Isoindoline-Based Nitroxides as Bioresistant Spin Labels for Protein Labeling through Cysteines and Alkyne-Bearing Noncanonical Amino Acids.

Electron paramagnetic resonance (EPR) spectroscopy in combination with site-directed spin labeling (SDSL) is a powerful tool in protein structural research. Nitroxides are highly suitable spin labeling reagents, but suffer from limited stability, particularly in the cellular environment. Herein we present the synthesis of a maleimide- and an azide-modified tetraethyl-shielded isoindoline-based nitroxide (M- and Az-TEIO) for labeling of cysteines or the noncanonical amino acid para-ethynyl-l-phenylalanine (pENF).

Site-directed spin labelling of proteins by Suzuki-Miyaura coupling via a genetically encoded aryliodide amino acid.

We report site-directed protein spin labelling via Suzuki-Miyaura coupling of a nitroxide boronic acid label with the genetically encoded amino acid 4-iodo-l-phenylalanine. The resulting spin label bears a rigid biphenyl linkage with lower flexibility than spin label R1. It is suitable to obtain defined electron paramagnetic resonance distance distributions and to report protein-membrane interactions and conformational transitions of α-synuclein.