Cryo-TEM imaging was performed using a JEOL JEM-2100 transmission electron microscope (JEOL GmbH, Eching, Germany). Specimens were prepared by casting a 4 μl droplet of the aliquot solutions onto lacey carbon-coated copper TEM grids (200 mesh, Electron Microscopy Sciences, Hatfield, PA), and plunge-freezed into liquid ethane using an FEI vitrobot Mark IV set at 4 °C and 95% humidity. Vitrified grids were either transferred directly to the microscope's cryo-transfer holder (Gatan 914, Gatan, Munich, Germany) or stored in liquid nitrogen. All grids were glow-discharged before use. Imaging was carried out at temperatures around -180 °C. The TEM was operated at an acceleration voltage of 200 kV, and an objective lens defocus of about 1.5-2 μm was used to increase the contrast.