A Set of Three Gd^III Spin Labels with Methanethiosulfonyl Groups for Bioconjugation Covering a Wide Range of EPR Line...

Spin labels based on Gd^III complexes are important tools for the elucidation of the structure, dynamics and interaction of biomolecules by electron paramagnetic resonance (EPR) spectroscopy. Their EPR spectroscopic properties line width and relaxation times influence their performance in a particular application. To be able to apply a complex well-suited for a specific application, a set of Gd^III complexes with different EPR spectroscopic properties ready-made for spin labeling will be highly useful. We prepared three Gd^III complexes with DO3APic, NO3Pic, and PyMTA as the basic ligand units. They cover a wide range of EPR line widths but have in common a cysteine-targeting methanethiosulfonyl (MTS) group connected to a pyridine ring, which is an intrinsic part of the ligand. The reaction with a cysteine-containing pentapeptide (0.45 mM in the peptide, pH ∼ 7) was complete within 90 s and chemoselective. The MTS group hydrolyzed with half-lives of >24, 8, 2, and 1 h at pH 5, 6, 7, and 8, respectively. The structurally related nicotinic acid-substituted disulfide (NDS) group was found to be hydrolytically much more stable. However, the MTS spin label clearly won the competition for the pentapeptide over the NDS spin label. If high reactivity is essential, MTS is clearly the better choice.