Unambiguous evidence for the participation of singlet oxygen ( 1 ) in photodynamic oxidation of amino acids

Abstract— A variety of experimental tests have been applied to the methylene-blue-sensitized photooxidation of amino acids to distinguish between singlet oxygen and non-singlet oxidation mechanisms. Conventional flash photolysis and laser photolysis were used to measure the rate constants for the quenching of excited triplet sensitizer and singlet oxygen by the amino acids histidine. tryptophan and methionine and the nucleotide guanosine-5′-monophosphate. In the case of histidine, the rate constants alone rule out an oxidation mechanism involving direct reaction with excited dye. With the other amino acids, and with guanosine monophosphate, the oxidation rates might be accounted for by either mechanism. The inhibition of the photo-oxidation of both tryptophan and methionine as well as histidine by the singlet-oxygen quenchers N₃^? and tetramethylethylene suggests that these reactions occur via a singlet-oxygen mechanism. A newly developed test of singlet oxygen reactions involving a comparison of photooxidation rates in normal and perdeuterated solvents has been used to establish that the photooxidation of tryptophan proceeds primarily by a singlet-oxygen mechanism. These experiments appear to constitute the first proof that singlet oxygen is involved in the photooxidation of the three amino acids tryptophan, methionine and histidine.     

Antioxidative properties of nitroxyl radicals and hydroxyamines in reactions with triplet and deaminated kynurenine

The reactions of triplet kynurenine and 4-(2-aminophenyl)-4-oxocrotonic acid, formed upon the thermal decomposition of kynurenine, with nitroxyl radicals and cyclic N-hydroxylamines were studied. Nitroxyl radicals were found to quench efficiently the triplet state of kynurenine (rate constant 3–6·.10⁸ L mol^-1 s^-1). The quenching proceeds via the spin-exchange mechanism and affords no new products. Neither nitroxyl radicals, nor hydroxyl-lamines react with 4-(2-aminophenyl)-4-oxocrotonic acid under conditions similar to physiological.     

Nuclear‐Spin Polarization in Electron‐Transfer Reactions of Amines

Chemically induced dynamic nuclear polarization (CIDNP) observed during electron transfer (ET) reactions of tertiary amines such as DABCO ( 1 ) or Et₃N ( 2 ) with a wide range of electron acceptors support the involvement of amine radical‐cations (e.g., 1^{. +} or 2^{. +} ) as key intermediates. Radical ions such as 2^{. +} may be deprotonated, generating neutral aminoalkyl radicals (e.g., 2^. ). When generated by reaction with an electron acceptor of energetically low triplet state such as naphthalene (¹Naph*), the resulting pair 2^{. +} /Naph^.? reacts mostly by reverse electron transfer (RET) from triplet pairs populating the naphthalene triplet state.     

Theoretical studies of electron and hydrogen transfer reactions between semiquinone radicals and oxygen

 To explore the interactions between ubiquinones and oxygen in living organisms, the thermodynamics of a series of electron and hydrogen transfer reactions between semiquinone radicals, as well as their corresponding protonated forms, and oxygen, singlet or triplet, were studied using the hybrid Hartree–Fock–density functional theory method Becke's three parameter hybrid method with the Lee, Yang, and Parr correlation functional. Effects of the solvent and of the isoprenyl tail on the electron and hydrogen transfer reactions were also investigated. It is found that semiquinone radicals (semiquinone anion radicals or protonated semiquinone radicals) cannot react with triplet oxygen to form the superoxide anion radical O₂ ⁻. In contrast, neutral quinones can scavenge O₂ ⁻ efficiently. In the gas phase, only protonated semiquinone radicals can react spontaneously with singlet oxygen to produce peroxyl radical (HO₂). However, both semiquinone anion radicals and protonated semiquinone radicals can react with singlet oxygen to produce harmful oxygen radicals (O₂ ^{− a l l b u l l} and HO₂, respectively) in aqueous and protein environments. The free-energy changes of the corresponding reactions obtained for different ubiquinone systems are very similar. It clearly shows that the isoprenyl tail does not influence the electron and hydrogen transfer reactions between semiquinone radicals and oxygen significantly. Results of electron affinities, vertical ionization potentials, and proton affinities also show that the isoprenyl tail has no substantial effect on the electronic properties of ubiquinones. Received: 3 July 2000 / Accepted: 6 September 2000 / Published online: 21 December 2000     

Ion-Pair LC Analysis of Pyrroloquinoline Quinone in Neurotransmitter Amino Acid Incubations: Determination of Chemical Kinetics

Neurotransmitters are the chemical messengers of the brain. Many neurodegenerative diseases of the central nervous system are related to abnormal neurotransmitter activity. Pyrroloquinoline quinine (PQQ) has previously been shown to be a promising candidate for preventing cognitive deficit in neurodegeneration. To investigate whether PQQ can modulate the levels of brain neurotransmitter amino acids, a rapid and reliable ion-pair liquid chromatographic method was established and validated for the analysis of PQQ in reaction mixtures containing specific neurotransmitter amino acids. The reaction mixtures were separated on an amethyst C18-P reverse-phase column with 35:65 (v/v) acetonitrile:20 mM potassium dihydrogen phosphate, pH 5.5, containing 20 mM tetrabutyl ammonium bromide as mobile phase at a flow rate of 0.8 mL min⁻¹. The validated method was applied successfully to study the chemical kinetics of PQQ reactions with five neurotransmitter amino acids. Order of reaction n, rate constant k, and activation energy E _a values for the reactions were calculated. This work provides important information for studying the possible protective mechanisms of PQQ in neurodegenerative diseases. Furthermore, the simplicity of this method combined with its sensitivity and reliability make it a novel contribution in the field of neurotransmitter research.

     

Reactions of gas phase amino acids with the prevailing radicals in biomass thermal treatments

Amino acids, N-containing compounds, hold a significant importance in various field. Within the biomass energy sector, amino acids constitute a large fraction of the biomass's nitrogen content. As such, it is essential to comprehend their combustion chemistry; most specifically their biomolecular interactions with governing radicals in the pyrolytic and combustion media that prevail during thermal utilization of biomass. Herein, we have employed quantum chemical calculations and reaction rate theory to investigate reactions of a selected set of amino acids with H, CH₃, NH₂, OH, HO₂, and HS radicals. Thermo-kinetic calculations have been performed to determine the rates of hydrogen abstraction by these six radicals across all possible reaction channels for three specific amino acids: alanine, cysteine, and methionine. The investigation of other amino acids like glycine, threonine, and other models have been carried out for α-C positions as the most probable abstractable sites. The study also examines the individual effects of different substituents (COOH, NH₂, HS, and CH₂) and uncovers significant insights. Notably, the presence of the COOH group introduces polar effects that counterintuitively deactivate the thermodynamically favored α-abstraction pathway. Presented thermo-kinetic values are anticipated to complement existing biomass kinetic models and to improve current understanding of chemical events that participate in the complex nitrogen transformation reactions in biomass.     

Theoretical study on photooxidation mechanism of ruthenium complex [Ru(II)‐(bpy)2(TMBiimH2)]2+ with molecular oxygen

Photoinduced reactions of ruthenium complexes with molecular oxygen have attracted a lot of experimental attention; however, the reaction mechanism remains elusive. In this work, we have used the density functional theory method to scrutinize the visible‐light induced photooxidation mechanism of the ruthenium complex [Ru(II)‐(bpy)₂(TMBiimH₂)]²⁺ (bpy: 2, 2‐bipyridine and TMBiimH₂: 4, 5, 4, 5‐tetramethyl‐2, 2‐biimidazole) initiated by the attack of molecular oxygen. The present computational results not only explain very well recent experiments, also provide new mechanistic insights. We found that: (1) the triplet energy transfer process between the triplet molecular oxygen and the metal‐ligand charge transfer triplet state of the ruthenium complex, which leads to singlet molecular oxygen, is thermodynamically favorable; (2) the singlet oxygen addition process to the S₀ ruthenium complex is facile in energy; (3) the chemical transformation from endoperoxide to epidioxetane intermediates can be either two‐ or one‐step reaction (the latter is energetically favored). These findings contribute important mechanistic information to photooxidation reactions of ruthenium complexes with molecular oxygen. © 2016 Wiley Periodicals, Inc.     

Characterization of transient species in laser photolysis of aromatic amino acids using acetone as photosensitizer

The photochemical processes of aromatic amino acids were investigated in aqueous solution using acetone as photosensitizer by KrF (248 nm) laser flash photolysis. Laser-induced transient species were characterized according to kinetic analysis and quenching experiments. The intermediates recorded were assigned to the excited triplet state of tryptophan, the radicals of tryptophan and tyrosine. The excited triplet state of tryptophan produced via a triplet-triplet excitation transfer and the radicals arising from electron transfer reaction has been identified. Neither electron transfer nor energy transfer between triplet acetone and phenylalanine can occur in photolysis of phenylalanine aqueous solution which contains acetone. Furthermore, triplet acetone-induced radical transformation: Trp/N-Tyr→Trp-Tyr/O was observed directly in photolysis of dipeptide (Trp-Tyr) aqueous solution containing acetone, and the transformation resulting from intramolecular electron transfer was suggested.     

A laser flash photolysis study of amino acids and dipeptides using 4-nitroquinoline 1-oxide as a photosensitizer: The pH dependence

The pH effects on the photochemical reaction of amino acids and related dipeptides with 4-nitroquinoline 1-oxide (4NQO) as a photosensitizer have been investigated by laser flash photolysis. The obtained kinetic parameters show that the electron transfer from Tryptophan (Trp), Tyrosine (Tyr) as well as dipeptides containing Trp and/or Tyr residue to triplet 4NQO (^T4NQO) are efficient, but inefficient from methionine (Met) and dipeptides containing neither Trp nor Tyr. The result was supported by the calculated values of the free energy change from measured oxidation potentials for the electron transfer. It was demonstrated that Trp and Tyr residues are initial reaction sites with ^T4NQO, while Tyr/O^? radical may be final species for Trp-Tyr dipeptide. In acidic aqueous solutions, the self-quenching rate constants of ^T4NQO and the rate constants of electron transfer from amino acids to ^T4NQO decrease with decreasing pH. In alkaline solutions, amino acids are easily oxidized by 4NQO under irradiation of laser pulse, and no transient absorption signal was observed.     

Immunological Detection of N‐formylkynurenine in Porphyrin‐Mediated Photooxided Lens α‐crystallin

Crystallin proteins are responsible for maintaining lens transparency and allowing the lens to focus light undistorted onto the retina. The α‐crystallins are the major lens crystallins, and function as both structural proteins and chaperones to protect all lens proteins from damage leading to lens deterioration. Because lens crystallin proteins do not turn over, the damage they accumulate can lead to cataracts, the world’s leading cause of blindness. Photosensitizing porphyrins can accumulate in the eye through either endogenous metabolism or through therapeutic or diagnostic procedures. Porphyrin buildup exacerbates lens aging through increased levels of singlet oxygen, resulting in protein polymerization and amino acid residue alteration. Tryptophans oxidize to kynurenine and N‐formylkynurenine (NFK) causing irreversible changes in the refractive index of the normally transparent lens, leading to development of cataracts. Additionally, NFK is itself a photosensitizer, and its presence exacerbates lens deterioration. This work uses anti‐NFK antiserum to study porphyrin‐facilitated photooxidation of α‐crystallin tryptophan residues. In vitro experiments show that four biologically interesting porphyrins mediate α‐crystallin polymerization and accumulation of both protein radicals and NFK. Confocal microscopy of cultured human lens epithelial cells indicates that while all four porphyrins photosensitize cellular proteins, not all oxidize the tryptophans of cellular α‐crystallin to NFK.     

Complex Formation Reactions of Divinyltin(IV) Complexes with Amino Acids,Peptides, Dicarboxylic acids and Related Compounds

Complex formation equilibria of divinyltin(IV) with amino acids, peptides, and dicarboxylic acids have been investigated. Stoichiometry and stability constants for the complexes formed were determined at 25°C and ionic strength 0.1 M NaNO₃. The results showed the formation of ML, MLH, and ML₂ (organotin : ligand : hydrogen) complexes with amino acids. Peptides form ML complexes and the corresponding deprotonated amide species MLH_?1. In the latter species the binding with divinyltin(IV) occurs through the terminal amino group, carboxylate oxygen, and the amide nitrogen atoms (CO^? ₂, N^? _amide, NH₂). The results showed the formation of ML and ML₂ complexes with dicarboxylic acids. The concentration distribution of the complexes in solution was evaluated. The bonding sites of the divinyltin(IV) complex in solid state with oxalic acid was investigated by means of elemental analyses, FTIR, and mass spectra. Non-isothermal decomposition of the above complex has been studied and the result was statistically analyzed. The main steps were identified for the thermal decomposition reaction and each step proved to be a first order reaction. The kinetic parameters E _a and A were calculated for each step in the reaction. The thermodynamic functions H, G, and S* were calculated for each step of the reaction.     

Construction of Cyclobutanes by Multicomponent Cascade Reactions in Homogeneous Solution through Visible-Light Catalysis

[2+2] Photocycloaddition of two olefins is a general method to assemble the core scaffold, cyclobutane, found in numerous bioactive molecules. A new approach to synthesize cyclobutanes through multicomponent cascade reactions by merging aldol reaction and Witting reaction with visible-light-induced [2+2] cycloaddition is reported. An array of cyclobutanes with high selectivity has been achieved from commercially available aldehydes, ketones (or phosphorus ylide), and olefins with visible-light irradiation of a catalytic amount of (fac-tris(2-phenylpyridinato-C₂,N)iridium) ([Ir(ppy)₃]) at room temperature. Control experiments and spectroscopic studies revealed that the triplet–triplet energy transfer from the excited [Ir(ppy)₃]* to enones, generated in situ from aldehyde and ketone or aldehyde and phosphorus ylide, is responsible for these simple and efficient muticomponent transformations.     

Mechanisms of reactions of flavin mononucleotide triplet with aromatic amino acids

Chemical reactions between the photoexcited triplet state of flavin mononucleotide and the aromatic amino acids, N-acetyl tryptophan (TrpH), N-acetyl tyrosine (TyrOH), and N-acetyl histidine (HisH) in aqueous solution have been studied in the pH range 2-12. Across the whole pH range, the principal mechanism of reaction of both TrpH and TyrOH is shown to be electron transfer. For HisH, the mechanism and rate of the reaction depend on the protonation state of the reactants. In acidic conditions (pH < 4), reaction does not occur. At 4 < pH < 11, the reaction proceeds via hydrogen atom abstraction with a rate constant varying from 3.0 x 10(6) to 2.5 x 10(8) M(-1) s(-1). In extremely basic solution (pH > 12) the mechanism switches to electron transfer.     

Synthesis,Characterization, Photophysics and Photochemistry of Pyrylogen Electron Transfer Sensitizers

A series of new dicationic sensitizers that are hybrids of pyrylium salts and viologens has been synthesized. The electrochemical and photophysical properties of these “pyrylogen” sensitizers are reported in sufficient detail to allow rationale design of new photoinduced electron transfer reactions. The range of their reduction potentials (+0.37–+0.05 V vs SCE) coupled with their range of singlet (48–63 kcal mol^?1) and triplet (48–57 kcal mol^?1) energies demonstrate that they are potent oxidizing agents in both their singlet and triplet excited states, thermodynamically capable of oxidizing substrates with oxidation potentials as high as 3.1 eV. The pyrylogens are synthesized in three steps from readily available starting materials in modest overall 11.4–22.3% yields. These sensitizers have the added advantages that: (1) their radical cations do not react on the CV timescale with oxygen bypassing the need to run reactions under nitrogen or argon and (2) have long wavelength absorptions between 413 and 523 nm well out of the range where competitive absorbance by most substrates would cause a problem. These new sensitizers do react with water requiring special precautions to operate in a dry reaction environment.     

Systematic study of the transfer of amino acids across the water/l,2-dichloroethane interface facilitated by dibenzo-18-crown-6

Failitated ion transfer reactions of 20 amino acids with dibenzo-18-crown-6 (DB18C6) at the water/1,2-dichloroethane (W/DCE) interfaces supported at the tips of micro- and nano-pipets were investigated systematically using cyclic voltammetry. It was found that there were only 10 amino acids, that is, Leu, Val, lle, Phe, Trp, Met, Ala, Gly, Cys, Gln (in brief), whose protonated forms as cations can give well-defined facilitated ion transfer voltammograms within the potential window, and the reaction pathway was proven to be consistent with the transfer by interfacial complexation/dissociation (TIC/TID) mechanisms. The association constants of DB 18C6 with different amino acids in the DCE (β°), and the kinetic parameters of reaction were evaluated based on the steady-state voltammetry of micro- or nano-pipets, respectively. The experimental results demonstrated that the selectivity of complexation of protonated amino acid by DB18C6 compared with that of alkali metal cations was low, which can be attributed to the vicinal effect arising from steric hindrance introduced by their side group and the steric bulk effect by lipophilic stabilization. Moreover, the association constants and the standard rate constants for different amino acids showed good correlations with their hydrophobicity (π), except Gly and Met, which inferred that the selectivity of such heterogeneous complex reaction for different amino acids with DB18C6, was not only affected by discrimination in binding these ions to the crown ether macro-cycle, but also significantly modified by the ion transfer Gibbs energy which was closely related to the structure of the transferred ions, protonated amino acids.     

Photochemische Reaktionen. 52. Mitteilung [1]. Zur Photochemie von α,β-ungesättigten cyclischen Ketonen: Spezifische Reaktionen der n,π*- und π,π*-Triplettzustände von O-Acetyl-testosteron und 10-Methyl-Δ1,9-octalon-(2)

The photochemistry of the conjugated cyclohexenones O-acetyl testosterone ( 1 ) and 10-methyl-Δ^1,9-octalone-(2) ( 24 ) has been investigated in detail. The choice of reaction paths of both ketones depends strongly on the solvent used. In t-butanol, a photostationary equilibrium 1 ? 3 is reached which is depleted solely by the parallel rearrangement 1 → 5 (Chart 1; for earlier results on these reactions see [2a] [6] [7]). In benzene, double bond shift 1 → 16 (Chart 3) occurs instead, which is due to hydrogen abstraction from a ground-state ketone by the oxygen of an excited ketone as the primary photochemical process. In toluene, the major reaction is solvent incorporation ( 1 → 17 , Chart 4) through hydrogen addition to the β-carbon of the enone, accompanied by double bond shift and formation of saturated dihydroketone as the minor reactions. Contrary in part to an earlier report [19], the photochemical transformation of the bicyclic enoné 24 exhibit a similar solvent dependence. The corresponding products 25 – 29 are summarized in Chart 5 and Table 1. Sensitization and quenching experiments established the triplet nature of the above reactions of 1 and 24 . Based on STERN -VOLMER analyses of the quenching data (cf. Figures 2, 4–8, and Table 3), rearrangement, double bond reduction and toluene addition are attributed to one triplet state of the enones which is assigned tentatively as ³(π, π*) state, and the double bond shift is attributed to another triplet assigned as ³(n, π*) state (cf. Figure 9). The stereospecific rearrangement of the 1α-deuterated ketone 2 to the 4β-deuterio isomer 4 shows the reaction to proceed with retention at C-1 and inversion at C-10. The 4-substituted testosterone derivatives 33 – 36 (Chart 8) were found to be much less reactive in general than 1 . In particular, 4-methyl ketone 33 remains essentially unchanged on irradiation in t-butanol, benzene and toluene.     

Chiral DMAP‐N‐oxides as Acyl Transfer Catalysts: Design,Synthesis, and Application in Asymmetric Steglich Rearrangement

A DMAP‐N‐oxide, featuring an α‐amino acid as the chiral source, was developed, synthesized and applied in asymmetric Steglich rearrangement. A series of O‐acylated azlactones afforded C‐acylated azlactones possessing a quaternary stereocenter in high yields (up to 97 % yield) and excellent enantioselectivities (up to 97 % ee). Compared to the widespread use of pyridine nitrogen, which serves as the nucleophilic site in the asymmetric acyl transfer reaction, we discovered that chiral DMAP‐N‐oxides, in which the oxygen now acts as the nucleophilic site, are efficient acyl transfer catalysts. Our finding might open a new door for the development of chiral DMAP‐N‐oxides for asymmetric acyl transfer reactions.     

Oxygen Transfer from Inorganic and Organic Peroxides to Organic Substrates: A Common Mechanism?

In this progress report an attempt is made to rationalize, from a mechanistic point of view, the different ways in which oxygen is transferred from inorganic and organic peroxides to nucleophilic substrates, particularly olefins. Oxygen transfer from transition-metal peroxides, which is relevant to catalytic oxidations using O₂, H₂O₂ or ROOH, occurs via a cyclic or “pseudocyclic” peroxymetalation in which a dioxametallacycle is formed. Owing to the wide discrepancy between peroxymetalation and the conventional oxidation mechanism, i.e. nucleophilic attack of the substrate at the electrophilic “active oxygen”, we propose an alternative mechanism involving dioxiranes as the reactive species. The generation of dioxiranes appears to be a common denominator in the reactions of most organic peroxides e.g. peroxy acids, the reaction of electrophilic ketones with H₂O₂, or ozonizations. Oxygen transfer from dioxirane reagents probably involves the formation of a charge-transfer π-complex between the substrate and the carbon atom of the dioxirane, and the subsequent formation of a cyclic peroxidic intermediate.     

Chiral DMAP‐N‐oxides as Acyl Transfer Catalysts: Design,Synthesis, and Application in Asymmetric Steglich Rearrangement

A DMAP‐N‐oxide, featuring an α‐amino acid as the chiral source, was developed, synthesized and applied in asymmetric Steglich rearrangement. A series of O‐acylated azlactones afforded C‐acylated azlactones possessing a quaternary stereocenter in high yields (up to 97 % yield) and excellent enantioselectivities (up to 97 % ee). Compared to the widespread use of pyridine nitrogen, which serves as the nucleophilic site in the asymmetric acyl transfer reaction, we discovered that chiral DMAP‐N‐oxides, in which the oxygen now acts as the nucleophilic site, are efficient acyl transfer catalysts. Our finding might open a new door for the development of chiral DMAP‐N‐oxides for asymmetric acyl transfer reactions.