Synergism between airborne singlet oxygen and a trisubstituted olefin sulfonate for the inactivation of bacteria

Tuning the 1O2 Oxidation of a Phenol at the Air/Solid Interface of a Nanoparticle: Hydrophobic Surface Increases Oxophilicity

Although silica surfaces have been used in organic oxidations for the production of peroxides, studies of airborne singlet oxygen at interfaces are limited and have not found widespread advantages. Here, with prenyl phenol-coated silica and delivery of singlet oxygen (¹O₂) through the gas phase, we uncover significant selectivity for dihydrofuran formation over allylic hydroperoxide formation. The hydrophobic particle causes prenyl phenol to produce an iso-hydroperoxide intermediate with an internally protonated oxygen atom, which leads to dihydrofuran formation as well as O atom transfer. In contrast, hydrophilic particles cause prenyl phenol to produce allylic hydroperoxide, due to phenol OH hydrogen bonding with SiOH surface groups. Mechanistic insight is provided by air/nanoparticle interfaces coated with the prenyl phenol, in which product yield was 6-fold greater on the hydrophobic nanoparticles compared to the hydrophilic nanoparticles and total rate constants (ASI-k_T) of ¹O₂ were 13-fold greater on the hydrophobic vs hydrophilic nanoparticles. A slope intersection method was also developed that uses the airborne ¹O₂ lifetime (τ_airborne) and surface-associated ¹O₂ lifetime (τ_surf) to quantitate ¹O₂ transitioning from volatile to non-volatile and surface boundary (surface···¹O₂). Further mechanistic insights on the selectivity of the reaction of prenyl phenol with ¹O₂ was provided by density functional theory calculations.

Probing the Transition State-to-Intermediate Continuum: Mechanistic Distinction between a Dry versus Wet Perepoxide in the Singlet Oxygen "Ene" Reaction at the Air-Water Interface

A mechanistic study is reported for the reactions of singlet oxygen (¹O₂) with alkene surfactants of tunable properties. Singlet oxygen was generated either top-down (photochemically) by delivery as a gas to an air-water interface or bottom-up (chemically) by transport to the air-water interface as a solvated species. In both cases, reactions were carried out in the presence of 7-carbon (7C), 9-carbon (9C), or 11-carbon (11C) prenylsurfactants [(CH₃)₂C═CH(CH₂)_nSO₃^- Na⁺ (n = 4, 6, 8)]. Higher "ene" hydroperoxide regioselectivities (secondary ROOH 2 to tertiary ROOH 3) were reached in delivering ¹O₂ top-down through air as compared to bottom-up via aqueous solution. In the photochemical reaction, ratios of 2:3 increased from 2.5:1 for 7C, to 2.8:1 for 9C, and to 3.2:1 for 11C. In contrast, in the bubbling system that generated ¹O₂ chemically, the selectivity was all but lost, ranging only from 1.3:1 to 1:1. The phase-dependent regioselectivities appear to be correlated with the "ene" reaction with photochemically generated, drier ¹O₂ at the air-water interface vs those with wetter ¹O₂ from the bubbling reactor. Density functional theory-calculated reaction potential energy surfaces (PESs) were used to help rationalize the reaction phase dependence. The reactions in the gas phase are mediated by perepoxide transition states with 32-41 kJ/mol binding energy for C═C(π)···¹O₂. The perepoxide species, however, evolve to well-defined stationary structures in the aqueous phase, with covalent C-O bonds and 85-88 kJ/mol binding energy. The combined experimental and computational evidence points to a unique mechanism for ¹O₂ "ene" tunability in a perepoxide continuum from a transition state to an intermediate.

A Singlet Oxygen Priming Mechanism: Disentangling of Photooxidative and Downstream Dark Effects

Airborne singlet oxygen obtained from photosensitization of triplet dioxygen is shown to react with an alkene surfactant (8-methylnon-7-ene-1 sulfonate) leading to "ene" hydroperoxides that in the dark inactivate planktonic Escherichia coli (E. coli). The "ene" hydroperoxide photoproducts are not toxic on their own, but they become toxic after the bacteria are pretreated with singlet oxygen. The total quenching rate constant (k_T) of singlet oxygen of the alkene surfactant was measured to be 1.1 × 10⁶ M^-1 s^-1 at the air/liquid interface. Through a new mechanism called singlet oxygen priming (SOP), the singlet oxygen leads to hydroperoxides then to peroxyl radicals, tetraoxides, and decomposition products, which also promote disinfection, and therefore offer a "one-two" punch. This offers a strong secondary toxic effect in an otherwise indiscernible dark reaction. The results provide an insight into assisted killing by an exogenous alkene with dark toxicity effects following exposure to singlet oxygen.

Structurally Characterized BODIPY-Appended Oxidovanadium(IV) β-Diketonates for Mitochondria-Targeted Photocytotoxicity

Mixed-ligand oxidovanadium(IV) β-diketonates having NNN-donor dipicolylamine-conjugated to boron-dipyrromethene (BODIPY in L₁) and diiodo-BODIPY (in L₂) moieties, namely, [VO(L₁)(acac)]Cl (1), [VO(L₂)(acac)]Cl (2), and [VO(L₁)(dbm)]Cl (3), where acac and dbm are monoanionic O,O-donor acetylacetone and 1,3-diphenyl-1,3-propanedione, were prepared, characterized, and tested for their photoinduced anticancer activity in visible light. Complexes 1 and 2 were structurally characterized as their PF₆ ^- salts (1a and 2a) by X-ray crystallography. They showed V^IVN₃O₃ six-coordinate geometry with dipicolylamine base as the facial ligand. The non-iodinated BODIPY complexes displayed absorption maxima at ∼501 nm, while it is ∼535 nm for the di-iodinated 2 in 10% DMSO-PBS buffer medium (pH = 7.2). Complexes 1 and 3 being green emissive (λ_em, ∼512 nm; λ_ex, 470 nm; Φ_F, ∼0.10) in 10% aqueous DMSO were used for cellular imaging studies. Complex 3 localized primarily in the mitochondria of the cervical HeLa cells with a co-localization coefficient value of 0.7. The non-emissive diiodo-BODIPY complex 2 showed generation of singlet oxygen (Φ_Δ ≈ 0.47) on light activation. Annexin-V assay showed singlet oxygen-mediated cellular apoptosis, making this complex a targeted PDT agent.

Secondary Dark Reactions Following Photodynamic Treatment are More Damaging Than Previously Thought

Photodynamic treatment is often thought to produce reactive oxygen species (ROS) that directly induce killing; the nomenclature and phrases revolve around such notions of light-dependency. Few studies reference the possible existence of oxidation products formed in secondary reactions, which bear cytotoxicity competitive to their ROS precursors. Here, we highlight the paper by Girotti and Korytowski in this issue of Photochemistry and Photobiology, which does just that. In this paper, they report on cholesterol hydroperoxides, which are formed after photosensitized oxidation and yield cytotoxic mixtures in dark reactions after the light's turned off. Some of the hydroperoxides are transported by protein carriers and damage tissue outside their site of origin. A similar dark cytotoxicity may be anticipated for biological peroxides from in vivo photodynamic therapy.

Targeted photodynamic therapy in visible light using BODIPY-appended copper(ii) complexes of a vitamin B6Schiff base

BODIPY-appended copper(ii) complexes of vitamin B₆derivatives localize in mitochondria and exhibit cancer cell selective photocytotoxicity by¹O₂mediated apoptosis.

Mechanistic and Kinetic Study of Singlet O2 Oxidation of Methionine by On-Line Electrospray Ionization Mass Spectrometry

We report a reaction apparatus developed to monitor singlet oxygen ((1)O2) reactions in solution using on-line ESI mass spectrometry and spectroscopy measurements. (1)O2 was generated in the gas phase by the reaction of H2O2 with Cl2, detected by its emission at 1270 nm, and bubbled into aqueous solution continuously. (1)O2 concentrations in solution were linearly related to the emission intensities of airborne (1)O2, and their absolute scales were established based on a calibration using 9,10-anthracene dipropionate dianion as an (1)O2 trapping agent. Products from (1)O2 oxidation were monitored by UV-Vis absorption and positive/negative ESI mass spectra, and product structures were elucidated using collision-induced dissociation-tandem mass spectrometry. To suppress electrical discharge in negative ESI of aqueous solution, methanol was added to electrospray via in-spray solution mixing using theta-glass ESI emitters. Capitalizing on this apparatus, the reaction of (1)O2 with methionine was investigated. We have identified methionine oxidation intermediates and products at different pH, and measured reaction rate constants. (1)O2 oxidation of methionine is mediated by persulfoxide in both acidic and basic solutions. Persulfoxide continues to react with another methionine, yielding methionine sulfoxide as end-product albeit with a much lower reaction rate in basic solution. Density functional theory was used to explore reaction potential energy surfaces and establish kinetic models, with solvation effects simulated using the polarized continuum model. Combined with our previous study of gas-phase methionine ions with (1)O2, evolution of methionine oxidation pathways at different ionization states and in different media is described.

Air-Water Interface Effects on the Regioselectivity of Singlet Oxygenations of a Trisubstituted Alkene

The regioselective synthesis of allylic hydroperoxide sulfonates by singlet oxygenation at the air-water interface has been found to depend on the concentration of the alkene sulfonate and added calcium salt. The regioselectivity is proposed to originate from an orthogonal alkene relative to the water surface for preferential methyl hydrogen abstraction by airborne singlet oxygen in an ene reaction. The findings hint that the air-water interface is a locale for synthetic reactions.

New singlet oxygen donors based on naphthalenes: synthesis, physical chemical data, and improved stability

Singlet oxygen donors are of current interest for medical applications, but suffer from a short half-life leading to low singlet oxygen yields and problems with storage. We have synthesized more than 25 new singlet oxygen donors based on differently substituted naphthalenes in only a few steps. The influence of functional groups on the reaction rate of the photooxygenations, thermolysis, half-life, and singlet oxygen yield has been thoroughly studied. We determined various thermodynamic data and compared them with density functional calculations. Interestingly, remarkable stabilities of functional groups during the photooxygenations and stabilizing effects for some endoperoxides during the thermolysis have been found. Furthermore, we give evidence for a partly concerted and partly stepwise thermolysis mechanism leading to singlet and triplet oxygen, respectively. Our results might be interesting for "dark oxygenations" and future applications in medicine.