During the catalytic cycle of ribonucleotide reductase, cysteine-439 forms one of these, which is then transferred to the substrate and to a tyrosine residue. Potassium nitrosodisulfonate is a persistent example of one. N-bromosuccinimide is used in substitutions that have one of these as an intermediate, while a technique used to image them commonly uses TEMPO to trap them. Besides EPR, they are 
During the catalytic cycle of ribonucleotide reductase, cysteine-439 forms one of these, which is then transferred to the substrate and to a tyrosine residue. Potassium nitrosodisulfonate is a persistent example of one. N-bromosuccinimide is used in substitutions that have one of these as an intermediate, while a technique used to image them commonly uses TEMPO to trap them. Besides EPR, they are commonly created by hemolytic bond cleavage in species like peroxides. Reactive oxygen species are a subset of, for 10 points, these chemical species that contain an unpaired electron.
During the catalytic cycle of ribonucleotide reductase, cysteine-439 forms one of these, which is then transferred to the substrate and to a tyrosine residue.
free {radicals}