1. Physics Department, Sapienza University, 00185 Rome, Italy
2. Institute of Crystallography, CNR, 00015 Monterotondo, Italy
3. LISC, CMM-FBK and University of Trento, 38123 Povo, Italy
4. Institute
of Physics, Catholic University, 00168
Rome, Italy
5. Structural
Biology Group, ESRF, 38043 Grenoble, France