= Electron crystallography
\b[Electron crystallography] determines ordered molecular or atomic structure using electrons interacting with a crystalline specimen. <Electron diffraction> and suitably resolved electron images provide complementary constraints on a structural model. The periodic order supplies repeated information; interpreting that information still requires a model of scattering and image formation.
An important biological application resolved the alpha/beta <tubulin> <heterodimer>. Zinc-induced tubulin sheets provided an ordered specimen for a model reported at $3.7$ angstrom resolution. The resulting structure distinguished the two related <protein subunits> and their different nucleotide environments.<Structure of the alpha beta tubulin dimer by E. Nogales and others>[Nature 391 (1998), 199–203; abstract and structural deposition 1TUB: electron crystallography of zinc-induced sheets.]{ref}
The specimen geometry matters. An ordered sheet is not the same object as an isolated <microtubule> in a living cell, even when its molecules reveal features relevant to microtubule assembly. The structural experiment answers questions about the molecular arrangement under its preparation conditions; biological interpretation then connects that arrangement to other observations.
Resolution also differs from the size of the object. A sheet may extend over many molecular repeats while the data constrain features on the angstrom scale. Quoting a nominal resolution does not mean every atom is independently located with that exact uncertainty or that every flexible region is equally well resolved.
<Electron crystallography> belongs to the wider family of <electron microscopy> and <crystallography> methods, but should not be used as a synonym for every electron micrograph. A dark spot, an image of a cellular outline and a diffraction-constrained molecular model are different types of evidence.
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