Contrast and radiation damage in point-projection electron imaging of purple membrane at 100 V

Experimental transmission point-projection images of edge regions of uncoated, unstained purple membrane (PM) protein have been obtained at a beam energy of 100 V. We find that very high electron doses may be tolerated at this electron energy without structural change at the 2 nm level, that the membrane (thickness about 5.0 nm) is transparent above 1 kV and opaque at 100 V, and that is shows no charging effects below about 200 V. Image interpretation and damage mechanisms are reviewed.