Chemical Properties of Singlet Oxygen
Unlike many other molecules, molecular oxygen is most stable in a triplet state rather than a singlet state, and this property makes life in an oxygenated environment possible. Triplet oxygen (3O2), the ground state of molecular oxygen, has two unpaired, spin-parallel electrons (Figure 1A), whereas SO, the lowest excited state of molecular oxygen, has two valence electrons spin-paired in a single orbital and a second orbital left empty (Figure 1B). The terms “triplet” and “singlet” oxygen refer to the possible number of electron spins that each form can take; the triplet form has three possible arrangements of electron spins, whereas SO has only one possible arrangement. The triplet configuration of molecular oxygen limits its ability to react directly with most stable organic molecules, which typically have singlet ground states. This limitation prevents runaway oxidation at moderate temperatures and makes life as we know it possible. Singlet oxygen reacts far more readily with organic compounds than triplet oxygen, and can participate in ene reactions and Diels-Alder cycloadditions that triplet oxygen cannot (Figure 1C-D). Consequently, the lifetime of SO in vitro in water and most organic solvents is in the order of microseconds, despite being relatively stable in gaseous form (Koh & Fluhr, 2016; Thorning et al., 2022). Due to this high reactivity, SO is among the most potent reactive oxygen species (ROS), and readily oxidizes molecules with carbon-carbon double bonds. It damages proteins by reacting with cysteine, histidine, methionine, tryptophan, and tyrosine residues, disrupts membranes by oxidizing polyunsaturated fatty acids to form lipid hydroperoxides, and mutates DNA, causing G to T point mutations (Di Mascio et al., 2019; Agnez-Lima et al., 2012). Hydroperoxides generated by SO can also cause free radical chain reactions, amplifying the oxidative response (Dogra & Kim, 2020). While its high chemical reactivity can make SO toxic, this same trait also enables it to mediate plant interactions with biotic stressors.