Elemental mapping in single-particle reconstructions by reconstructed electron energy-loss analysis

For macromolecular structures determined by cryogenic electron microscopy, no technique currently exists for mapping elements to defined locations, leading to errors in the assignment of metals and other ions, cofactors, substrates, inhibitors and lipids that play essential roles in activity and regulation. Elemental mapping in the electron microscope is well established for dose-tolerant samples but is challenging for biological samples, especially in a cryo-preserved state. Here we combine electron energy-loss spectroscopy with single-particle image processing to allow elemental mapping in cryo-preserved macromolecular complexes. Proof-of-principle data show that our method, reconstructed electron energy-loss (REEL) analysis, allows a three-dimensional reconstruction of electron energy-loss spectroscopy data, such that a high total electron dose is accumulated across many copies of a complex. Working with two test samples, we demonstrate that we can reliably localize abundant elements. We discuss the current limitations of the method and potential future developments.