Facile Reduction of 1,2-Dioxetanes by Thiols as Potential Protective Measure against Photochemical Damage of Cellular DNA

Near-Infrared Chemiluminescent Theranostics

Molecular chemiluminescence probes with near-infrared (NIR) emission offer promising benefits in deciphering complex pathological processes in a living system, as NIR chemiluminescence minimizes autofluorescence, enhances deep-tissue penetration, and improves signal-to-noise ratio. Molecular engineering using single-luminophore design and dual-luminophore design with intramolecular energy transfer provides ways to develop conventional chemiluminophore scaffolds into NIR chemiluminescence probes with ideal chemiluminescence quantum yield and half-life. By virtue of the structural diversity, 1,2-dioxetane-based NIR chemiluminophores with biomarker activity have been developed. This review summarizes the molecular design strategies of NIR chemiluminescence theranostic probes (NCTPs), followed by introducing activatable NCTPs with their biomedical applications for disease theranostics. Lastly, future perspectives and potential challenges of NIR chemiluminescence imaging in preclinical research and clinical translational potential are discussed.

Chemiexcitation: Mammalian Photochemistry in the Dark†

Light is one way to excite an electron in biology. Another is chemiexcitation, birthing a reaction product in an electronically excited state rather than exciting from the ground state. Chemiexcited molecules, as in bioluminescence, can release more energy than ATP. Excited states also allow bond rearrangements forbidden in ground states. Molecules with low-lying unoccupied orbitals, abundant in biology, are particularly susceptible. In mammals, chemiexcitation was discovered to transfer energy from excited melanin, neurotransmitters, or hormones to DNA, creating the lethal and carcinogenic cyclobutane pyrimidine dimer. That process was initiated by nitric oxide and superoxide, radicals triggered by ultraviolet light or inflammation. Several poorly understood chronic diseases share two properties: inflammation generates those radicals across the tissue, and cells that die are those containing melanin or neuromelanin. Chemiexcitation may therefore be a pathogenic event in noise- and drug-induced deafness, Parkinson's disease, and Alzheimer's; it may prevent macular degeneration early in life but turn pathogenic later. Beneficial evolutionary selection for excitable biomolecules may thus have conferred an Achilles heel. This review of recent findings on chemiexcitation in mammalian cells also describes the underlying physics, biochemistry, and potential pathogenesis, with the goal of making this interdisciplinary phenomenon accessible to researchers within each field.

Malonic Anhydrides, Challenges from a Simple Structure

After many years of unsuccessful attempts, monomeric malonic anhydrides were prepared by ozonolysis of ketene dimers, a procedure validated by model studies. The structure proof relied most heavily on IR absorption at 1820 cm^-1 and a Raman band at 1947 cm^-1. Malonic anhydrides are unstable, decomposing below room temperature to a ketene plus carbon dioxide. Surprisingly, according to kinetic studies, the dimethyl derivative is slightly less unstable than the parent, and the monomethyl is the fastest to decompose, with an enthalpy of activation of only 12.6 kcal/mol. Computations rationalize this behavior in terms of a concerted [2_s + 2_a] cycloreversion that requires a more highly organized transition state, as also manifested by a negative entropy of activation.

Triplet-Energy Quenching Functions of Antioxidant Molecules

UV-like DNA damage is created in the dark by chemiexcitation, in which UV-activated enzymes generate reactive oxygen and nitrogen species that create a dioxetane on melanin. Thermal cleavage creates an electronically excited triplet-state carbonyl whose high energy transfers to DNA. Screening natural compounds for the ability to quench this energy identified polyenes, polyphenols, mycosporine-like amino acids, and related compounds better known as antioxidants. To eliminate false positives such as ROS and RNS scavengers, we then used the generator of triplet-state acetone, tetramethyl-1,2-dioxetane (TMD), to excite the triplet-energy reporter 9,10-dibromoanthracene-2-sulfonate (DBAS). Quenching measured as reduction in DBAS luminescence revealed three clusters of 50% inhibitory concentration, ~50 μM, 200–500 μM, and >600 μM, with the former including sorbate, ferulic acid, and resveratrol. Representative triplet-state quenchers prevented chemiexcitation-induced “dark” cyclobutane pyrimidine dimers (dCPD) in DNA and in UVA-irradiated melanocytes. We conclude that (i) the delocalized pi electron cloud that stabilizes the electron-donating activity of many common antioxidants allows the same molecule to prevent an electronically excited species from transferring its triplet-state energy to targets such as DNA and (ii) the most effective class of triplet-state quenchers appear to operate by energy diversion instead of electron donation and dissipate that energy by isomerization.

Oxidative Formation of Disulfide Bonds by a Chemiluminescent 1,2-Dioxetane under Mild Conditions

The oxidation of alkyl thiols to disulfides has been achieved under mild conditions using a chemiluminescent 1,2-dioxetane as a stoichiometric oxidant. Besides the mild and biocompatible reaction conditions, this approach offers the possibility to monitor the presence of thiols through oxidation and chemiluminescence of the remaining dioxetane.

Photodegradation pathways and the in vitro phototoxicity of pyrazinamide, a phototoxic antitubercular drug

The phototoxic antitubercular drug pyrazinamide (1) is photolabile under irradiation with UV-A light as well as with a N2 laser (at 337 nm) in aerobic conditions. Irradiation in methanolic and in aqueous solutions of 1 produces four and three photoproducts, respectively. Their formation involves primary alpha-cleavage between the excited carbonyl of the amido group and the aromatic ring followed by hydrogen abstraction and dimerization. Pyrazinamide was able to cause photohemolysis in human erythrocytes and peroxidation of linoleic acid. Inhibition of both processes on addition of reduced glutathione (GSH) or ascorbic acid suggests the involvement of radicals. The absence of inhibition of the photohemolysis and lipid peroxidation processes in the presence of sodium azide (NaN3), or irradiation under argon, and the absence of singlet oxygen during the photolysis confirmed with 2,5-dimethylfuran rules out the possibility of participation of 1O2 in this process. Glutathione depletion was also observed. A radical intermediate was evidenced by thiobarbituric acid that was used as a radical probe, as well as by the dimerization of cysteine. No photohemolysis was detected in presence of the isolated photoproduct. We have also determined the relative efficiencies for the formation of single strand breaks after the irradiation of pBR322 DNA and pyrazinamide, which was also reduced in the presence of GSH.

Induced thermolysis of tert-butyl phenylperacetates by thiophenol: simultaneous occurrence of homolysis and single electron transfer

Thermolysis of tert-butyl phenylperacetates in the presence of thiophenol takes place via dual mechanism. The two-bond homolysis indicates rho(+)(4)= -1.16, testifying to polar transition states. The single electron transfer yields a radical anion intermediate which undergoes fragmentation with rho(ET) = 1.01.

Photodegradation and phototoxicity studies of furosemide. Involvement of singlet oxygen in the photoinduced hemolysis and lipid peroxidation

The phototoxic diuretic drug furosemide (1), a 5-(aminosulfonyl)-4-chloro-2-[(2-furanylmethyl)-amino] benzoic acid is photolabile under aerobic and anaerobic conditions. Irradiation of a methanol solution of 1 under oxygen produces photoproducts 2, 3, 4 and singlet oxygen, while under argon the photoproducts 2 and 4 were isolated. A peroxidic unstable photoproduct was detected during the photolysis under oxygen atmosphere. The formation of singlet oxygen by photolysis of 1 was evidenced by trapping with 2,5-dimethylfuran (GC-mass), furfuryl alcohol and 1,3-cyclohexadiene-1,4-diethanoate (HPLC) as 1O2 scavengers and by the histidine test. Furosemide was screened in vitro at different concentrations for UV-Vis-induced phototoxic effects in a photohemolysis test, in the presence and absence of different radical scavengers, singlet oxygen and hydroxyl radical quenchers. However, furosemide photosensitized the peroxidation of linoleic acid, as monitored by the UV-detection of dienic hydroperoxides and it also photosensitized the oxidation of histidine. The photodegradation was catalyzed in the presence of human serum albumin. Studies on peripheral blood mononuclear and polymorphonuclear cells (lymphocytes and neutrophils) demonstrated no phototoxicity on these cell lines.

The oxidation of cyclic sulfides by tetramethyldioxetane and the isobutanal/O2/peroxidase system: oxygen transfer versus electron transfer

The oxidation of chlorpromazine (CPZ) by tetramethyldioxetane (TMD) and isobutanal (IBAL)/O2/horseradish peroxidase (HRP) system was investigated. The reaction with TMD proved to be of the oxygen transfer type, generating chlorpromazine-5-oxide (CPZO) and tetramethylethylene-oxide, and not by single-electron transfer, as previously reported. In contrast, the reaction of CPZ with IBAL/O2/HRP leads to formation of chlorpromazine cation radical, through reaction with active intermediates Compound I and II, following its dismutation and hydrolysis to CPZO. For comparison, 10-methylphenothiazine was also tested. Despite the fact that both systems are known to generate oxidizing triplet acetone, this species does not participate in the oxidation path in either case.

Iron-thiolate induced oxidation of methionine to methionine sulfoxide in small model peptides. Intramolecular catalysis by histidine

Peptides containing either glycine and methionine, or glycine, methionine and histidine at various locations were oxidized by the dithiothreitol/ferric chloride system in phosphate buffer. The yields of peptide degradation and sulfoxide formation were measured as a function of peptide sequence and pH. In general little change of the final yields of peptide degradation is observed whereas the final yields of sulfoxide formation progressively decrease on going from pH 6.0 to 8.0. The pH profiles vary with the structure of the respective peptide. Efficient sulfoxide formation occurred when histidine and methionine were present within the same peptides sequence, and particularly when methionine was located at the C-terminus of the peptide. Added superoxide dismutase, catalase, and methanol did neither promote nor inhibit both the degradation of peptide and the formation of sulfoxide excluding free superoxide, hydrogen peroxide, and hydroxyl radicals as responsible reactive oxygen species. The observations are rationalized by invoking a pH-dependent conversion of an efficiently sulfoxide yielding oxidant into another oxidant which still degrades peptides but does not form methionine sulfoxide. The first might be a metal-bound peroxide or peroxyl species which converts into a metal-bound or 'complexed' hydroxyl radical.

Photodegradation and in vitro phototoxicity of fenofibrate, a photosensitizing anti-hyperlipoproteinemic drug

The phototoxic anti-hyperlipoproteinemic drug fenofibrate was found to be photolabile under aerobic and anaerobic conditions. Irradiation under argon of a methanol solution of this drug produced the photoproducts isopropyl 4-(1-[4-chlorophenyl]-1,2-dihydroxy)ethylphenoxyisobutyrate, 1,2-bis(4-chlorophenyl)-1,2bis (4-[isopropoxycarbonylisopropoxy]phenyl)ethane-1,2-diol and 4-(4-chlorobenzoyl)phenol, while under oxygen the photoproducts were 4-chloroperbenzoic acid, methyl 4-chlorobenzoate, 4-chlorobenzoic acid and singlet oxygen, as evidenced by trapping with 2,5-dimethylfuran. These results can be rationalized through hydrogen abstraction by excited fenofibrate, to afford a free radical as key intermediate. Biologically active antioxidants such as glutathione and cysteine efficiently reduced 4-chloroperbenzoic acid to 4-chlorobenzoic acid. The involvement of an electron transfer mechanism is suggested by detection (UV-vis spectrophotometry) of the radical cation TMP+. during the oxidation of tetramethylphenylenediamine (TMP) with 4-chloroperbenzoic acid. Fenofibrate was phototoxic in vitro when examined by the photohemolysis test, both under oxygen and argon atmosphere, although the photohemolysis rate was markedly lower under anaerobic conditions. The photoproducts 4-(1-[4-chlorophenyl]-1,2-dihydroxy)ethylphenoxyisobutyrate and 4-chloroperbenzoic acid induced hemolysis in the dark; however, this effect was quantitatively less important than photohemolysis by fenofibrate. On the other hand, fenofibrate photosensitized peroxidation of linoleic acid, monitored by the UV detection of dienic hydroperoxides. Based on the inhibition of this process upon addition of butylated hydroxyanisole, a radical chain (type I) mechanism appears to operate. In summary, fenofibrate is phototoxic in vitro. This behavior can be explained through the involvement of free radicals, singlet oxygen and stable photoproducts.

Formation of a perbenzoic acid derivative in the photodegradation of fenofibrate: phototoxicity studies on erythrocytes

The phototoxic antihyperlipoproteinemic drug fenofibrate (1) is photolabile under aerobic conditions. Irradiation of a methanol solution of 1 produces, under oxygen, photoproducts 2, 3, and 4. A peroxidic photoproduct 2 was isolated and identified. The biologically active antioxidants glutathione and cysteine efficiently reduce 2 to its acid. This photoproduct was also capable of efficiently oxidizing tetramethyl phenylendiamine (TMP) through an electron transfer mechanism, detecting a TMP+ species by UV-visible spectrometry. Fenofibrate was screened in vitro at different concentrations for UV-visible-induced phototoxic effects in a photohemolysis test, under oxygen as well as argon. The photohemolysis rate was low under anaerobic conditions. No hemolysis occurred without irradiation. The isolated photoproduct 2 induced hemolysis without irradiation.

Singlet oxygen induced single-strand breaks in plasmid pBR322 DNA: the enhancing effect of thiols

The biologically occurring thiols, glutathione, cysteamine and cysteine, significantly enhance the single-strand breaks in plasmid pBR322 DNA induced by singlet molecular oxygen (1O2) generated by the thermodissociation of the endoperoxide of 3,3'-(1,4-naphthylidene)dipropionate. The enhancing effect was also observed with chemically related sulfhydryl compounds but not by disulfides. In contrast, dihydrolipoate and its disulfide lipoate protected the plasmid DNA. Metal chelators as well as superoxide dismutase or catalase had no effect, whereas mannitol or sodium azide, decreased the thiol-1O2-induced strand breaks. It is concluded that the observed effects are mediated by reactive oxidation products arising from the 1O2-oxidation of thiols.

The peroxidase/oxidase activity of soybean lipoxygenase--II. Triplet carbonyls and red photoemission during polyunsaturated fatty acid and glutathione oxidation

During the aerobic reaction of soybean lipoxygenase with polyunsaturated fatty acids (linoleic, linolenic, and arachidonic acid) oxygen uptake is followed by excited carbonyl photoemission. The chemiluminescence yield of phi cl = 10(-10) photons/O2 molecule consumed is enhanced 2-3 orders of magnitude by the carbonyl sensitizers 9,10-dibromo-anthracene-2-sulfonate (kET tau 0 = 10(4) M-1; phi cl = 10(-8) photons/O2) and chlorophyll-a (kET tau 0 = 10(6) M-1; phi cl = 10(-7) photons/O2), respectively. alpha,beta-Saturated triplet excited carbonyls as from 1,2-dioxetane cleavage are discussed to arise from a secondary peroxidase/oxidase reaction with aldehydes formed in the course of enzymic lipid peroxidation. When 1 mM glutathione is added to the aerobic lipoxygenase/arachidonate reaction, carbonyl emission (375-455 nm) is replaced by intense red bands (630-645 nm and 695-715 nm) resembling the characteristic spectrum of (1 delta g)O2-singlet oxygen dimol-emission. The quantum yield (phi cl = 10(-8) photons/O2) remains unaffected by chlorophyll indicating that the red emission is independent of excited carbonyls. The effect of GSH is attributed to dioxetane interception and subsequent glutathione peroxidation generating 1O2 by electron transfer from the superoxide anion radical to a peroxysulfenyl radical.

The peroxidase/oxidase activity of soybean lipoxygenase--I. Triplet excited carbonyls from the reaction with isobutanal and the effect of glutathione

Soybean lipoxygenase shows a secondary peroxidase/oxidase activity: The aerobic reaction with isobutanal, enhanced by hydrogen peroxide as a cosubstrate, yields acetone, exhibits chemiluminescence and consumes oxygen (phi cl = 1.3 x 10(-9) photons/O2 molecule consumed). 9,10-Dibromoanthracene-2-sulfonate increases the photoemission (kET tau 0 = 2 x 10(4) M-1; phi cl = 0.9 x 10(-7) photons/O2), whereas it is diminished by sorbate, tryptophan, 2-methyl-1,4-naphthoquinone, glutathione, and superoxide dismutase. In the presence of hydrogen peroxide the lipoxygenase reaction with glutathione yields yet another excited state. From the well-known reactions promoted by horseradish-peroxidase, these features are concluded to indicate the novel activity of soybean lipoxygenase. With isobutanal as a substrate lipoxygenase acts as an oxidase and as a peroxidase. The mechanism suggested leads to photoemissive triplet excited acetone as expected from the cleavage of an intermediate dioxetane.

Single-electron-transfer in the reduction of 1,2-dioxetanes by biologically active substrates

Substances of low oxidation potential, which can also make available protons and hydrogen atoms, e.g. phenothiazines, NADH, and ascorbic acid efficiently reduce 1,2-dioxetanes to their vic-diols by single-electron-transfer; a significant side reaction is catalytic decomposition of dioxetanes into the corresponding ketone fragments.