Chemistry
How Neutron Scattering Reveals Hydrogen Positions in Protein Structures
Quick fact
Even though hydrogen makes up about half of the atoms in a protein, it is almost completely invisible in X-ray crystal structures because X-rays scatter from electrons, and hydrogen has only one electron. Neutrons, however, scatter from the atomic nuclei, so hydrogen and its isotope deuterium have strong and distinct neutron signals, making them directly observable.
Why this is interesting
You've probably seen stunning 3D models of proteins that seem to show every atom—but they're actually missing the smallest and arguably most important ones. Why are hydrogen atoms 'invisible' in most protein structures, and how can they finally be seen?
Read the full explanation
Understanding How Neutron Scattering Reveals Hydrogen Positions in Protein Structures
To understand how neutron scattering reveals hydrogen, imagine trying to see a ghost: you need a special 'camera' that detects something the ghost can't hide. For X-rays, the 'camera' detects electrons, and hydrogen—with just one electron—is nearly transparent. Neutrons, on the other hand, are uncharged particles that interact with the atomic nucleus. When a beam of neutrons hits a protein crystal, the neutrons bounce off the nuclei of atoms, including hydrogen. Because the strength of this scattering (called the scattering length) is different for hydrogen than for deuterium (an isotope with an extra neutron), scientists can use deuterium to enhance or suppress certain features. By collecting the diffracted neutrons, they build a 'neutron density map' that shows where the nuclei—and hence the hydrogen atoms—are located. This technique is called neutron crystallography. Even small-angle neutron scattering (SANS) uses similar principles to study hydrogen in proteins in solution, though at lower resolution.
A deeper explanation
The mechanism hinges on two key properties: (1) neutrons scatter from nuclei, not electrons, and (2) the scattering length is isotope-dependent. The scattering length of hydrogen (≈ -3.74 fm) is actually negative and quite different from deuterium (≈ 6.67 fm). This means hydrogen atoms produce a distinct 'negative' density in a neutron map, while deuterium produces a strong positive density. In a typical neutron diffraction experiment, a large crystal of a protein is exposed to a neutron beam, often at a specialized facility like a nuclear reactor or spallation source. The resulting diffraction pattern is collected and mathematically transformed into a three-dimensional map of nuclear scattering density. Since the map shows nuclei, hydrogen atoms appear as clear peaks—something X-ray maps almost never achieve. This capability is critical because knowing the exact position of a hydrogen atom tells you the protonation state of an amino acid side chain (e.g., is that histidine protonated?) and reveals the network of hydrogen bonds that stabilize the protein or dictate its catalytic activity. For instance, in enzyme active sites, whether a residue donates or accepts a hydrogen bond can change the reaction mechanism. Neutron scattering is therefore a unique tool that complements X-ray structures, providing the missing hydrogen atoms that are essential for a complete understanding of protein function.