Electron diffraction has historically played an important role in the advancement of crystallographic approaches for the determination of complex small molecule structures. A broad array of atomic structures has now been determined by micro crystal electron diffraction, MicroED. They include naturally occurring peptides, synthetic protein fragments and peptide-based natural products. However, de novo structure determination by MicroED remains problematic for all but ideal crystals. Automated, fragment-based approaches to structure determination eliminate the need for atomic resolution diffraction, instead enforcing stereochemical constraints through libraries of small model fragments.

Determination of atomic resolution peptide and small molecule structures by frontier electron diffraction methods. Electron diffraction patterns, A, can be collected from nanoscale peptide crystals as inset, phased by fragment-based methods, B, and lead to accurate atomic structures, C and D.

We have demonstrated the application of fragment-based phasing on various macromolecular structures including some for which all traditional phasing methods have failed, see image above. Most recently, we have endeavored to determine structures where resolution is far beyond the reach of direct methods, yielding novel solutions with 1.4-2.2Å data. The determined structures are accurate and in test cases, reflect structures determined by traditional X-ray crystallographic approaches. New nanobeam electron diffraction methods are further advancing our ability to determine novel structures, by yielding atomic insights from regions of lattices containing as few as 1000 molecules. Together, these approaches advance the frontier of electron diffraction-based structure determination methods, and are increasingly accessible to the broader field of structural science.