Complex sentence: NNK is a compound that is naturally synthesized in tobacco leaves that are exposed to light: the pyrrolidine ring in the nicotine opens and turns the nicotine into NNK, but most of the NNK found in tobacco smoke originates from nicotine burning.
Split into simpler sentences: NNK is a compound that is naturally synthesized in tobacco leaves that are exposed to light: the pyrrolidine ring in the nicotine opens and turns the nicotine into NNK. Most of the NNK found in tobacco smoke originates from nicotine burning.