Mass Spectrometric Study of Acoustically Levitated Droplets Illuminates Molecular-Level Mechanism of Photodynamic Therapy for Cancer involving Lipid Oxidation

Even though the general mechanism of photodynamic cancer therapy is known, the details and consequences of the reactions between the photosensitizer-generated singlet oxygen and substrate molecules remain elusive at the molecular level. Using temoporfin as the photosensitizer, here we combine field-induced droplet ionization mass spectrometry and acoustic levitation techniques to study the wall-less oxidation reactions of 18:1 cardiolipin and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1 '-rac-glycerol) (POPG) mediated by singlet oxygen at the air-water interface of levitated water droplets. For both cardiolipin and POPG, every unsaturated oleyl chain is oxidized to an allyl hydroperoxide, which surprisingly is immune to further oxidation. This is attributed to the increased hydrophilicity of the oxidized chain, which attracts it toward the water phase, thereby increasing membrane permeability and eventually triggering cell death.