A nitroxide-maleimide spin label.

was chemically bonded to poly-L-lysine and to bovine serum albumin (BSA) and showed a paramagnetic resonance spectrum that was sensitive to known structural changes in these molecules, such as those produced by changes in pH.2 This nitroxide-isocyanate is unfortunately not an ideal spin-labeling reagent since it is unstable and thus difficult to purify, and moreover the isocyanate group is rapidly hydrolyzed by water in competition with the labeling reaction. In the present paper we describe the preparation and some labeling properties of a new, much more stable and generally useful nitroxide spin-labeling compound, N-(1-oxyl-2,2,5,5tetramethyl pyrrolidinyl)-maleimide,