A VG Microscopes field-emission scanning transmission electron microscope (STEM) has been used to image and analyze rapidly frozen isolated macromolecules. Despite structural degradation due to radiation damage, elemental analysis of metal atoms bound to proteins was achievable at a resolution of about 100 angstroms using parallel-detection electron energy loss spectroscopy (PEELS). Copper was measured in hemocyanin molecules and the four iron atoms in hemoglobin were detectable if very high electron doses were used (about 5x10(8)e/angstroms(2)). Phosphorus was detectable in the tobacco mosaic virus containing a single strand of RNA, suggesting that PEELS could be used to measure the degree of phosphorylation of proteins such as neurofilaments. Elemental mapping was achieved using a digital acquisition system interfaced to the parallel EELS and it was demonstrated that images could be obtained from ferritin molecules giving the iron distribution in these molecules.
