Department of Biochemistry and Biotechnology, Royal Institute of Technology, S-100 44 Stockholm, Sweden, Department of Structural Biochemistry, Pharmacia & Upjohn AB, S-112 87 Stockholm, Sweden, Center for Advanced Biotechnology and Medicine and Department of Molecular Biology and Biochemistry, Rutgers University, Piscataway, New Jersey 08854-5638, and Departamento de Quimica Fisica, Fac. Quim., Universidad de Valencia, Dr. Moliner 50. 46100 Burjassot, Valencia, Spain