Free Radicals in Tobacco Smoke

A critical examination of the literature on gas-phase free radicals and TPM-associated EPFRs in cigarette smoke is presented. These radicals participate in catalytic cycles that generate ROS that can result in oxidative stress. It seems that primary radicals originate from the direct decomposition of tobacco constituents and are linked to the chemical structure of the constituents and the temperature of pyrolysis. On the other hand, secondary radicals are generated during aging of smoke. Their EPR spectra are indicative of hydroquinone and catechol. Lignin, chlorogenic acid, and proteins are major precursors of EPFRs in TPM, but combinations of components form more secondary radicals than the sum of the individual components, suggesting a synergistic role of TPM in the formation and the stabilization of secondary radicals. Matrix isolation EPR has identified cyclopentadienyl, phenoxyl, and semiquinone radicals as additional free radicals formed from phenolic precursors that may be responsible for gas-phase alkoxy radicals previously identified using spin-trapping techniques. EPFRs are found to initiate catalytic cycles that generate ROS, and the overall concentrations of ROS generated by EPFRS are expected to be much higher than that generated by hydroquinones/quinones.