The advantage of EM over X-ray diffraction when investigating the molecular structure of biological matter is its ability to gather an image (i.e. a phase and amplitude representation) and not just a diffraction pattern (i.e. amplitudes only) of the specimen. Its disadvantage is given by the fact that, despite the near-atomic (i.e. 2-3 Å) resolution performance of state-of-the-art instruments, the practical resolution with most biological specimens is typically limited to 10-30 Å. However, with a few "cooperative" specimens significant structural detail at the 5-10 Å resolution level has been obtained. Hence, in most cases studied to date, the EM has only provided us with relative representations (i.e. the overall size and shape) of protein molecules rather than with their absolute (i.e. near-atomic) structure as this is now commonly achieved by X-ray diffraction analysis. These limitations are primarily specimen-dependent: (1) biological matter has relatively low inherent contrast in an (e.g. 100 kV) electron beam; (2) the alterations (e.g. denaturation and collapse) accompanying the preparation (e.g. dehydration) of biological material for inspection in the high vacuum of an EM cause serious specimen preparation artifacts; (3) biological specimens are extremely radiation sensitive when bombarded with electrons.
U. Aebi, U. Aebi,
A. Engel, A. Engel,
"Three-Dimensional (3D) Electron Microscopy (EM) Of Biological Matter At The Molecular Level", Proc. SPIE 1030, Biostereometrics '88, (14 April 1989); doi: 10.1117/12.950484; https://doi.org/10.1117/12.950484