Correspondence on "A Mitochondrion-Localized Two-Photon Photosensitizer Generating Carbon Radicals Against Hypoxic Tumors".

It is shown that data presented in a paper by Chao and co-workers do not support the formation of active "Carbon Radicals" as claimed according to the title. The assignments of observed ESR spectra and the mechanistic interpretation are severely flawed. Hence, other reactive intermediates must be responsible for the observed tumor-damaging effects.

Comment on "Disclosing Cyclic(Alkyl)(Amino)Carbenes as a One-Electron Reductant: Synthesis of Acyclic(Amino)(Aryl)Carbene-Based Kekulé Diradicaloids".

This correspondence addresses a misassignment of an EPR spectrum of 2 in a recent publication (Chem. Eur. J. 2022, 28, e202104567) by Dr. Jana and co-workers. The original authors have prepared this correspondence together with Dr. Korth.

Solvent Cage Concept for the Homolytic Fragmentation of the Peroxynitrite-CO2 Adduct, ONOOCO2.

The toxicity of peroxynitrite, ONOO-, is directed by carbon dioxide via the formation of the corresponding adduct, ONOOCO2-. Entity ONOOCO2- is believed to be a highly unstable compound that primarily decomposes to nitrate and carbon dioxide, but it also undergoes fractional homolysis to generate carbonate radical anion, CO3•-, and nitrogen dioxide, NO2•, in a so-called solvent (radical) cage reaction. Recently, Koppenol et al. reviewed their proposal that ONOOCO2- is a relatively long-lived intermediate, arguing that "the solvent cage as proposed is physically not realistic". To further address whether ONOOCO2- could be a long-lived species, bond dissociation enthalpies (BDE) were calculated by the composite reference method (SMD)W1BD. Anion ONOOCO2- can exist in two conformers, s-cis-gauche and s-trans-gauche with predicted gas-phase O-O BDEs of about 10.8 and 9.5 kcal mol-1, respectively. Therefore, both conformers should have very short lifetimes. The (SMD)W1BD method was also used to evaluate the thermodynamic parameters of interest, revealing that the homolytic decomposition of ONOOCO2- is the most reasonable pathway. Moreover, previously reported experimental chemically induced dynamic nuclear polarization data also support the intermediacy of the radical cage and the formation of products CO2 and NO3- at a total yield of about 70%. Because the solvent radical cage concept for the decay of ONOO- in the presence of CO2 is supported by a variety of spectrometric methods as well as by quantum chemical calculations at high levels of theory, it provides strong evidence against the "out-of-cage" construct. For clarification of the nature of the transient UV/vis absorption(s) between 600 and 700 nm, as observed by Koppenol et al., several experimental approaches are suggested.

Comment on "Photochemical Activities of N-Nitroso Carboxamides and Sulfoximides and Their Application to DNA Cleavage".

This Correspondence reports on a structural and mechanistic misinterpretation of an observed EPR spectrum, a misleading presentation of DNA cleavage data, and the physicochemically unsupported assignment of nitric oxide as a major DNA cleaving agent in the article cited in the title.

Comment on "Persistent Room-Temperature Radicals from Anionic Naphthalimides: Spin Pairing and Supramolecular Chemistry" and on "Persistent Radical Pairs between N-Substituted Naphthalimide and Carbanion Exhibit pKa-Dependent UV/Vis Absorption".

EPR spectroscopic evidence for intramolecular electron transfer in anionic N-substituted naphthalimides to yield persistent diradical anions and intermolecular electron transfer from a variety of carbanions to 6-bromo-N-phenyl-naphthalimide to yield persistent radical-radical anion pairs was recently claimed in two papers by Zhang et al. In this comment, it is shown that the EPR spectra published in both papers do not agree with the proposed triplet-state species. Rather, the spectra are due to various doublet-state radicals, deriving from minor side reactions. The misinterpretations invalidate the general conclusions of the papers.

Phenolic Hydrogen Transfer by Molecular Oxygen and Hydroperoxyl Radicals. Insights into the Mechanism of the Anthraquinone Process.

Hydrogen atom transfer (HAT) by 3O2 and HO2• from arenols (ArOH), aryloxyls (ArO•), their tautomers (ArH), and auxiliary compounds has been investigated by means of CBS-QB3 computations. With 3O2, excellent linear correlations have been found between the activation enthalpy and the overall reaction enthalpy. Different pathways have been discerned for HATs involving OH or CH moieties. The results for ArOH + HO2• → ArO• + H2O2 neither afford a linear correlation nor agree with the experiment. The precise mechanism for the liquid-phase autoxidation of anthrahydroquinone (AnH2Q) appears to be not fully understood. A kinetic analysis shows that the HAT by chain-carrying HO2• occurs with a high rate constant of ≥6 × 108 M-1 s-1 (toluene). The second propagation step pertains to a diffusion-controlled HAT by 3O2 from the 10-OH-9-anthroxyl radical. Oxanthrone (AnOH) is a more stable tautomer of AnH2Q with a ratio of 13 (298 K) in non-hydrogen-bonding (HB) solvents, but the reactivity toward 3O2/HO2• is much lower. Combination of the computed free energies and Abrahams' HB donating (α2H) and accepting (ß2H) parameters has afforded an α2H(HO2•) of 0.86 and an α2H(H2O2) of 0.50.

Another Unprecedented Wieland Mechanism Confirmed: Hydrogen Formation from Hydrogen Peroxide, Formaldehyde, and Sodium Hydroxide.

In 1923, Wieland and Wingler reported that in the molecular hydrogen producing reaction of hydrogen peroxide with formaldehyde in basic solution, free hydrogen atoms (H. ) are not involved. They postulated that bis(hydroxymethyl)peroxide, HOCH2 OOCH2 OH, is the intermediate, which decomposes to yield H2 and formate, proposing a mechanism that would nowadays be considered as a "concerted process". Since then, several other (conflicting) "mechanisms" have been suggested. Our NMR and Raman spectroscopic and kinetic studies, particularly the determination of the deuterium kinetic isotope effect (DKIE), now confirm that in this base-dependent reaction, both H atoms of H2 derive from the CH2 hydrogen atoms of formaldehyde, and not from the OH groups of HOCH2 OOCH2 OH or from water. Quantum-chemical CBS-QB3 and W1BD computations show that H2 release proceeds through a concerted process, which is strongly accelerated by double deprotonation of HOCH2 OOCH2 OH, thereby ruling out a free radical pathway.

Comment on "trans-1,2-Disiloxybenzocyclobutene, an adequate partner for the auto-oxidation: EPR/spin trapping and theoretical studies" by J. Drujon et al., Phys. Chem. Chem. Phys., 2014, 16, 7513.

Despite the claim of the contrary, (i) the facile formation of a ring-opened cyclic peroxide from reaction of a 1,2-disubstituted benzocyclobutene with molecular oxygen has been reported before, and (ii) the analysis of the mechanistic steps of this process is incomplete, as only the symmetry-allowed conrotatory ring-opening of the benzocyclobutene has been considered. The probable involvement of biradical intermediates/biradicaloid transition states in a formally symmetry-forbidden reaction sequence has not been taken into account.

Tautomerization of Some Methylacenes and the Role of Reverse Radical Disproportionation.

The thermokinetics for the tautomerization of a series of methylenedihydroacenes to the corresponding methylacenes (toluene to 6-methylpentacene) have been investigated by means of CBS-QB3 calculations. Only for 6-methylpentacene does the methylenedihydro form predominate at room temperature. The obtained equilibrium ratios are consistent with various theoretical methods, but the agreement with the scarce experimental data is only qualitative. The noncatalyzed thermal tautomerization of the methylenedihydroacene in an inert solvent may proceed by means of a reverse radical disproportionation reaction (RRD) as the rate-determining step. The benzylic BDE(C-H)s and the hydrogen atom affinities (HA) of the tautomers have been used to calculate the reaction enthalpy, ΔRRDH. It appears that the Ea,RRD is substantially higher than ΔRRDH. This implies that the opposite reaction (and the tautomer forming step), a radical-radical disproportionation (RD), is an activated process. This is an often ignored or overlooked kinetic feature. The consequence is that although the RRD reaction may be kinetically feasible at elevated temperatures, the products are not the tautomers but rather dimers stemming from radical-radical recombination reactions, with p-isotoluene as a clear exception. It is further shown that the RRD self-reaction of phenalene is too slow at 298 K, despite claims to the contrary.

Comment on "synthesis, characterization, and structures of a persistent aniline radical cation": a new interpretation is necessary.

Further scrutiny for the aniline radical cation: The author of this Correspondence claims that the original spectrometric data for 2,4,6-tri-tert-butylaniline (TBA) reported in 2012 do not correspond to a stoichiometrically pure TBA(.+) SbF6 (-) salt, the quantum-chemical data do not support the reported structural data, and the interpretation of the apparent temperature-dependent structural changes in the solid state is open to interpretations other than the Jahn-Teller effect.

Anthrone and related hydroxyarenes: tautomerization and hydrogen bonding.

The keto-enolization of hydroxyl-substituted naphthols and 9-anthrols has been investigated by means of CBS-QB3 calculations. An excellent agreement between experiment and theory is found for the energetics for the anthrone (5) ⇌ anthrol (6) equilibrium, with an enthalpy of tautomerization, Δ(t)H, of 3.8 kcal mol(-1). In contrast, 1-naphthol is the preferred tautomer with a Δ(t)H = -9.0 kcal mol(-1). Substitution of the hydrogens at the adjacent carbons by hydroxyl groups leads to the formation of strong intramolecular hydrogen bonds within a six-membered ring in the enones and the enols. Due to the difference in the intramolecular hydrogen bond enthalpy, Δ(HB)H(intra), the equilibrium shifts further to the enone. Thus, for 1,8-dihydroxy-anthrone (anthralin, dithranol) Δ(t)H increases to 12.7 kcal mol(-1) with an enol/enone ratio of 10(-10). The solvent effect on the 5 ⇌ 6 equilibrium has been quantified by considering the formation of intermolecular hydrogen bond(s), leading to an acidity parameter α2(H) for anthrol of 0.42. It is shown that the hydrogen bond donating ability of bulk methanol is greatly attenuated through the formation of cyclic oligomers. The benzylic and phenolic bond dissociation enthalpies for anthrone up to anthralin suggest some antioxidant potency but the precise (radical) mechanism of action remains uncertain.

Cooperative self-assembly of discoid dimers: hierarchical formation of nanostructures with a pH switch.

Derivatives of the self-complementary 2-guanidiniocarbonyl pyrrole 5-carboxylate zwitterion (1) (previously reported by us to dimerize to 1•1 with an aggregation constant of ca. >10(10) M(-l) in DMSO) aggregate in a diverse manner depending on, e.g., variation of concentration or its protonation state. The mode of aggregation was analyzed by spectroscopic (NMR, UV) and microscopic (AFM, SEM, HIM, and TEM) methods. Aggregation of dimers of these zwitterions to higher supramolecular structures was achieved by introduction of sec-amide substituents at the 3-position, i.e., at the rearward periphery of the parent binding motif. A butyl amide substituent as in 2b enables the discoid dimers to further aggregate into one-dimensional (rod-like) stacks. Quantitative UV dilution studies showed that this aggregation is strongly cooperative following a nucleation elongation mechanism. The amide hydrogen seems to be essential for this rod-like aggregation, as neither 1 nor a corresponding tert-amide congener 2a form comparable structures. Therefore, a hydrogen bond-assisted π-π-interaction of the dimeric zwitterions is suggested to promote this aggregation mode, which is further affected by the nature of the amide substituent (e.g., steric demand), enabling the formation of bundles of strands or even two-dimensional sheets. By exploiting the zwitterionic nature of the aggregating discoid dimers, a reversible pH switch was realized: dimerization of all compounds is suppressed by protonation of the carboxylate moiety, converting the zwitterions into typical cationic amphiphiles. Accordingly, typical nanostructures like vesicles, tubes, and flat sheets are formed reversibly under acidic conditions, which reassemble into the original rod-like aggregates upon readjustment to neutral pH.

Ascorbic acid reduction of compound I of mammalian catalases proceeds via specific binding to the NADPH binding pocket.

Mammalian (Clade 3) catalases utilize NADPH as a protective cofactor to prevent one-electron reduction of the central reactive intermediate Compound I (Cpd I) to the catalytically inactive Compound II (Cpd II) species by re-reduction of Cpd I to the enzyme's resting state (ferricatalase). It has long been known that ascorbate/ascorbic acid is capable of reducing Cpd I of NADPH-binding catalases to Cpd II, but the mode of this one-electron reduction had hitherto not been explored. We here demonstrate that ascorbate-mediated reduction of Cpd I, generated by addition of peroxoacetic acid to NADPH-free bovine liver catalase (BLC), requires specific binding of the ascorbate anion to the NADPH binding pocket. Ascorbate-mediated Cpd II formation was found to be suppressed by added NADPH in a concentration-dependent manner, for the achievement of complete suppression at a stoichiometric 1:1 NADPH:heme concentration ratio. Cpd I → Cpd II reduction by ascorbate was similarly inhibited by addition of NADH, NADP(+), thio-NADP(+), or NAD(+), though with 0.5-, 0.1-, 0.1-, and 0.01-fold reduced efficiencies, respectively, in agreement with the relative binding affinities of these dinucleotides. Unexpected was the observation that although Cpd II formation is not observed in the presence of NADP(+), the decay of Cpd I is slightly accelerated by ascorbate rather than retarded, leading to direct regeneration of ferricatalase. The experimental findings are supported by molecular mechanics docking computations, which show a similar binding of NADPH, NADP(+), and NADH, but not NAD(+), as found in the X-ray structure of NADPH-loaded human erythrocyte catalase. The computations suggest that two ascorbate molecules may occupy the empty NADPH pocket, preferably binding to the adenine binding site. The biological relevance of these findings is discussed.

Advances in switchable supramolecular nanoassemblies.

Supramolecular nanoassemblies are gaining increasing importance as promising new materials with considerable potential for novel and promising applications. Within supramolecular nanoassemblies the connectivity of the monomeric units is based on reversible noncovalent interactions, like van der Waals interactions, hydrogen bonding, or ionic interactions. As the strength of these interactions depends on the molecular surrounding, the formation of nanoassemblies in principle can be controlled externally by changing the environment and/or the molecular shape of the underlying monomer. This way it is not only possible to switch the self-assembly on or off, but also to change between different aggregation states. In this minireview we present some recent selected approaches to supramolecular stimuli-responsive nanoassemblies.

Pyrene-based fluorescent nitric oxide cheletropic traps (FNOCTs) for the detection of nitric oxide in cell cultures and tissues.

The synthesis and the structural and spectroscopic characterization of nonfluorescent, pyrene-based cyclic o-quinodimethanes are reported. These compounds react efficiently with nitric oxide (NO) in a formal cheletropic manner, by which the fluorescent aromatic pyrene system is regenerated. The NO trapping capabilities and kinetics of the fluorescent nitric oxide cheletropic traps (FNOCTs) are assessed in THF and buffered aqueous solution by ESR, UV/Vis, and fluorescence spectroscopy, by employing NO solutions and NO released from N-diazeniumdiolates (NONOates). Prototypal biological applications include the quantitation of NO production from cultured rat alveolar macrophages and the endothelium of porcine aorta, which demonstrate a sensitivity for NO detection in the nanomolar range.

UVA-induced phenoxyl radical formation: A new cytotoxic principle in photodynamic therapy.

Psoralens are regularly used in therapy in combination with ultraviolet A light irradiation (PUVA) to treat skin diseases such as psoriasis, vitiligo, and mycosis fungoides. PUVA therapy is also used within the scope of extracorporeal photopheresis to treat a variety of diseases that have a suspected involvement of pathogenic T cells, including rejection of organ transplants, graft-vs-host disease, cutaneous T cell lymphoma, and autoimmune disorders. Because psoralens are the only photosensitizers used in PUVA therapies and are considered to be responsible for a number of side effects, the identification of alternative drugs is of practical interest. Here we investigated the impact of activated Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), a hydrophilic vitamin E analog lacking the phytyl tail, as an alternative photoactivatable agent with T cell cytotoxic properties. Despite the well-known antioxidative capacity of Trolox, we found that at low UVA doses and in the presence of supraphysiological concentration of nitrite, a natural constituent of human skin, this compound selectively enhances radical-mediated cytotoxicity toward T cells but not toward human skin fibroblasts, keratinocytes, or endothelial cells. The cytotoxic mechanism comprises a reaction of Trolox with photo-decomposition products of nitrite, which leads to increased Trolox phenoxyl radical formation, increased intracellular oxidative stress, and a consecutive induction of apoptosis and necrosis in fast proliferating T cells. Thus, the identified UVA/nitrite-induced phenoxyl radical formation provides an opportunity for a new cytotoxic photodynamic therapy.

Localization of hydrophobic N-diazeniumdiolates in aqueous micellar solution.

The interaction of phenyl-substituted zwitterionic N-diazeniumdiolates PhCH(2)N[N(O)NO](-)(CH(2))(2)NH(3)(+) (1) and PhCH(2)N[N(O)NO](-)(CH(2))(2)NH(2)(+)CH(2)Ph (2) with aqueous micellar solutions of prototypal surfactants was investigated by means of UV/vis and (1)H NMR spectroscopy in order to establish the localization of hydrophobic N-diazeniumdiolates in micelles as a model for the binding of the NO donors in biological membranes. In the presence of sodium dodecyl sulfate (SDS), significant shifts of the apparent pK(a) values of 1 and 2 were observed, suggesting strong electrostatic interaction between the diazeniumdiolates and the negatively charged SDS micelles. No effect on both pK(a) and rate of NO release was found in the presence of Triton X-100. The solubilization site of micellar bound N-diazeniumdiolates was established by (1)H NMR spectroscopic studies, taking advantage of the spectroscopic effects induced by CH-pi interactions. The spectra indicate that in alkaline solutions of SDS 1 resides preferably at the micellar surface within the interfacial region, whereas the more hydrophobic NO donor 2 penetrates into the apolar region of the micelle. This suggests hydrophobic interaction as the main driving force for micellar binding of 2 in alkaline solution. Similar studies in presence of Triton X-100 indicate that 1 and 2 are adsorbed within the poly(oxyethylene) layer of the micellar surface rather than penetrating the palisade layer of the micelles. In alkaline solutions of hexadecyltrimethylammonium bromide (CTAB), 1 and 2 bind to the cationic micellar aggregates, whereby the solubilization site strongly depends on the hydrophobicity of the substrate. Up to a moderate pH of 8, the hydrophobic NO donor 2 penetrates the hydrocarbon region of the micelles. As a result, the rate of NO release from 2 is noticeably inhibited by the micellar aggregates due to the higher local concentration of hydroxide ions along the micelle-water interface. From solubilization studies, guidelines for the development and application of future NONOates can be derived. The rate of NO release from micellar bound diazeniumdiolates is determined by the surface charge of the micelles. This ability to tune stability is significant for the design and selection of potential NO delivery systems (drug formulations).