3-DIMENSIONAL STRUCTURE OF LIPID VESICLES EMBEDDED IN VITREOUS ICE AND INVESTIGATED BY AUTOMATED ELECTRON TOMOGRAPHY

Automated electron tomography is shown to be a suitable means to Visualize the shape of phospholipid vesicles embedded in vitrified ice. With a slow-scan charge-coupled device camera as a recording device, the cumulative electron dose needed to record a data set of 60 projections at a magnification of 20,000x can be kept as low as 15 e(-)/Angstrom(2) (or 1500 electrons/nm(2)). The membrane of the three-dimensionally reconstructed vesicles is clearly Visible in two-dimensional sections through the three-dimensionally reconstructed volume. Some edges indicating a polygonal shape of the vesicles, frozen from the gel phase, are also clearly recognized. Because of the presently limited tilt angle range (+/-60 degrees), the upper and lower ''caps'' of the vesicles (representing about 35% of the surface of the ellipsoidal particles) remain invisible in the three-dimensional reconstruction.