Sigmatropic rearrangement enables access to a highly stable spirocyclic nitroxide for protein spin labelling†
Mateusz P.
Sowiński, 
a
Elena M.
Mocanu,
b
Hannah
Ruskin-Dodd,
b
David B.
Cordes,
c
Janet E.
Lovett
b
and
Marius
Haugland-Grange
*a
a
Elena M.
Mocanu,
b
Hannah
Ruskin-Dodd,
b
David B.
Cordes,
c
Janet E.
Lovett
b
and
Marius
Haugland-Grange
*a
Abstract
Spin labelling enables the study of biomolecules using electron paramagnetic resonance (EPR) spectroscopy. Here, we describe the synthesis of a cysteine-reactive spin label based on a spirocyclic pyrrolidinyl nitroxide containing an iodoacetamide moiety. The spin label was shown to be highly persistent under reducing conditions while maintaining excellent EPR relaxation parameters up to a temperature of 180 K. After successful double spin labelling of a calmodulin variant, interspin distances were measured by the EPR experiment double electron–electron resonance (DEER) at 120 K.
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