Airborne singlet oxygen obtained from photosensitization of triplet dioxygen is shown to react with an alkene surfactant (8-methylnon-7-ene-1 sulfonate) leading to “ene” hydroperoxides that in the dark inactivate planktonic Escherichia coli (E. coli). The “ene” hydroperoxide photoproducts are not toxic on their own, but they become toxic after the bacteria are pretreated with singlet oxygen. The total quenching rate constant (k T) of singlet oxygen of the alkene surfactant was measured to be 1.1 × 106 M–1 s–1 at the air/liquid interface. Through a new mechanism called singlet oxygen priming (SOP), the singlet oxygen leads to hydroperoxides then to peroxyl radicals, tetraoxides, and decomposition products, which also promote disinfection, and therefore offer a “one-two” punch. This offers a strong secondary toxic effect in an otherwise indiscernible dark reaction. The results provide an insight into assisted killing by an exogenous alkene with dark toxicity effects following exposure to singlet oxygen.

A mechanistic study is reported for the reactions of singlet oxygen ( 1 O 2 ) with alkene surfactants of tunable properties. Singlet oxygen was generated either topdown (photochemically) by delivery as a gas to an air−water interface or bottom-up (chemically) by transport to the air−water interface as a solvated species. In both cases, reactions were carried out in the presence of 7-carbon (7C), 9-carbon (9C), or 11-carbon (11C) prenylsurfactants [(CH 3 ) 2 CCH(CH 2 ) n SO 3 − Na + (n = 4, 6, 8)]. Higher "ene" hydroperoxide regioselectivities (secondary ROOH 2 to tertiary ROOH 3) were reached in delivering 1 O 2 top-down through air as compared to bottom-up via aqueous solution. In the photochemical reaction, ratios of 2:3 increased from 2.5:1 for 7C, to 2.8:1 for 9C, and to 3.2:1 for 11C. In contrast, in the bubbling system that generated 1 O 2 chemically, the selectivity was all but lost, ranging only from 1.3:1 to 1:1. The phase-dependent regioselectivities appear to be correlated with the "ene" reaction with photochemically generated, drier 1 O 2 at the air−water interface vs those with wetter 1 O 2 from the bubbling reactor. Density functional theory-calculated reaction potential energy surfaces (PESs) were used to help rationalize the reaction phase dependence. The reactions in the gas phase are mediated by perepoxide transition states with 32−41 kJ/mol binding energy for CC(π)••• 1 O 2 . The perepoxide species, however, evolve to well-defined stationary structures in the aqueous phase, with covalent C−O bonds and 85−88 kJ/mol binding energy. The combined experimental and computational evidence points to a unique mechanism for 1 O 2 "ene" tunability in a perepoxide continuum from a transition state to an intermediate.