Cryo-electron microscopy (cryo-EM) has transformed how scientists see the molecules of life, earning its pioneers the 2017 Nobel Prize in Chemistry. Yet it has a stubborn blind spot: most human proteins are too small to image clearly. Physicists at the University of California, Berkeley, have now built a device that could close much of that gap, a laser phase plate that uses an extraordinarily intense laser to sharpen contrast. Their results appear in the journal Science.
An old idea, applied to electrons
The concept dates back nearly a century. In 1930, Dutch physicist Frits Zernike realized that light passing through transparent biological material is slightly delayed, shifting its phase. By shifting the unscattered light by 90 degrees, he turned those invisible delays into visible differences in brightness. That invention, phase-contrast microscopy, won the 1953 Nobel Prize in Physics.
The Berkeley team, led by physics professor Holger Müller, has brought the same principle to electron microscopes, which magnify roughly 10,000 times more than light microscopes. Instead of a physical plate, they use light itself: a laser interacts directly with the electron beam, shifting its phase to boost contrast, according to the UC Berkeley release.
Engineering the brightest continuous laser focus
Shifting an electron beam’s phase by 90 degrees takes enormous light intensity in a tiny spot. Müller spent about 15 years on the problem after first proposing it in 2010. The solution was to trap a continuous laser inside a mirrored cavity, where the light bounces more than 10,000 times, building up power while being focused to a minuscule point. “It’s 75 kilowatts focused to a few microns,” Müller said, calling it “the brightest continuous laser focus ever.”
With support from Biohub, the team fitted the laser phase plate to a customized Thermo Fisher Krios cryo-EM instrument. Müller named the resulting microscope Theia, after the Greek Titaness of light.
What the results show
The researchers tested six samples of varying size and preparation quality. The laser consistently improved resolution, with the biggest gains for the hardest cases. “For the most challenging cases, small particles, bad specimens, the laser produces a very considerable advantage,” Müller said.
To demonstrate, the team reconstructed two proteins: aldolase, which cryo-EM already handles well, and hemoglobin, which sits near the lower size limit of conventional instruments. Both improved, and the gain was especially pronounced for hemoglobin.
The numbers explain why this matters. Conventional cryo-EM struggles with proteins smaller than about 70 kilodaltons, and proteins below that threshold make up about 90% of the human proteome. With the laser phase plate, researchers can now image proteins as small as 50 kilodaltons, though it remains challenging. Müller believes the approach could eventually reach about 17 kilodaltons, the size of myoglobin.
A bigger payoff inside cells
The most dramatic impact may come in cryo-electron tomography (cryo-ET), which combines images from many angles to build 3D views of molecules inside their natural cellular surroundings. Inside a cell, the scene is crowded. “It’s like a forest of trees, and you’re trying to find one leaf on one tree in there,” said Bridget Carragher, founding technical director of imaging at Biohub, where a related two-laser microscope is being built. “Cryo-ET needs a dramatic step forward in contrast.”
Both teams are working with Thermo Fisher Scientific, the leading cryo-EM manufacturer, which could ease a path to wider availability. The paper, “Laser phase plate improves structure determination of small proteins by cryo-EM,” was published in Science.
Practical takeaways
- For structural biologists: Small and hard-to-prepare proteins that were out of reach may become accessible, especially for cryo-ET work.
- For research organizations: Watch for commercial versions through instrument makers before planning major upgrades.
- For innovation watchers: This is a classic example of physics tools unlocking biology, and of long-horizon research paying off after 15 years.
The bottom line
The laser phase plate takes a Nobel-winning optical idea and makes it work for electrons using one of the most intense continuous laser foci ever built. If it scales, scientists could see far more of the molecular machinery inside cells. As Biohub’s Stephani Otte put it, “What was once invisible will become visible.”