Structures of the nitroxide side chains produced by reactions of the sulfhydryl group of a cysteine residue with common spin labels. (A) (1-oxyl-2,2,5,5-tetramethylpyrroline-3-methyl)methanethiosulfonate [MTSL], 1; (B) 3-(2-iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyl-1-oxyl [3-(2-iodoacetamido)-PROXYL], 4; and (C) 3-maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyl-1-oxyl [3-maleimido-PROXYL], 7.