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Concept: Singlet oxygen


Water-Soluble Chlorophyll Proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins which tetramerize upon binding four chlorophyll (Chl) molecules. The bound Chls are highly photostable, despite the lack of bound carotenoids known, in Chl-containing photosynthetic proteins, to act as singlet oxygen and Chl triplet ((3)Chl) quenchers. Although the physiological function of WSCPs is still unclear, it is likely to be related to their biochemical stability and their resistance to photodegradation. To get insight into the origin of this photostability, the properties of the (3)Chl generated in WSCPs upon illumination were investigated. We found that, unlike the excited singlet states, which are excitonic states, the triplet state is localized on a single Chl molecule. Moreover, the lifetime of the (3)Chl generated in WSCPs is comparable to that observed in other Chl-containing systems and is reduced in presence of oxygen. In contrast to previous observations, we found that WSCP actually photosensitizes singlet oxygen with an efficiency comparable to that of Chl in organic solvent. We demonstrated that the observed resistance to photooxidation depends on the conformation of the phytyl moieties, which in WSCP are interposed between the rings of Chl dimers, hindering the access of singlet oxygen to the oxidizable sites of the pigments.

Concepts: Photosynthesis, Oxygen, Acetic acid, Solvent, Excited state, Singlet oxygen, Singlet state, Triplet oxygen


Cell-penetrating peptides such as TAT or R9 labeled with small organic fluorophores can lyse endosomes upon light irradiation. The photo-endosomolytic activity of these compounds can in turn be used to deliver proteins and nucleic acids to the cytosol of live cells with spatial and temporal control. In this report, we examine the mechanisms by which such fluorescent peptides exert a photolytic activity using red blood cells as a membrane model. We show that the peptides TAT and R9 labeled with tetramethylrhodamine photolyse red blood cells by promoting the formation of singlet oxygen in the vicinity of the cells' membranes. In addition, unlabeled TAT and R9 accelerate the photolytic activity of the membrane-bound photosensitizer Rose Bengal in trans, suggesting that the cell-penetrating peptides participate in the destabilization of photo-oxidized membranes. Peptides and singlet oxygen generators therefore act in synergy to destroy membranes upon irradiation. © 2012 Wiley Periodicals, Inc. Photochemistry and Photobiology © 2012 The American Society of Photobiology.

Concepts: Photosynthesis, Protein, Oxygen, Cell nucleus, Blood, Cell membrane, Red blood cell, Singlet oxygen


Upconverting nanoparticles (UCNPs) have attracted considerable attention as potential photosensitizer carriers for photodynamic therapy (PDT) in deep tissues. In this work, a new and efficient NIR photosensitizing nanoplatform for PDT based on red-emitting UCNPs is designed. The red emission band matches well with the efficient absorption bands of the widely used commercially available photosensitizers (Ps), benefiting the fluorescence resonance energy transfer (FRET) from UCNPs to the attached photosensitizers and thus efficiently activating them to generate cytotoxic singlet oxygen. Three commonly used photosensitizers, including chlorine e6 (Ce6), zinc phthalocyanine (ZnPc) and methylene blue (MB), are loaded onto the alpha-cyclodextrin-modified UCNPs to form Ps@UCNPs complexes that efficiently produce singlet oxygen to kill cancer cells under 980 nm near-infrared excitation. Moreover, two different kinds of drugs are co-loaded onto these nanoparticles: chemotherapy drug doxorubicin and PDT agent Ce6. The combinational therapy based on doxorubicin (DOX)-induced chemotherapy and Ce6-triggered PDT exhibits higher therapeutic efficacy relative to the individual means for cancer therapy in vitro.

Concepts: Spectroscopy, Oxygen, Ultraviolet, Chemotherapy, Förster resonance energy transfer, Methylene blue, Singlet oxygen, Photodynamic therapy


Vertical excitation energies obtained with state-specific multi-reference coupled cluster (MRCC) methods are reported for the low-lying singlet and triplet excited of the ozone molecule. The MRCC results are also compared with those obtained with high-order equation-of-motion coupled cluster methods.

Concepts: Oxygen, Quantum mechanics, Atom, Excited state, Singlet oxygen, Singlet state, Coupled cluster, Ozone


We examined the effect of the oxygenation level on efficacy of two photosensitizing agents, both of which target lysosomes for photodamage but via different photochemical pathways. Upon irradiation, the chlorin termed NPe6 forms singlet oxygen in high yield while the bacteriopheophorbide WST11 forms only oxygen radicals (in an aqueous environment). Photokilling efficacy by WST11 in cell culture was impaired when the atmospheric oxygen concentration was reduced from 20% to 1%, while photokilling by NPe6 was unaffected. Studies in a cell-free system revealed that rates of photobleaching of these agents, as a function of the oxygenation level, were correlated with results described above. Moreover, the rate of formation of oxygen radicals by either agent was more sensitive to the level of oxygenation than was singlet oxygen formation by NPe6. These data indicate that the photochemical process that leads to oxygen radical formation is more dependent on the oxygenation level than is the pathway leading to formation of singlet oxygen. © 2012 Wiley Periodicals, Inc. Photochemistry and Photobiology © 2012 The American Society of Photobiology.

Concepts: Oxygen, Oxidative stress, Reactive oxygen species, Radical, Physical chemistry, Singlet oxygen, Photochemistry, Free radicals


2-Demethoxy-2,3-ethylenediamino hypocrellin B (EDAHB) is a diamino-substituted hypocrellin B (HB) with high absorption of red light and high quantum yield of both singlet oxygen ((1)O(2)) and superoxide anions (O(2)(-)). Here we reported the cellular uptake, subcellular location, and cytotoxicity of EDAHB, as well as EDAHB-mediated photodynamic therapy (PDT) efficiency, and cell apoptosis. Results showed that EDAHB accumulated in HeLa cells rapidly up to 1h, with a subsequent decrease in the rate of uptake. EDAHB distributed with well-defined spots throughout the cytoplasm of the cells. EDAHB showed a much higher photopotentiation factor than HB. The phototoxicity of EDAHB to HeLa cells occurred via a mitochondria/caspase apoptosis pathway. This study showed EDAHB to be a promising candidate of photosensitizer for anti-tumor PDT.

Concepts: Protein, Oxygen, Cell, Apoptosis, Cell culture, Vesicle, Singlet oxygen, Photodynamic therapy


BODIPY dyes tend to be highly fluorescent, but their emissions can be attenuated by adding substituents with appropriate oxidation potentials. Substituents like these have electrons to feed into photoexcited BODIPYs, quenching their fluorescence, thereby generating relatively long-lived triplet states. Singlet oxygen is formed when these triplet states interact with (3)O(2). In tissues, this causes cell damage in regions that are illuminated, and this is the basis of photodynamic therapy (PDT). The PDT agents that are currently approved for clinical use do not feature BODIPYs, but there are many reasons to believe that this situation will change. This review summarizes the attributes of BODIPY dyes for PDT, and in some related areas.

Concepts: Fluorescence, Electron, Spectroscopy, Oxygen, Ultraviolet, Singlet oxygen, Photodynamic therapy, Triplet oxygen


Gold nanorods with three different aspect ratios were prepared and their dual capabilities for two-photon imaging and two-photon photodynamic therapy have been demonstrated. These gold nanorods exhibit large two-photon absorption action cross-sections, about two orders of magnitude larger than small organic molecules, which makes them suitable for two-photon imaging. They can also effectively generate singlet oxygen under two-photon excitation, significantly higher than traditional photosensitizers such as Rose Bengal and Indocyanine Green. Such high singlet oxygen generation capability under two-photon excitation was ascribed to their large two-photon absorption cross-sections. Polyvinylpyrrolidone (PVP) coated gold nanorods displayed excellent biocompatibility and high cellular uptake efficiency. The two-photon photodynamic therapy effect and two-photon fluorescence imaging properties of PVP coated gold nanorods have been successfully demonstrated on HeLa cells in vitro using fluorescence microscopy and indirect XTT assay method. These gold nanorods thus hold great promise for imaging guided two-photon photodynamic therapy for the treatment of various malignant tumors.

Concepts: Protein, Oxygen, Cancer, Two-photon excitation microscopy, Aspect ratio, Singlet oxygen, Photodynamic therapy, Rose bengal


The photo-inactivation rate of bacteria by methylene blue, MB, was found to be significantly lower in plasma than in water, saline, and PBS solutions. The spectroscopic data and ultrafast time resolved transient spectra and kinetics presented show that methylene blue photo-bleaches faster and to a larger degree in plasma and the MB excited singlet and triplet state populations in plasma are much lower in plasma than in water and PBS solutions. The optical density, OD, of MB in plasma was found to decrease by ∼50% after a minute of illumination with 661 nm light, while under identical conditions the OD in PBS solution decreased by only 1%. Based on these data and the effect of the plasma proteins on MB photochemistry, a mechanism is proposed that accounts for the low inactivation rate of bacteria in plasma.

Concepts: Spectroscopy, Light, Chemistry, Physical chemistry, Methylene blue, Singlet oxygen, Singlet state, Doublet state


HPLC and emission spectroscopy were used to investigate UVA photosensitization of methylene blue (MB) or naproxen (NAP) towards bovine serum albumin (BSA). In addition, time resolved singlet oxygen measurements were carried out. The most stable drug : protein adducts stoichiometry of MB-BSA (1 : 1) and NAP-BSA (9 : 1) were verified by means of binding constant determination. UVA photosensitization of MB or NAP on BSA was studied by monitoring tryptophan (Trp) residue integrity. The sensitized photodegradation of the BSA resulted in different degrees of Trp damage. Thus, protein damage was determined by quantitative measurements of the different Trp (photo)-products. Indeed, many of these Trp derivatives are diagnostic for the photosensitization mechanism and some of them, for the first time in this work, were obtained by UVA photosensitization in proteins. The analysis of quantum yields of the photoproduct distribution allowed to weigh up the type I/II contribution to the UVA photosensitization mechanism. As expected, additional experiments in deuterated solvent resulted in an increase of the photodegradation quantum yields for those species where a singlet oxygen mechanism was involved. The UVA mediated generation of these Trp derivatives is consistent with the occurrence of singlet oxygen formation (almost dominant in MB), and photoionization (significant in NAP) within the protein matrix. Additional experiments at lower NAP concentration, as well as with human serum albumin (which differs for Trp content and, partially, localization), support further the molecular mechanism of photosensitization proposed. The results obtained in the case of this more complex system are in agreement with the free Trp model, even if, in almost all cases, the Trp photoproduct formation quantum yields are lower, due to the higher number of sensitization targets in the proteins.

Concepts: Proteins, Protein, Spectroscopy, Oxygen, Serum albumin, Physical chemistry, Methylene blue, Singlet oxygen