EPR spectroscopy of diamagnetic bio‐macromolecules is based on site‐directed spin labeling (SDSL). Herein, a novel labeling strategy for proteins is presented. A nitroxide‐based spin label has been developed and synthesized that can be ligated to proteins by an inverse‐electron‐demand Diels–Alder (DAinv) cycloaddition to genetically encoded noncanonical amino acids. The nitroxide moiety is shielded by a photoremovable protecting group with an attached tetra(ethylene glycol) unit to achieve water solubility. SDSL is demonstrated on two model proteins with the photoactivatable nitroxide for DAinv reaction (PaNDA) label. The strategy features high reaction rates, combined with high selectivity, and the possibility to deprotect the nitroxide in Escherichia coli lysate.

Kugele et al. report site-directed spin labelling via Suzuki–Miyaura coupling of a nitroxide boronic acid label with the genetically encoded amino acid 4-iodo-l-phenylalanine.

The membrane transporter BtuB is site-directedly spin labelled on the surface of living bacteria via Diels–Alder click chemistry.

The ethylene receptor 1 transmembrane domain was site-directedly spin labelled to obtain distance restraints.

EPR spectroscopy of diamagnetic bio-macromolecules is based on site-directed spin labeling (SDSL). Here, we present a novel labeling strategy for proteins. We developed and synthesized a nitroxide-based spin label that can be ligated to proteins by an inverse-electron-demand Diels-Alder (DAinv) cycloaddition to genetically encoded non-canonical amino acids (ncAA). The nitroxide moiety is shielded by a photoremovable protecting group (PPG) with an attached tetraethylene glycol unit to achieve water solubility. We demonstrate SDSL of two model proteins with the PaNDA (Photoactivatable Nitroxide for DAinv reaction) label. Our strategy features high reaction rates combined with high selectivity, as well as high stability of the nitroxide in E. coli lysate.