Biological inactivation by singlet oxygen: distinguishing O2(1 delta g) and O2(1 sigma g+)

Singlet Oxygen Photophysics: From Liquid Solvents to Mammalian Cells

Molecular oxygen, O2, has long provided a cornerstone for studies in chemistry, physics, and biology. Although the triplet ground state, O2(X3Σg-), has garnered much attention, the lowest excited electronic state, O2(a1Δg), commonly called singlet oxygen, has attracted appreciable interest, principally because of its unique chemical reactivity in systems ranging from the Earth's atmosphere to biological cells. Because O2(a1Δg) can be produced and deactivated in processes that involve light, the photophysics of O2(a1Δg) are equally important. Moreover, pathways for O2(a1Δg) deactivation that regenerate O2(X3Σg-), which address fundamental principles unto themselves, kinetically compete with the chemical reactions of O2(a1Δg) and, thus, have practical significance. Due to technological advances (e.g., lasers, optical detectors, microscopes), data acquired in the past ∼20 years have increased our understanding of O2(a1Δg) photophysics appreciably and facilitated both spatial and temporal control over the behavior of O2(a1Δg). One goal of this Review is to summarize recent developments that have broad ramifications, focusing on systems in which oxygen forms a contact complex with an organic molecule M (e.g., a liquid solvent). An important concept is the role played by the M+•O2-• charge-transfer state in both the formation and deactivation of O2(a1Δg).

X-ray Activated Nanoplatforms for Deep Tissue Photodynamic Therapy

Photodynamic therapy (PDT), the use of light to excite photosensitive molecules whose electronic relaxation drives the production of highly cytotoxic reactive oxygen species (ROS), has proven an effective means of oncotherapy. However, its application has been severely constrained to superficial tissues and those readily accessed either endoscopically or laparoscopically, due to the intrinsic scattering and absorption of photons by intervening tissues. Recent advances in the design of nanoparticle-based X-ray scintillators and photosensitizers have enabled hybridization of these moieties into single nanocomposite particles. These nanoplatforms, when irradiated with diagnostic doses and energies of X-rays, produce large quantities of ROS and permit, for the first time, non-invasive deep tissue PDT of tumors with few of the therapeutic limitations or side effects of conventional PDT. In this review we examine the underlying principles and evolution of PDT: from its initial and still dominant use of light-activated, small molecule photosensitizers that passively accumulate in tumors, to its latest development of X-ray-activated, scintillator-photosensitizer hybrid nanoplatforms that actively target cancer biomarkers. Challenges and potential remedies for the clinical translation of these hybrid nanoplatforms and X-ray PDT are also presented.

Lifetime and diffusion distance of singlet oxygen in air under everyday atmospheric conditions

Singlet oxygen is a toxic chemical but powerful oxidant, exploited in many chemical and biological applications. However, the lifetime of singlet oxygen in air under atmospheric conditions is yet to be known. This has limited safe usage of singlet oxygen in air, despite being a strong antimicrobial agent with the unique property of relaxing to breathable oxygen after serving its purpose. Here, we solve this long-standing problem by combining experimental and theoretical research efforts; we generate singlet oxygen using a photosensitizer at a local source and monitor the time-dependent extent of singlet oxygen reaction with probe molecules at a detector, precisely controlling the detector distance from the source. To explain our experimental results, we employ a theoretical model that fully accounts for singlet oxygen diffusion, radiative and nonradiative relaxations, and the bimolecular reaction with probe molecules at the detector. For all cases investigated, our model, with only two adjustable parameters, provides an excellent quantitative explanation of the experiment. From this analysis, we extract the lifetime of singlet oxygen in the air to be 2.80 s at 23 °C under 1 atm, during which time singlet oxygen diffuses about 0.992 cm. The correctness of this estimation is confirmed by a simple mean-first-passage time analysis of the maximum distance singlet oxygen can reach from the source. We also confirm the sterilization effects of singlet oxygen for distances up to 0.6-0.8 cm, depending on the bacteria strain in question, between the bacteria and the singlet oxygen source.

Hydrogels: soft matters in photomedicine

Photodynamic therapy (PDT), a shining beacon in the realm of photomedicine, is a non-invasive technique that utilizes dye-based photosensitizers (PSs) in conjunction with light and oxygen to produce reactive oxygen species to combat malignant tissues and infectious microorganisms. Yet, for PDT to become a common, routine therapy, it is still necessary to overcome limitations such as photosensitizer solubility, long-term side effects (e.g., photosensitivity) and to develop safe, biocompatible and target-specific formulations. Polymer based drug delivery platforms are an effective strategy for the delivery of PSs for PDT applications. Among them, hydrogels and 3D polymer scaffolds with the ability to swell in aqueous media have been deeply investigated. Particularly, hydrogel-based formulations present real potential to fulfill all requirements of an ideal PDT platform by overcoming the solubility issues, while improving the selectivity and targeting drawbacks of the PSs alone. In this perspective, we summarize the use of hydrogels as carrier systems of PSs to enhance the effectiveness of PDT against infections and cancer. Their potential in environmental and biomedical applications, such as tissue engineering photoremediation and photochemistry, is also discussed.

Light treatments of nail fungal infections

Nail fungal infections are notoriously persistent and difficult to treat which can lead to severe health impacts, particularly in the immunocompromized. Current antifungal treatments, including systemic and topical drugs, are prolonged and do not effectively provide a complete cure. Severe side effects are also associated with systemic antifungals, such as hepatotoxicity. Light treatments of onychomycosis are an emerging therapy that has localized photodynamic, photothermal or photoablative action. These treatments have shown to be an effective alternative to traditional antifungal remedies with comparable or better cure rates achieved in shorter times and without systemic side effects. This report reviews significant clinical and experimental studies in the field, highlighting mechanisms of action and major effects related to light therapy; in particular, the impact of light on fungal genetics.

Design features for optimization of tetrapyrrole macrocycles as antimicrobial and anticancer photosensitizers

Photodynamic therapy (PDT) uses non-toxic dyes called photosensitizers (PS) and harmless visible light that combine to form highly toxic reactive oxygen species that kill cells. Originally, a cancer therapy, PDT, now includes applications for infections. The most widely studied PS are tetrapyrrole macrocycles including porphyrins, chlorins, bacteriochlorins, and phthalocyanines. The present review covers the design features in PS that can work together to maximize the PDT activity for various disease targets. Photophysical and photochemical properties include the wavelength and size of the long-wavelength absorption peak (for good light penetration into tissue), the triplet quantum yield and lifetime, and the propensity to undergo type I (electron transfer) or type II (energy transfer) photochemical mechanisms. The central metal in the tetrapyrrole macrocycle has a strong influence on the PDT activity. Hydrophobicity and charge are important factors that govern interactions with various types of cells (cancer and microbial) in vitro and the pharmacokinetics and biodistribution in vivo. Hydrophobic structures tend to be water insoluble and require a drug delivery vehicle for maximal activity. Molecular asymmetry and amphiphilicity are also important for high activity. In vivo some structures possess the ability to selectively accumulate in tumors and to localize in the tumor microvasculature producing vascular shutdown after illumination.

Mechanisms in photodynamic therapy: part one-photosensitizers, photochemistry and cellular localization

The use of non-toxic dyes or photosensitizers (PS) in combination with harmless visible light that is known as photodynamic therapy (PDT) has been known for over a hundred years, but is only now becoming widely used. Originally developed as a tumor therapy, some of its most successful applications are for non-malignant disease. In a series of three reviews we will discuss the mechanisms that operate in the field of PDT. Part one discusses the recent explosion in discovery and chemical synthesis of new PS. Some guidelines on how to choose an ideal PS for a particular application are presented. The photochemistry and photophysics of PS and the two pathways known as Type I (radicals and reactive oxygen species) and Type II (singlet oxygen) photochemical processes are discussed. To carry out PDT effectively in vivo, it is necessary to ensure sufficient light reaches all the diseased tissue. This involves understanding how light travels within various tissues and the relative effects of absorption and scattering. The fact that most of the PS are also fluorescent allows various optical imaging and monitoring strategies to be combined with PDT. The most important factor governing the outcome of PDT is how the PS interacts with cells in the target tissue or tumor, and the key aspect of this interaction is the subcellular localization of the PS. Examples of PS that localize in mitochondria, lysosomes, endoplasmic reticulum, Golgi apparatus and plasma membranes are given. Finally the use of 5-aminolevulinic acid as a natural precursor of the heme biosynthetic pathway, stimulates accumulation of the PS protoporphyrin IX is described.

Singlet-oxygen reactions sensitized on solid surfaces of lignin or titanium dioxide: Product studies from hindered secondary amines and from lipid peroxidation

Product analyses and kinetic methods were used to determine the role of singlet oxygen in lignin-catalyzed oxidations of organic substrates. Method A used the ESR analysis of nitroxide radicals formed by singlet oxygen (Type II) on 2,2,6,6-tetramethylpiperidine, 1, or tetramethylpiperidone, 2. Method B used HPLC analysis of the 9- and 13-linoleate chain hydroperoxides formed on oxidation of methyl linoleate to distinguish free-radical peroxidation (Type I) from singlet-oxygen oxidation (Type II) on the basis of different cis,trans (kinetic) to trans,trans (thermodynamic) product ratios. Applications of method A to solid dispersions of lignin or titanium dioxide (TiO2, a known singlet-oxygen sensitizer) indicated singlet-oxygen reactions. In addition to the nitroxide triplet, irradiation of lignin produces a persistent broad signal in the solid attributed to phenoxyl radicals. Benzophenone and 3,5-di-tert-butyl-ortho-benzoquinone, 5, coated on silica gel were used as models to compare the effects of irradiating such compounds on the products and kinetics of methyl linoleate oxidation. Benzophenone acted as an initiator, giving free-radical peroxidation, whereas 5 or lignin coated with methyl linoleate acted as singlet-oxygen sensitizers, according to both product studies (method B) and the kinetic order in oxygen consumption during UV photolysis. Photolysis of phase-separated sensitizer (TiO2 or lignin) and substrate (methyl linoleate) resulted in typical singlet-oxygen products. These results indicate that singlet oxygen plays a significant role in the photo-yellowing of high-lignin-content wood pulps. Key words: lignin, singlet oxygen, mechanism, peroxidation, products.

On the inactivation of bacteria by singlet oxygen--another view

Midden and Dahl, in a recent paper, have presented important data on the inactivation of bacteria by singlet oxygen. In analyzing the data, use was made of a theory published earlier by the present author. The purpose of this paper is to point out that theory and experiment can be brought into better agreement by assuming that the interaction of singlet oxygen with the bacteria takes place in an essentially lipid environment rather than aqueous.