(A) MTSSL (2,5-dihydro-3,4-bis(methanesulfonylthiomethyl)-2,2,5,5-tetramethyl-1H-pyrrol-1-yloxyl radical) and (B) bis-MTSL (trans-3,4-bis(methanesulfonylthiomethyl)-2,2,5,5-tetramethylpyrrolidin-1-yloxyl radical) spin labels, commercially available from Toronto Research Chemicals, 2 Brisbane Rd., Toronto, Ontario, Canada M3J 2J8.