Maleimide and Isomaleimide Pyrrolidine‐Nitroxide Spin Labels

2,2,5,5‐Tetramethylpyrrolidine‐1‐oxyl‐3‐maleimide and the corresponding isomaleimide have been synthesized and characterised. The electron‐spin‐resonance spectra exhibited by these labels when bound to poly‐L‐lysine and bovine‐plasma albumin are discussed in terms of their structures and their relative degrees of binding to thiol and amino groups. Contamination of male‐imide spin labels with isomaleimide is shown to be a real possibility. Emphasis is thus laid on the need for careful characterisation of spin labels prior to their use.