1. EMBL, Meyerhofstrasse 1, 69117 Heidelberg, Germany, Departamento de Quimica Fisica, Facultad de Ciencias, Universidad de Granada, 18071-Granada, Spain, EMBL, DESY, Notkestrasse 85, D-22603 Hamburg, Germany and Institute of Protein Research, Russian Academy of Sciences, Pushino, 142292 Russia