DEER spectroscopy (double electron-electron resonance) is a pulsed EPR technique that measures the distance between two unpaired electrons, usually spin labels attached to chosen sites on a protein, across a range of roughly 1.5 to 8 nanometers. Because every molecule in the frozen sample contributes to the signal, DEER EPR returns a full distribution of distances instead of a single average, which shows how many conformations a protein adopts and how flexible it is. This article is for protein scientists and drug discovery teams deciding whether DEER fits a project. It covers how the experiment works, how DEER relates to PELDOR, the distance limits, and where DEER is used in drug discovery.

Protein dimerization is one of the most widespread and functionally important forms of protein–protein interaction. Proteins rarely act in isolation in a biological environment; instead, they associate as homodimers, heterodimers or higher-order oligomers to form functional assemblies. This self-assembly occurs in every cellular compartment, from membranes to the nucleus and cytosol, and regulates processes ranging from enzymatic activation and signal transduction to pathogenic pathways. A dimer observed in a crystal structure, however, may represent either a genuine biological assembly or a contact created by crystal packing. A crystal structure provides detailed information about molecular architecture, but it does not by itself establish whether two molecules associate under relevant solution conditions, how strong that interaction is, or whether the subunits retain the same arrangement outside the lattice. Distinguishing between these possibilities is therefore an important task in structural biology, biophysics and pharmaceutical research, where protein–protein interactions can influence function, stability and the binding of potential drug candidates.

Protein dimerization occurs when two protein molecules associate to form a dimer. A crystal structure can reveal how two protein molecules fit together, identifying an interface that may influence protein function or drug discovery. However, a dimer observed in a crystal does not necessarily represent the dominant assembly in solution. Crystal packing can stabilize contacts that are weak, transient, or absent under physiologically relevant conditions. For pharmaceutical researchers, this distinction matters because a proposed dimer interface may form a binding site, regulate protein activity, or provide a target for small molecules. Before using the structure to guide drug design, researchers need to establish whether the protein associates in solution and whether the arrangement of its subunits is consistent with the proposed model.