Synthesis of two series of cysteine‐specific spin labels containing a 15 N substituted nitroxide

Abstract Synthesis of two series of spin labels with a cysteine‐reactive group, either maleimide or methanethiosulfonate, and a 15 N substituted nitroxide separated at variable distance is reported. In the synthesis, the cysteine‐reactive groups were first incorporated into the target molecule and the 15 N substituted nitroxide was introduced in the last step via a mild reaction without destroying the cysteine‐reactive groups. The synthetic routes described here are applicable to the synthesis of spin labels with either 15 N substituted or “normal” 14 N nitroxide. Spin labeling of cysteine residues in rhodopsin by the newly synthesized spin labels was also demonstrated.