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.