Hi Alex, Welcome to the field of microED! From a practical standpoint, microED also suffers from the phase problem, and somewhat moreso compared to X-ray crystallography because anomalous signal is very limited. It is true that a TEM microscope operated in imaging mode produces images that contain both phase and amplitude information, which you correctly infer means that single particle cryoEM does not suffer from the phase problem. This is why a map from cryoEM is generally of higher quality than one determined by X-ray crystallography at the same resolution, because we don't have to guess at the phases.
In classic microED data collection, we don't actually take advantage of the phase information that can be measured using imaging mode. We quickly find our crystals in imaging mode and then flip the switch to diffraction mode and collect a dataset devoid of phase information. It has been suggested that we could use imaging mode to get at the phases of the diffraction patterns, but I am not aware of anyone actually doing this. Practically speaking, this would add significant additional time to the data collection and we likely would only be able to reliably use the phases for the lower resolution range (though, that might not be a deal breaker). Thankfully though, molecular replacement and ab initio methods for small molecules work pretty well on these datasets. Plus, most X-ray structures are solved this way anyways. There have been efforts to use radiation damage as a means to phase a small peptide bound to zinc (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7313391/). And there is some interesting work being done with dynamical scattering as a way to see differences in Friedel pairs (see Tim Gruene's post here: https://www.jiscmail.ac.uk/cgi-bin/wa-jisc.exe?A2=CCP4BB;b22eae56.2007), but these methods are not "classic microED" and require completely different data processing software that is unfamiliar to most X-ray crystallographers. For now, we just crank up the cycles in SHELXT/SHELXD or hope for a good molecular replacement model. Best of luck, Jessica On Sat, Aug 15, 2020 at 6:03 PM Alex Lee <alexlee198...@gmail.com> wrote: > Hi, All, > > I am new to MicroED (microcrystal electron diffraction). I know that X-ray > crystallography has phase problem, and I think MicroED has phase problem > too (it is diffraction of electron instead of x-ray). However, when I read > the Wikipedia, I could not understand the following description of MicroED: > One > of the main difficulties in X-ray crystallography is determining phases in > the diffraction pattern. Because of the complexity of X-ray lenses, it is > difficult to form an image of the crystal being diffracted, and hence phase > information is lost. Fortunately, electron microscopes can resolve atomic > structure in real space and the crystallographic structure factor phase > information can be experimentally determined from an image's Fourier > transform. The Fourier transform of an atomic resolution image is similar, > but different, to a diffraction pattern—with reciprocal lattice spots > reflecting the symmetry and spacing of a crystal. > > Does the above description mean that MicroED, or more broadly electron > crystallogaphy does NOT suffer from phase problem? How about single > particle cryo electron microscopy, it should NOT have phase problem, right? > > Thanks for any input in it. > > Best, > Alex > > ------------------------------ > > To unsubscribe from the CCP4BB list, click the following link: > https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 > -- Jessica Bruhn, Ph.D Principal Scientist NanoImaging Services, Inc. 4940 Carroll Canyon Road, Suite 115 San Diego, CA 92121 Phone #: (888) 675-8261 www.nanoimagingservices.com ######################################################################## To unsubscribe from the CCP4BB list, click the following link: https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 This message was issued to members of www.jiscmail.ac.uk/CCP4BB, a mailing list hosted by www.jiscmail.ac.uk, terms & conditions are available at https://www.jiscmail.ac.uk/policyandsecurity/
