No abstract

The preparation, spectroscopic, and electrochemical characterization of a family of 16 new bodipy dyes with tunable redox potentials and versatile functional groups is reported. Electron-withdrawing or -donating groups (Et, H, Cl, or CN) at positions C2 and C6 enabled tuning the redox potentials within a ca. 0.7 eV window without significantly affecting either the HOMO-LUMO gap or the absorption and emission spectra. Hydroxymethyl or formyl groups at the meso (C8) position in turn provided a handle for covalent tethering to receptors and biomolecules of interest, which dispenses with the more commonly used meso-aryl moiety as a means to tag molecules. The dyes can thus be coupled to both electrophiles and nucleophiles. Importantly, it is shown that meso-formyl bodipy dyes are nonemissive and have significantly lower molar extinction coefficients compared to their meso-hydroxymethyl and meso-acetoxymethyl counterparts (which in turn are bright, with emission quantum yields in the range of 0.7-1). The nonemissive meso-formyl bodipy dyes thus provide unique opportunities as fluorogenic probes of nucleophilic attack and as fluorescent labeling agents where uncoupled fluorophores will not contribute to the fluorescence background. Overall, the new bodipy dyes reported here are promising candidates for the preparation of fluorescent sensors relying on photoinduced electron transfer and may find use in a number of fluorescent-labeling protocols.

The preparation of two highly sensitive fluorogenic α-tocopherol (TOH) analogues which undergo >30-fold fluorescence intensity enhancement upon reaction with peroxyl radicals is reported. The probes consist of a chromanol moiety coupled to the meso position of a BODIPY fluorophore, where the use of a methylene linker (BODIPY-2,2,5,7,8-pentamethyl-6-hydroxy-chroman adduct, H(2)B-PMHC) vs an ester linker (meso-methanoyl BODIPY-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid, H(2)B-TOH) enables tuning their reactivity toward H-atom abstraction by peroxyl radicals. The development of a high-throughput fluorescence assay for monitoring kinetics of peroxyl radical reactions in liposomes is subsequently described where the evolution of the fluorescence intensity over time provides a rapid, facile method to conduct competitive kinetic studies in the presence of TOH and its analogues. A quantitative treatment is formulated for the temporal evolution of the intensity in terms of relative rate constants of H-atom abstraction (k(inh)) from the various tocopherol analogues. Combined, the new probes, the fluorescence assay, and the data analysis provide a new method to obtain, in a rapid, parallel format, relative antioxidant activities in phospholipid membranes. The method is exemplified with four chromanol-based antioxidant compounds differing in their aliphatic tails (TOH, PMHC, H(2)B-PMHC, and H(2)B-TOH). Studies were conducted in six different liposome solutions prepared from poly- and mono-unsaturated and saturated (fluid vs gel phase) lipids in the presence of either hydrophilic or lipophilic peroxyl radicals. A number of key insights into the chemistry of the TOH antioxidants in lipid membranes are provided: (1) The relative antioxidant activities of chromanols in homogeneous solution, arising from their inherent chemical reactivity, readily translate to the microheterogeneous environment at the water/lipid interface; thus similar values for k(inh)(H(2)B-PMHC)/k(inh)(H(2)B-TOH) in the range of 2-3 are recorded both in homogeneous solution and in liposome suspensions with hydrophilic or lipophilic peroxyl radicals. (2) The relative antioxidant activity between tocopherol analogues with the same inherent chemical reactivity but bearing short (PMHC) or long (TOH) aliphatic tails, k(inh)(PMHC)/k(inh)(TOH), is ~8 in the presence of hydrophilic peroxyl radicals, regardless of the nature of the lipid membrane into which they are embedded. (3) Antioxidants embedded in saturated lipids do not efficiently scavenge hydrophilic peroxyl radicals; under these conditions wastage reactions among peroxyl radicals become important, and this translates into larger times for antioxidant consumption. (4) Lipophilic peroxyl radicals show reduced discrimination between antioxidants bearing long and short aliphatic tails, with k(inh)(PMHC)/k(inh)(TOH) in the range of 3-4 for most lipid membranes. (5) Lipophilic peroxyl radicals are scavenged with the same efficiency by all four antioxidants studied, regardless of the nature of their a...

The reactivity, electrochemistry, and photophysics of the novel antioxidant indicator B-TOH, a BODIPY-alpha-tocopherol adduct, were investigated. We also studied a newly prepared BODIPY-3,5-di-tert-butyl-4-hydroxybenzoic acid adduct (B-BHB) and compared the results for both sets of probes. Our results highlight the potential of B-TOH as a fluorescent antioxidant indicator and help illustrate the considerations to be taken into account in preparing a receptor-reporter-type fluorescent antioxidant indicator. Based on the experimental values of the redox potentials for the reporter BODIPY and from the redox potentials estimated for the phenol receptor segment, the off-to-on emission enhancement recently reported for B-TOH upon peroxyl radical scavenging can be unequivocally assigned to the deactivation of an intramolecular photoinduced electron transfer (PeT) which operates in the reduced form of B-TOH. Theoretical calculations performed at the B3LYP/6-31G(d) level on HOMO energy levels relative to vacuum further support the deactivation of a PeT mechanism upon peroxyl radical scavenging by B-TOH. Fluorescence lifetimes and fluorescence quantum yields measured in a range of solvent polarities, from hexane to acetonitrile, for B-TOH, B-BHB, and their BODIPY precursors PM605 or PMOH, are consistent with an intramolecular nonradiative decay pathway operative in B-TOH. This pathway is not operative in B-BHB where PeT is deemed highly endergonic based on electrochemical studies. A subsequent analysis on the antioxidant properties of both fluorophore-phenol adducts studied herein indicates that B-TOH antioxidant activity is on par with that of alpha-tocopherol, the most potent naturally occurring lipid soluble antioxidant, whereas B-BHB is a poor antioxidant. Oxygen uptake studies upon peroxyl radical initiated styrene autoxidation and laser flash photolysis studies on the rate of H-atom abstraction by cumyloxyl radicals reveal similar reactivity patterns for B-TOH and 2,2,5,7,8-pentamethyl-6-hydroxychroman (PMHC), an alpha-tocopherol analogue lacking the phytil tail. Analogous reactivity studies on B-BHB underscore its poor antioxidant activity. In general, this work provides substantial amount of information useful in designing off/on lipid soluble fluorescent antioxidant indicators based on phenol moieties.

We report here the preparation of a lipophilic fluorogenic antioxidant (Mito-Bodipy-TOH) that targets the inner mitochondrial lipid membrane (IMM) and is sensitive to the presence of lipid peroxyl radicals, effective chain carriers in the lipid chain autoxidation. Mito-Bodipy-TOH enables monitoring of the antioxidant status, i.e., the antioxidant load and ability to prevent lipid chain autoxidation, within the inner mitochondrial membrane of live cells. The new probe consists of 3 segments: a receptor, a reporter, and a mitochondria-targeting element, constructed, respectively, from an α-tocopherol-like chromanol moiety, a BODIPY fluorophore, and a triphenylphosphonium cation (TPP). The chromanol moiety ensures reactivity akin to that of α-tocopherol, the most potent naturally occurring lipid soluble antioxidant, while the BODIPY fluorophore and TPP ensure partitioning within the inner mitochondrial membrane. Mechanistic studies conducted either in homogeneous solution or in liposomes and in the presence of free radical initiators show that the antioxidant activity of Mito-Bodipy-TOH is on par with that of α-tocopherol. Studies conducted on live fibroblast cells further show the antioxidant depletion in the presence of methyl viologen (paraquat), a known agent of oxidative stress and source of superoxide radical anion (and indirectly, a causative of lipid peroxidation) within the mitochondria matrix. We recorded a ca. 8-fold emission enhancement with Mito-Bodipy-TOH in cells stressed with methyl viologen, whereas no enhancement was observed in control studies with untreated cells. Our findings underscore the potential of the new fluorogenic antioxidant Mito-Bodipy-TOH to study the chemical link between antioxidant load, lipid peroxidation and mitochondrial physiology.