Mn(II) tags for DEER distance measurements in proteins via C-S attachment.

Mn2+ chelating tags for Mn2+–Mn2+ distance measurements by pulse EPR spectroscopy were developed. They feature a stable C–S conjugation to the protein, high reactivity towards cysteine thiols and short and rigid linkers that can be used in distance measurements with high resolution under reductive conditions. Double electron–electron resonance measurements at 95 GHz on ubiquitin labeled with these tags showed the expected narrow distance distribution.