Temporal Resolving Power

Biomolecules perform many amazing functions, and to understand their actions, it is necessary to determine how their amino acids and atoms evolve over time.

Understanding Muybridge's The Horse in Motion

The Horse in Motion (1878)

Eadweard Muybridge's The Horse in Motion, a motion picture depicting a horse's movement, is informative about how LCLS studies biomolecules.

This film is a series of images scrolled through with several cameras used to take snapshots to understand the horse's movement and action. Each image can be considered one point in time of the horse running. To understand a biomolecule, similar multiple spatially resolved time points during a biological reaction or dynamics must be seen.

Graphic showing individual images from Muybridge movie
Individual images from Muybridge's the Horse in Motion

If the camera exposure time is longer than the motion of interest, the motion in the produced image is blurred because there would be contributions from all images in time.

When presented with the averaged-out image and asked to ascertain parts of the horse that are important for the action (i.e., running down the track), it may seem as if the easily resolved parts (the body, tail, and head) are the most important for the action, but this would be an incomplete takeaway. 

Time-resolved structures
Averaged-out image of the motion picture, The Horse in Motion

How does this relate to LCLS?

X-rays and electrons are probes used to determine the high-resolution structures of biological macromolecules. However, most of these measurements are performed with long pulses with samples at cryogenic temperatures because X-rays and electrons are inherently destructive to macromolecules.

The freezing process can alter the state of the biomolecule, and since freezing is not instantaneous (10s of µs for single particles and ms+ for 3D crystals), different biomolecules in a crystal or ensemble might end up in different conformations that end up being averaged-out in the final structure, which is equivalent to more than one (up to all) of the different images of the horse running being combined into the determined structure.

A visualization of the biological motions, the relevant time scales, and the approximate times it takes to freeze different samples. The freezing time can be considered an effective camera shutter speed allowing only the processes to be resolvable.

LCLS is fundamentally different from synchrotrons or cryo-EM sources. X-rays produced by LCLS are nominally 40 fs long and can be even shorter, surpassing conventional radiation damage and allowing samples to be measured at room temperature under near-physiological conditions. The 40 fs pulse duration is the equivalent of the camera shutter.

Atomic motion occurs minimally in 40 fs, therefore LCLS allows greater time resolution of biomolecules than any other source in the world, allowing the individual images of Muybridge's horse to be much better than any other tool.

X-rays produced by LCLS are nominally 40 fs long, allowing for a greater time resolution of biomolecules than any other source in the world.