How Phenol and α-Tocopherol React with Ambient Ozone at Gas/Liquid Interfaces

Abstract

The exceptional ability of α-tocopherol (α-TOH) for scavenging free radicals is believed to also underlie its protective functions in respiratory epithelia. Phenols, however, can scavenge other reactive species. Herein, we report that α-TOH/α-TO^− reacts with closed-shell O_3(g) on the surface of inert solvent microdroplets in <1 ms to produce persistent α-TO−O_n^−(n = 1−4) adducts detectable by online thermospray ionization mass spectrometry. The prototype phenolate PhO^−, in contrast, undergoes electron transfer under identical conditions. These reactions are deemed to occur at the gas/liquid interface because their rates: (1) depend on pH, (2) are several orders of magnitude faster than within microdroplets saturated with O_3(g). They also fail to incorporate solvent into the products: the same α-TO−On^− species are formed on acetonitrile or nucleophilic methanol microdroplets. α-TO−O_n(=1−3)^− signals initially evolve with [O_3(g)] as expected from first-generation species, but α-TO−O^− reacts further with O_3(g) and undergoes collisionally induced dissociation into a C_(19)H_(40) fragment (vs C_(19)H_(38) from α-TO^−) carrying the phytyl side chain, whereas the higher α-TO−O_(n≥2)^− homologues are unreactive toward O_3(g) and split CO_2 instead. On this basis, α-TO−O^− is assigned to a chroman-6-ol (4a, 8a)-ene oxide, α-TO−O2^− to an endoperoxide, and α-TO−O3^− to a secondary ozonide. The atmospheric degradation of the substituted phenols detected in combustion emissions is therefore expected to produce related oxidants on the aerosol particles present in the air we breathe.

Files

Additional details