1. Istituto di Scienze Fisiche and INFM, Università di Ancona, Via Ranieri 65, I-60131 Ancona, Italy, European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France, Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5, I-34016 Basovizza (Trieste), Italy, and Institute of Biophysics and X-ray Structure Research, Austrian Academy of Sciences, Graz, Austria