Unveiling the triplet state of a 4-amino-7-nitrobenzofurazan derivative in cyclohexane.

The nitrobenzofurazan (NBD) moiety has gained tremendous popularity over the last decades due to its fluorogenic nature. Indeed, upon interaction with aliphatic amines, it generates a stable fluorescent adduct, which has been used for protein and lipid labeling. In fact the 4-amino substituted NBD belongs to the broad family of intramolecular charge transfer molecules, with the amino group acting as an electron donor upon photoexcitation, and the nitro group as an electron acceptor. Although the singlet excited state of 4-amino NBD derivatives has been abundantly studied, investigation of its triplet manifold is scarce and even the absence of intersystem crossing for this type of molecules has been suggested. However, intramolecular charge transfer molecules are known to undergo intersystem crossing and high phosphorescence quantum yields have been reported in a nonpolar solvent. In the present paper, we have investigated the photophysical and photochemical properties of N-hexyl-7-nitrobenzo[c][1,2,5]xadiazole-4-amine. We have shown the existence of a triplet state for this molecule in cyclohexane via nanosecond laser flash photolysis. Interestingly, deactivation of the triplet state leads to photoproducts formation, which are only present in the absence of oxygen.

Vitamin E prevents lipid raft modifications induced by an anti-cancer lysophospholipid and abolishes a Yap1-mediated stress response in yeast.

We have previously established that the anti-cancer lysophospholipid edelfosine (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine, Et-18-OCH(3)) induces cell death in yeast by selective modification of lipid raft composition at the plasma membrane. In this study we determined that alpha-tocopherol protects cells from the edelfosine cytotoxic effect, preventing the internalization of sterols and the plasma membrane proton pump ATPase, Pma1p. Two non-mutually exclusive hypotheses were considered to explain the protective effect of alpha-tocopherol: (i) its classical antioxidant activity is necessary to break progression of lipid peroxidation, despite the fact Saccharomyces cerevisiae does not possess polyunsaturated fatty acids and (ii) due to its complementary cone shape, insertion of alpha-tocopherol could correct membrane curvature stress imposed by edelfosine (inverted cone shape). We then developed tools to distinguish between these two hypotheses and dissect the structural requirements that confer alpha-tocopherol its protective effect. Our results indicated its lipophilic nature and the H donating hydroxyl group from the chromanol ring are both required to counteract the cytotoxic effect of edelfosine, suggesting edelfosine induces oxidation of membrane components. To further support this finding and learn more about the early cellular response to edelfosine we investigated the role that known oxidative stress signaling pathways play in modulating sensitivity to the lipid drug. Our results indicate the transcription factors Yap1 and Skn7 as well as the major peroxiredoxin, Tsa1, mediate a response to edelfosine. Interestingly, the pathway differed from the one triggered by hydrogen peroxide and its activation (measured as Yap1 translocation to the nucleus) was abolished by co-treatment of the cells with alpha-tocopherol.