Spin-labeled biomolecules.

are remarkably stable and inert,1 and show sharp, well-resolved, and simple paramagnetic resonance spectra2 that are sensitive to molecular motion,3 and, to a lesser extent, sensitive to polarity of the molecular environment.4 As shown below, such nitroxide radicals containing an isocyanate group can be bound to proteins and synthetic polypeptides in a manner similar to that used previously to attach fluorescent dyes.5 Here we report a preliminary study of the paramagnetic resoiiance of a nitroxide radical bonded to bovine serum albumin and to poly-L-lysine. It is evident from the present study that there are many other potential applicatioiis of this method to biological systems. Materials and Methods.-Preparation of 2, 2, 6, 5-tetramethyl-3-isocyanatopyrrolidine-1-oxyl (II): 2, 2, 5, 5-Tetramethyl-3-aminopyrrolidone-1-oxyl (III) was synthesized from triacetonamine by the method of Rosantzev and Krivitzkaya.6 NCO NH2