Radiation exposure and recognition of electron microscopic images of protamine at high resolution.

The effect of radiation exposure on visual recognition of individual macromolecules of the protein protamine was examined using high resolution electron microscopy in scanning transmission, fixed beam transmission, dark field and bright field, and recording of images on plates or via video imaging systems including image intensifiers and digital image storage. Loss of recognition of protamine, including its 5 A substructure, followed approximately three-hit kinetics with a Do on the exponential portion of the curve of about 470 e/A2. In spite of the inevitable chemical damage at high doses, virtually all molecules in orientations that exhibited the characteristic protamine structure could still be recognized in dark field scanning transmission or fixed beam bright field at 100-200 e/A2. Recognition fell from 30 to 6% when doses were increased from 1000 to 2000 e/A2 in fixed beam dark field. Exposure rates, varied over seven orders of magnitude from 0.15 e/A2-s to 3 X 10(6) e/A2-s, had no effect at all on the recognition of structures.