Double Nitroxide Labeling by Copper-Catalyzed Azide–Alkyne Cycloadditions with Noncanonical Amino Acids for Electron Paramagnetic Resonance Spectroscopy

[Image: see text] Electron paramagnetic resonance spectroscopy in combination with site-directed spin labeling (SDSL) is an important tool to obtain long-range distance restraints for protein structural research. We here study a variety of azide- and alkyne-bearing noncanonical amino acids (ncAA) in terms of protein single- and double-incorporation efficiency via nonsense suppression, metabolic stability, yields of nitroxide labeling via copper-catalyzed [3 + 2] azide–alkyne cycloadditions (CuAAC), and spectroscopic properties in continuous-wave and double electron–electron resonance measurements. We identify para-ethynyl-l-phenylalanine and para-propargyloxy-l-phenylalanine as suitable ncAA for CuAAC-based SDSL that will complement current SDSL approaches, particularly in cases in which essential cysteines of a target protein prevent the use of sulfhydryl-reactive spin labels.