Abstract

Single heavy atoms of potential usefulness as heavy atom labels for biological molecules are presently being studied by two techniques. A small computer interfaced directly to a high resolution 30 kv scanning transmission electron microscope (STEM) is used to measure the elastic scattering cross-sections of each heavy atom (e.g. Thorium) or aggregates of heavy atoms in a selected field of the microscope. Histograms of the scattering cross-sections can be made to determine the average crosssection. Statistical analysis of the parameters listed below for the carbon film background and heavy atom signals give the most useful evaluation of heavy atom visibility. Model compounds with covalently bound iodine and mercury atoms spaced greater than 5 Å apart are also being studied to determine the elastic scattering cross-sections and to evaluate radiation damage from molecules with a well defined geometry.

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