Photosensitized Oxidative Damage from a New Perspective: The Influence of Before-Light and After-Light Reaction Conditions.

Photooxidative damage is heavily influenced by the presence of bioactive agents. Conversely, bioactive agents influence the local environment, which in turn is perturbed by photooxidative damage. These sorts of processes give rise to a version of the "chicken-and-egg" quandary. In this Perspective, we probe this issue by referring to photooxidative damage in one direction as the light-dark (L-D) sequence and in a second direction as the dark-light (D-L) sequence with a reversed cause and effect. The L-D sequence can lead to the downstream production of reactive molecular species (RMS) in the dark, whereas the D-L sequence can be a pre-irradiation period, such as an additive to limit cellular iron levels to enhance biosynthesized amounts of a protoporphyrin sensitizer. A third direction comes from L-D or D-L sequences, or both simultaneously, which can also be useful for optimizing photodynamics. Photodynamic optimization will benefit from understanding and quantitating unidirectional L-D and D-L pathways, and bidirectional L-D/D-L pathways, for improved control over photooxidative damage. Photooxidative damage, which occurs during anticancer photodynamic therapy (PDT), will be shown to involve RMS. Such RMS include persulfoxides (R2S+OO-), NO2•, peroxynitrate (O2NOO-), OOSCN-, SO3•-, selenocyanogen [(SeCN)2], the triselenocyanate anion [(SeCN)3-], I•, I2•-, I3-, and HOOI, as well as additives to destabilize membranes (e.g., caspofungin and saponin A16), inhibit DNA synthesis (5-fluorouracil), or sequester iron (desferrioxamine). In view of the success that additive natural products and repurposed drugs have had in PDT, a Perspective of additive types is expected to reveal mechanistic details for enhanced photooxidation reactions in general. Indeed, strategies for how to potentiate photooxidations with additives remain highly underexplored.

Role of curvature in acridone for 1 O2 oxidation of a natural product homoallylic alcohol: A novel iso-hydroperoxide intermediate.

A density functional theoretical (DFT) study is presented, implicating a 1 O2 oxidation process to reach a dihydrobenzofuran from the reaction of the natural homoallylic alcohol, glycocitrine. Our results predict an interconversion between glycocitrine and an iso-hydroperoxide intermediate [R(H)O+ -O- ] that provides a key path in the chemistry which then follows. Formations of allylic hydroperoxides are unlikely from a 1 O2 'ene' reaction. Instead, the dihydrobenzofuran arises by 1 O2 oxidation facilitated by a 16° curvature of the glycocitrine ring imposed by a pyramidal N-methyl group. This curvature facilitates the formation of the iso-hydroperoxide, which is analogous to the iso species CH2 I+ -I- and CHI2 + -I- formed by UV photolysis of CH2 I2 and CHI3 . The iso-hydroperoxide is also structurally reminiscent of carbonyl oxides (R2 C=O+ -O- ) formed in the reaction of carbenes and oxygen. Our DFT results point to intermolecular process, in which the iso-hydroperoxide's fate relates to O-transfer and H2 O dehydration reactions for new insight into the biosynthesis of dihydrobenzofuran natural products.

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-kT) 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 (τairborne) and surface-associated 1O2 lifetime (τsurf) 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.

Sensitized Photooxidation of Ortho-Prenyl Phenol: Biomimetic Dihydrobenzofuran Synthesis and Total 1O2 Quenching.

The sensitized photooxidation of ortho-prenyl phenol is described with evidence that solvent aproticity favors the formation of a dihydrobenzofuran [2-(prop-1-en-2-yl)-2,3-dihydrobenzofuran], a moiety commonly found in natural products. Benzene solvent increased the total quenching rate constant (kT ) of singlet oxygen with prenyl phenol by ~10-fold compared to methanol. A mechanism is proposed with preferential addition of singlet oxygen to prenyl site due to hydrogen bonding with the phenol OH group, which causes a divergence away from the singlet oxygen 'ene' reaction toward the dihydrobenzofuran as the major product. The reaction is a mixed photooxidized system since an epoxide arises by a type I sensitized photooxidation.

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.

Induced Disassembly of a Virus-like Particle under Physiological Conditions for Venom Peptide Delivery.

Virus-like particles (VLPs) show considerable promise for the in vivo delivery of therapeutic compounds such as bioactive venom peptides. While loading and targeting protocols have been developed for numerous VLP prototypes, induced disassembly under physiological conditions of neutral pH, moderate temperature, and aqueous medium remain a challenge. Here, we implement and evaluate a general mechanism, based on ring-opening metathesis polymerization (ROMP), for controllable VLP disassembly. This mechanism is independent of cell-specific factors or the manipulation of environmental conditions such as pH and temperature that cannot be readily controlled in vivo. The ROMP substrate norbornene is covalently conjugated to surface-exposed lysine residues of a P22 bacteriophage-derived VLP, and ROMP is induced by treatment with the water-soluble ruthenium catalyst AquaMet. Disruption of the P22 shell and release of a GFP reporter is confirmed via native agarose electrophoresis, TEM, and dynamic light scattering (DLS) analyses. Our ROMP disassembly strategy does not depend on the particular structure or morphology of the P22 nanocontainer and is adaptable to other VLP prototypes for the potential delivery of venom peptides for pharmacological applications.

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 (kT) 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.

Why are the Elemental Nonmetals (F2 , Cl2 , Br2 , I2 , S8 , P4 ) of so Many Hues or of Any Hues and Where is the Chromophore? Insight into Intera-X-X Bonds.

A unique approach is used to relate the HOMO-LUMO energy difference to the difference between the ionization potential (IP) and electron affinity (EA) to assist in deducing not only the colors, but also chromophores in elemental nonmetals. Our analysis focuses on compounds with lone pair electrons and σ electrons, namely X2 (X = F, Cl, Br, I), S8 and P4 . For the dihalogens, the [IP - EA] energies are found to be: F2 (12.58 eV), Cl2 (8.98 eV), Br2 (7.90 eV), I2 (6.78 eV). We suggest that the interahalogen X-X bond itself is the chromophore for these dihalogens, in which the light absorbed by the F2 , Cl2 , Br2 , I2 leads to longer wavelengths in the visible by a π â†’ σ* transition. Trace impurities are a likely case of cyclic S8 which contains amounts of selenium leading to a yellow color, where the [IP - EA] energy of S8 is found to be 7.02 eV. Elemental P4 with an [IP - EA] energy of 9.09 eV contains a tetrahedral and σ aromatic structure. In future work, refinement of the analysis will be required for compounds with π electrons and σ electrons, such as polycyclic aromatic hydrocarbons (PAHs).

Adjuvants that Empower the Action of Photodynamic Therapy.

Compounds have been devised whose supportive actions make them important adjuvants in the priming of photosensitization to selectively target cancer cells. Here, we highlight the paper by Maytin and Hasan in this issue of Photochemistry & Photobiology, which describes adjuvants methotrexate, 5-fluorouracil, vitamin D and its analogs leading to improved photodynamic therapy outcome. These small molecule adjuvants act by different mechanisms to enhance the cytotoxicity in tumor cells and the therapeutic effect in cancers. These findings add to the list of strategies for enhancement of photodynamic therapy.