Electron Dose Series

Electron-dose series recorded on vitrified chromatophore vesicles in Rhodobacter sphaeroides, a purple bacterium. The total tolerable electron dose for vitrified samples is 50–100 e A−2. In electron tomography, this dose has to be fractioned over the total number of projections in the tilt series, thus producing single images with low signal-to-noise ratios (SNRs). Accurate alignment of the individual projections requires a minimum SNR that is typically obtained at an electron dose of 1–2 e A−2. The dose series was recorded on an FEI Titan Krios transmission electron microscope (TEM) operated at 300 keV and equipped with a direct electron detector. The ~150-nm-thick sample was prepared by plunge freezing and subsequent thinning by focused-ion-beam (FIB) milling. The defocus was approximately −10 μm.