Kinetic study on the singlet oxygen quenching activity of nordihydroguaiaretic acid (NDGA) using methylene blue sensitized photooxidation of α-terpinene

Photocleavable Protecting Groups Using a Sulfite Self-Immolative Linker for High Uncaging Quantum Yield and Aqueous Solubility

Using light as an external stimulus to control (bio)chemical processes offers many distinct advantages. Most importantly, it allows for spatiotemporal control simply through operating the light source. Photocleavable protecting groups (PPGs) are a cornerstone class of compounds that are used to achieve photocontrol over (bio)chemical processes. PPGs are able to release a payload of interest upon light irradiation. The successful application of PPGs hinges on their efficiency of payload release, captured in the uncaging Quantum Yield (QY). Heterolytic PPGs efficiently release low pKa payloads, but their efficiency drops significantly for payloads with higher pKa values, such as alcohols. For this reason, alcohols are usually attached to PPGs via a carbonate linker. The self-immolative nature of the carbonate linker results in concurrent release of CO2 with the alcohol payload upon irradiation. We introduce herein novel PPGs containing sulfites as self-immolative linkers for photocaged alcohol payloads, for which we discovered that the release of the alcohol proceeds with higher uncaging QY than an identical payload released from a carbonate-linked PPG. Furthermore, we demonstrate that uncaging of the sulfite-linked PPGs results in the release of SO2 and show that the sulfite linker improves water solubility as compared to the carbonate-based systems.

Kinetics of Squalene Quenching Singlet Oxygen and the Thermal Degradation Products Identification

Squalene has been proven to possess various bioactive functions that are widely present in vegetable oils. A more comprehensive understanding of the reaction behavior of squalene under oxidative conditions was achieved by studying its antioxidant capacity and thermal degradation products. The total singlet oxygen quenching rate constant (kr + kq) of squalene was 3.8 × 107 M-1 s-1, and both physical and chemical quenching mechanisms equally contribute to the overall singlet oxygen quenching. Fourteen degradation products of squalene were identified at 180 °C by using gas chromatography-mass spectrometry (GC-MS). Combining with DFT calculations, the thermal degradation pathway of squalene was proposed: the aldehydes, ketones, and alcohols, and epoxy compounds were formed by the homolytic cleavage of squalene hydroperoxides to form alkoxy radicals, followed by β-scission of the alkoxyl radicals at adjacent C-C bonds or intramolecular cyclization.

Stilbenes: A new strategy for protecting light-sensitive foods, a review of their structure classification and singlet oxygen quenching mechanism

Natural stilbenes have been studied extensively as a result of their complicated structures and diverse biological activities. Singlet oxygen (1O2), a kind of reactive oxygen species (ROS) has a strong destructive effect on food systems (especially for light-sensitive foods). Many cutting-edge scientific studies have found that some stilbenes not only have extensive quenching properties for ROS, but also can selectively quench 1O2. However, the industry devoted too much energy on the development of more new stilbenes, lacking in-depth summaries and reflections on the characteristics of their basic structure and the mechanism of their extraordinary 1O2 quenching abilities. Therefore, we summarized the classification methods for stilbene compounds and evaluated similarities, differences and possible limitations of different classification methods. In addition, we described the role of different functional groups in stilbenes in quenching of 1O2 and summarized the quenching mechanism of 1O2 by stilbenes. By the way, the current application of stilbene compounds and their potential risks in the food industry were also mentioned in this article. The stilbenes can be used as antioxidants (especially new strategies against 1O2 oxidation) in food systems to improve the shelf life. At this stage, it is necessary to develop more effective and safe food antioxidant stilbenes based on their quenching mechanism.

Assessment of the antioxidant activities of representative optical and geometric isomers of astaxanthin against singlet oxygen in solution by a spectroscopic approach

Astaxanthin (AST) is a natural antioxidant and has been widely applied as a food supplement. While astaxanthin has many isomers, there are few studies comparing its physicochemical properties. In this work, we were concerned about their antioxidant activities against external oxidative stresses, and specifically, the singlet oxygen (¹O₂) quenching capacities of the representative optical and geometric isomers of astaxanthin were examined. Methylene blue (MB) was used as the photosensitizer to produce ¹O₂, and 1,3-diphenylisobenzofuran (DPBF) was used to probe ¹O₂. Our results showed that the ¹O₂ quenching capacities of the optical isomers, including 3S,3'S, 3R,3'S, and 3R,3'R all-trans-astaxanthin, are identical. In contrast, the ¹O₂ quenching capacity of cis-astaxanthin is higher than that of all-trans-astaxanthin. As such, this work provides an effective spectroscopic approach to assessing the antioxidant activities of various forms of astaxanthin against singlet oxygen, and demonstrates the remarkable difference among the geometric isomers.

Quenching effects of (-)-Epigallocatechin gallate for singlet oxygen production and its protection against oxidative damage induced by Ce6-mediated photodynamic therapy in vitro

BACKGROUND

Singlet oxygen (¹O₂) is highly reactive to biological components such as lipids, proteins and DNA, which induces oxidative damage to cells and tissues. Natural antioxidants may function as ¹O₂ quencher to prevent ¹O₂ involved photosensitized oxidation in biological system.

METHODS

Time-resolved measurement of ¹O₂ luminescence was employed to evaluate the ¹O₂ quenching abilities of natural antioxidants in air-statured phosphate buffered saline (PBS), including (-)-Epigallocatechin gallate (EGCG), Proanthocyanidins, L-carnosine and Vitamin C. The ¹O₂ quenching effects and rate constant of EGCG were investigated by detecting the absorption, fluorescence and ¹H-NMR spectroscopy and ¹O₂ luminescence decay curves, respectively. In addition, the protective activity of EGCG against ¹O₂ oxidative damage caused by Ce6-mediated photodynamic therapy (PDT) was verified in cells.

RESULTS

EGCG, proanthocyanidins, L-carnosine and Vitamin C efficiently quenched ¹O₂ luminescence at 1270 nm. The triplet-state quenching rate constants of EGCG for Rose Bengal (RB), Chlorin e6, AlPcS and HiPorfin are 2.21 × 10⁹, 4.90 × 10⁸, 3.30 × 10⁸, 1.78 × 10⁹ M^-1s^-1, while the ¹O₂ quenching rate constants are 2.80 × 10⁸, 1.50 × 10⁸, 1.30 × 10⁸, 1.70 × 10⁸ M^-1s^-1, respectively. Furthermore, EGCG could effectively quench ¹O₂ production to prevent NIH/3T3 cells oxidative damage induced by Ce6-mediated PDT.

CONCLUSIONS

EGCG is an efficient quencher for both triplet-state photosensitizers and ¹O₂. The quenching ability of EGCG during photosensitization for selected photosensitizers is: RB > HiPorfin > Ce6 > AlPcS. EGCG could be used to protect normal cells and tissue against oxidative damage.

Simultaneous Determination of the Chemical (kr ) and the Physical (kq ) Quenching Rate Constants of Singlet Oxygen in Aqueous Solution by the Chemiluminescence-quenching Method†

This work reports a novel and visual method for the simultaneous determination of the chemical (k_r ) and the physical (k_q ) quenching rate constants of singlet oxygen (¹ O₂ ,¹ ∆_g ) in aqueous media. It is based on the disruption, by a water-soluble substrate S, of the ¹ O₂ chemiluminescence (CL) generated by the H₂ O₂ /Na₂ MoO₄ catalytic system. A mathematical analysis of the CL signal at 1270 nm vs time provides separately the overall (k_r  + k_q ) and the chemical (k_r ) quenching rate constants. In ordinary water (H₂ O), ¹ O₂ lifetime is short and the CL intensity is weak allowing solely the investigation of very reactive substrates for which (k_r  + k_q ) > 3 × 10⁶  m^-1  s^-1 while, in D₂ O, ¹ O₂ lifetime is significantly longer lifetime and the CL signal is much stronger allowing the study of poorly reactive substrates for which (k_r  + k_q ) > 4 × 10⁵  m^-1  s^-1 . The method has been successfully tested on a series of anionic and nonionic water-soluble naphthalene derivatives commonly used as bio-compatible ¹ O₂ carriers. The obtained k_r and k_q values are in good agreement with the values determined by conventional techniques, namely, flash photolysis and competitive kinetics with a reference quencher.

Carbon-Carbon Double Bond and Resorcinol in Resveratrol and Its Analogues: What Is the Characteristic Structure in Quenching Singlet Oxygen?

Stilbenes, particularly resveratrol and resveratrol dimers, could effectively quench singlet oxygen (¹O₂). It was reported that both resorcinol and carbon-carbon double bond quenching ¹O₂ can participate in the mechanism. However, it is still not clear which structure plays a dominant role in quenching ¹O₂. To investigate the characteristic structure in the mechanism of quenching ¹O₂, the resveratrol, pterostilbene and piceatannol quenching ¹O₂ abilities were compared by UHPLC-QTOF-MS² and UHPLC-QQQ-MS². Results showed that catechol, carbon-carbon double bond and resorcinol participated in the quenching of ¹O₂. Catechol ring plays a leading role in the mechanism, and the contribution of the structures in quenching ¹O₂ activity are as follows: catechol ring > carbon-carbon double bond > resorcinol ring, which is supported by the calculation of energy. Our findings will contribute to the future screening of stilbenes with higher activity, and those stilbenes may have great therapeutic potential in ¹O₂-mediated diseases.