Singlet Oxygen Flux, Associated Lipid Photooxidation, and Membrane Expansion Dynamics Visualized on Giant Unilamellar Vesicles

Molecular jackhammers eradicate cancer cells by vibronic-driven action

Through the actuation of vibronic modes in cell-membrane-associated aminocyanines, using near-infrared light, a distinct type of molecular mechanical action can be exploited to rapidly kill cells by necrosis. Vibronic-driven action (VDA) is distinct from both photodynamic therapy and photothermal therapy as its mechanical effect on the cell membrane is not abrogated by inhibitors of reactive oxygen species and it does not induce thermal killing. Subpicosecond concerted whole-molecule vibrations of VDA-induced mechanical disruption can be achieved using very low concentrations (500 nM) of aminocyanines or low doses of light (12 J cm-2, 80 mW cm-2 for 2.5 min), resulting in complete eradication of human melanoma cells in vitro. Also, 50% tumour-free efficacy in mouse models for melanoma was achieved. The molecules that destroy cell membranes through VDA have been termed molecular jackhammers because they undergo concerted whole-molecule vibrations. Given that a cell is unlikely to develop resistance to such molecular mechanical forces, molecular jackhammers present an alternative modality for inducing cancer cell death.

Alkylation converts riboflavin into an efficient photosensitizer of phospholipid membranes

A new decyl chain [-(CH₂)₉CH₃] riboflavin conjugate has been synthesized and investigated. A nucleophilic substitution (S_N2) reaction was used for coupling the alkyl chain to riboflavin (Rf), a model natural photosensitizer. As expected, the alkylated compound (decyl-Rf) is significantly more lipophilic than its precursor and efficiently intercalates within phospholipid bilayers, increasing its fluorescence quantum yield. The oxidative damage to lipid membranes photoinduced by decyl-Rf was investigated in large and giant unilamellar vesicles (LUVs and GUVs, respectively) composed of different phospholipids. Using a fluorogenic probe, fast radical formation and singlet oxygen generation was demonstrated upon UVA irradiation in vesicles containing decyl-Rf. Photosensitized formation of conjugated dienes and hydroperoxides, and membrane leakage in LUVs rich in poly-unsaturated fatty acids were also investigated. The overall assessment of the results shows that decyl-Rf is a significantly more efficient photosensitizer of lipids than its unsubstituted precursor and that the association to lipid membranes is key to trigger phospholipid oxidation. Alkylation of hydrophilic photosensitizers as a simple and general synthetic tool to obtain efficient photosensitizers of biomembranes, with potential applications, is discussed.