For cryo-TEM studies, a sample droplet of 2 µL was put on a lacey carbon filmed copper grid (Science Services, Munich, Germany). Subsequently, most of the liquid was removed with blotting paper leaving a thin film stretched over the lace holes. The specimens were instantly shock frozen by rapid immersion into liquid nitrogen in a temperature-controlled freezing unit (Zeiss Cryobox, Carl Zeiss Microscopy GmbH, Jena, Germany). The temperature was monitored and kept constant in the chamber during all the sample preparation steps. The specimen was inserted into a cryotransfer holder (CT3500, Gatan, Munich, Germany) and transferred to a Zeiss / LEO EM922 Omega EFTEM (Zeiss Microscopy GmbH, Jena, Germany). Examinations were carried out at temperatures around 90 K. The TEM was operated at an acceleration voltage of 200 kV. Zero-loss filtered images ( E = 0 eV) were taken under reduced dose conditions (100 -1000 e nm -2 ). All images were registered digitally by a bottom mounted CCD camera system (Ultrascan 1000, Gatan, Munich, Germany) combined and processed with a digital imaging processing system (Digital Micrograph GMS 1.9, Gatan, Munich, Germany).