Hydroxyl mechanism of the antimalarial effect of artemisinin and its analogs

Kinetic schemes for the intramolecular oxidation of four artemisinin analogs, which are used as drugs against malaria, were developed. Each stage of the kinetic scheme is characterized by the enthalpy, activation energy, and rate constant calculated using the model of intersecting parabolas. The competition of mono- and bimolecular radical reactions was taken into account when developing the schemes. The hydroperoxide groups are formed as a result of the intramolecular oxidation of these compounds and generate free radicals in the reaction with Fe^II. Among these free radicals, hydroxyl radicals play the key role, since their yield (n _OH) correlates with the antimalarial activity of the peroxide compound. The efficiency of the drug (index IC₅₀) exponentially depends on n _OH and is expressed by the formula IC₅₀(Artemisinin)/IC₅₀(Compound) = 1.54·10⁻⁶exp(3.9n _OH). The elementary reactions resulting in the generation of hydroxyl radicals are considered. It is supposed that DNA of a malaria parasite is the main biological target for hydroxyl radicals.     

Generation of hydroxyl radicals by the intramolecular oxidation of tricyclic artemisinin analogs and their antimalarial activity

The kinetic schemes were constructed for the intramolecular oxidation of four tricyclic artemisinin derivatives differed in number and arrangement of the methyl groups. Each step of the scheme was characterized by the enthalpy. The activation energies and rate constants were calculated by using the intersecting parabolas model. Three of the four tricyclic derivatives were found to undergo intramolecular oxidation, and the hydroperoxide groups formed generate free radicals. Owing to this, the compounds possess antimalarial activity. The fourth compound is not substantially oxidized due to certain specific features of its structure and exhibits no antimalarial activity. The latter correlates with the number of hydroxyl radicals generated by the compound (n _OH). The dep endence of the IC₅₀ index on n _OH is nonlinear. Three elementary reactions leading to the generation of reactive hydroxyl radicals were identified.     

Iron(II)-induced degradation of antimalarial beta-sulfonyl endoperoxides: evidence for the generation of potentially cytotoxic carbocations

Reactions of antimalarial beta-sulfonyl endoperoxides 9 and 10, which, like yingzhaosu A (2), derive from the 2,3-dioxabicyclo[3.3.1]nonane system 3, with iron(II) salts were studied. Product analysis of the iron(II)-induced degradations provided evidence for the intermediacy of carbon-centered cyclohexyl radicals 20 and 31 and their possible oxidation to the corresponding carbocations 21 and 32. It is conceivable that the antimalarial activity of beta-sulfonyl endoperoxides of type 5 may derive from alkylation of vital intraparasitic biomolecules by free radicals and/or carbocations, generated within the malaria parasite through a similar iron(II)-induced degradation process.     

4-N,N-Dimethylamino-4'-Isopropylbenzophenone as polymerization photoinitiator. Effect of solvent and photoinitiator concentration on its photoreactivity and on the polymerization process

Several kinetics aspects of the methyl methacrylate (MMA) polymerization using 4-dimethylamino-4'-isopropylbenzophenone (PI) as photoinitiator have been studied. The order of the polymerization reaction with respect to monomer and initiator concentrations have been investigated, as well as the polymerization behavior under well-stirred and unstirred conditions; values of initiation quantum yield (?_i) and k_p/k_t^1/2 have also been determined. It has been found that the nature of the polymerization-initiating radicals depends on the type of solvent and the photoinitiator concentration ([PI]). In cyclohexane solution and at low [PI] (< 5 x 10^-5M), the cyclohexyl radical is practically the only polymerization initiating radical, while at higher [PI] both radicals, cyclohexyl and the aminoalkyl derived from PI, participate in the initiation step, increasing the participation of the later as the [PI] increases. When benzene is used as solvent both phenyl and aminoalkyl radicals participate in the initiation step at any [PI] employed. Efficiencies of the radicals derived from solvent and photoinitiator have been determined.     

Rate constants for the reactions of isobutyl,neopentyl, cyclopentyl,and cyclohexyl radicals with molecular oxygen

Rate constants have been measured for the reactions of four hydrocarbon radicals with O₂ in the gas phase at room temperature. Laserflash photolysis was used to generate low concentrations of radicals. A photoinization mass spectrometer followed the radical loss as a function of time. The measured pseudo first-order decay rate of the radical and the absolute oxygen concentration were combined to give the absolute rate constants (in units of 10^?12 cm³ molec^?1 s^?1): isobutyl (2.9 ± 0.7); neopentyl (1.6 ± 0.3); cyclopentyl (17 ± 3); and cyclohexyl (14 ± 2). The cycloalkyl radicals have rate constants similar to those of other secondary radicals. However, the isobutyl and neopentyl radicals react more slowly than similar primary radicals. These new rate constants are compared in Figure 2 with the recently published correlation of reactive cross section with radical ionization potential.     

Generation of radicals and antimalarial activity of dispiro-1,2,4-trioxolanes

The kinetic schemes of the intramolecular oxidation of radicals generated from substituted dispiro-1,2,4-trioxolanes (seven compounds) in the presence of Fe²⁺ and oxygen were built. Each radical reaction was defined in terms of enthalpy, activation energy, and rate constant. The kinetic characteristics were calculated by the intersecting parabolas method. The competition between the radical reactions was considered. The entry of radicals generated by each compound into the volume was calculated. High antimalarial activity was found for 1,2,4-trioxolanes, which generated hydroxyl radicals. The structural features of trioxolanes responsible for the generation of hydroxyl radicals were determined.     

Formation of alkyl nitrates from the reaction of branched and cyclic alkyl peroxy radicals with NO

The yields of C₅ and C₆ alkyl nitrates from neopentane, 2-methylbutane, 2-methylpentane, 3-methylpentane, and cyclohexane have been measured in irradiated CH₃ONONO-alkane-air mixtures at 298 ± 2 K and 735-torr total pressure. Additionally, OH radical rate constants for neopentyl nitrate, 3-nitro-2-methylbutane, 2-nitro-2-methylpentane, 2-nitro-3-methylpentane, and cyclohexyl nitrate, relative to that for n-butane, have been determined at 298 ± 2 K. Using a rate constant for the reaction of OH radicals with n-butane of 2.58 × 10^?12 cm³ molecule^?1 s^?1, these OH radical rate constants are (in units of 10^?12 cm³ molecule^?1 s^?1): neopentyl nitrate, 0.87 ± 0.21; cyclohexyl nitrate, 3.35 ± 0.36; 3-nitro-2-methylbutane, 1.75 ± 0.06; 2-nitro-2-methylpentane, 1.75 ± 0.22; and 2-nitro-3-methylpentane, 3.07 ± 0.08. After accounting for consumption of the alkyl nitrates by OH radical reaction and for the yields of the individual alkyl peroxy radicals formed in the reaction of OH radicals with the alkanes studied, the alkyl nitrate yields (which reflect the fraction of the individual RO₂ radicals reacting with NO to form RONO₂) determined were: neopentyl nitrate, 0.0513 ± 0.0053; cyclohexyl nitrate, 0.160 ± 0.015; 3-nitro-2-methylbutane, 0.109 ± 0.003; 2-nitro-2methylbutane, 0.0533 ± 0.0022; 2-nitro-2-methylpentane, 0.0350 ± 0.0096; 3- + 4-nitro-2-methylpentane, 0.165 ± 0.016; and 2-nitro-3-methylpentane, 0.140 ± 0.014. These results are discussed and compared with previous literature values for the alkyl nitrates formed from primary and secondary alkyl peroxy radicals generated from a series of n-alkanes.     

Absolute rate constants for the reactions between amino and alkyl radicals at 298 K

The absolute rate constants for the reactions of NH₂ radicals with ethyl, isopropyl, and t-butyl radicals have been measured at 298 K, using a flash photolysis–laser resonance absorption method. Radicals were generated by flashing ammonia in the presence of an olefin. A new measurement of the NH₂ extinction coefficient and oscillator strength at 597.73 nm was performed. The decay curves were simulated by adjusting the rate constants of both the reaction of NH₂ with the alkyl radical and the mutual interactions of alkyl radicals. The results are k(NH₂ + alkyl) = 2.5 (±0.5), 2.0 (±0.4), and 2.5 (±0.5) × 10¹⁰ M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively. The best simulations were obtained when taking k(alkyl + alkyl) = 1.2, 0.6, and 0.65 × 10¹⁰M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively, in good agreement with literature values.     

“Complex” versus “free radical” mechanism for the catalytic decomposition of H2O2 by ferric ions

Kinetic and spectrophotometric measurements made during the Fe³⁺ ion catalyzed decomposition of H₂O₂ have been analyzed using the computer simulation method. Improved values of the rate constants of the “complex scheme” and of the molar absorptivities ofthe intermediates were obtained: k₃/K_M = 4.94 M^?1 min^?1, k₄ = 193 M^?1 min^?1, ε_I/K_M = 52.3 M^?2 cm^?1, ε_II = 25.7 M^?1 cm^?1. The simulation revealed details of the reaction which were unavailable by other means and which explained several features of its kinetics. The total amount of O₂ evolved in the reaction using [H₂O₂] ～ 10^?2 M has been calculated and found to be nearly stoichiometric. O₂ evolution experiments in this region cannot, thus, distinguish between the “complex mechanism” predicting nearly stoichiometric evolution of O₂ and the “free radical mechanism” predicting exactly stoichiometricamounts of O₂. There are discrepancies within the “free radical scheme” with regard to the correct values of the rate constants to fit the reactions of H₂O₂ both with Fe²⁺ and Fe³⁺ ions, as well as other reactions assumed to proceed via free radicals.     

Combination and disproportionation reactions of allylic radicals with ethyl,propyl, and t-Butyl Radicals

1-Methylallyl, 1,1-dimethylallyl, 1,2-dimethylallyl, 1,3-dimethylallyl, 1,1,2-trimethylallyl, and 1-ethylallyl radicals have been generated in the gas phase at 20 ± 1°C by addition of H atoms, formed by Hg(6³P₁) photosensitization of H₂, to appropriate dienes. Their combination reactions with ethyl radicals have been studied and the relative reactivities of the reaction centers in each allylic radical determined. Similar measurements have been made for some combination reactions of n-propyl, i-propyl, and t-butyl with 1-methylallyl and 1,1,2-trimethylallyl radicals. The more substituted reaction centers are found to be the less reactive. In addition the self-combination and disproportionation of 1-methylallyl radicals has been investigated, as has cross disproportionation of each allylic radical with ethyl. The results establish a general pattern of reactivity for these radicals, which is interpreted primarily in terms of the effects of steric interaction during reaction.     

Matrix effects in fast atom bombardment mass spectrometry of cationic iridium(III) and rhodium(III) coordination complexes

Fast atom bombardment mass spectra of cationic iridium(III) and rhodium(III) coordination complexes (M⁺Cl₂L₂, X^?; where the ligand L is a dinitrogenous aromatic system) have been obtained with thioglycerol, glycerol or tetraglyme as a matrix. Two kinds of reactions, initiated by particle bombardment, have been discovered between these complexes and the matrix. First, with thioglycerol one or two chlorine atoms are substituted by a thioglycerol radical, more rapidly for rhodium compounds; secondly, when the ligand L possesses a diazo function, this function is hydrogenated depending on the ability of the matrix to generate hydrogen radicals by bombardment.     

Bimolecular self-reactions of 2-arylindandione- 1,3-yl radicals studied by flash photolysis

Kinetic and thermodynamic data for reaction (1) of certain C-centered aromatic radicals (referred to in this paper by the numbers I to X) in chlorobenzene: have been obtained. The k₁ values of radicals varied between (1.1 ± 0.2) × 10⁶M^?1·sec^?1 (radical VIII) and (3.6 ± 0.7) × 10⁹M^?1 sec^?1 (radical VI) at 20°C. An investigation of the relationship between the recombination rates of radicals I–VIII and X and the solvent viscosity (mixture of toluene and dibutylphthalate, 0.6 < η < 18.4 cP) has shown that the recombination reactions involving radicals I–IV are limited by diffusion in solvents having a viscosity η> 10 cP and are activation reactions in solvents having a viscosity η < 10 cP. The recombination of radicals VIII and IX is an activation reaction, while that of radicals V–VII is diffusion-controlled in the entire viscosity range. The recombination of radical X is limited, in the viscosity range of 18.4 to 2 cP, by intrusion into the first coordination sphere of the partner, the effect of viscosity on the radical X recombination rate in the specified range being the same as its effect on diffusion-controlled reactions. The possible reasons of the discrepancies between the experimental fast recombination rate constants and the theoretical values calculated by the Debye–Smoluchowski theory are discussed. The equilibrium constant depends strongly on the nature of the substituent in the phenyl fragment: the substituents which increase unpaired electron delocalization in the radical intensify the dissociation of the respective dimer. Long-wave absorption bands have been recorded for radicals I–X and their extinction coefficients obtained. Dimers I–V are thermo- and photochromic compounds.     

The influence of monomer chemistry on radical formation and secondary reactions during electron-beam polymerization

The vast majority of industrial electron-beam (EB) polymerizations are initiated via radical mechanism. Because radicals drive EB polymerization, understanding their formation and secondary reactions can provide insight into polymerization kinetics and property development. Primary and initiating radicals were quantified by measuring G(R^•) and G(M^•) , respectively, for various acrylate and methacrylate monomers. Monomer chemistry was shown to impact primary radical formation; however, increased primary radical concentration did not necessarily correlate to increased conversion. Despite exhibiting high values of G(R^•) , methacrylates achieved very little conversion and had G(M^•) values near zero. Acrylates achieved much higher G(M^•) values and rates of polymerization compared to their methacrylate counterparts. Additionally, the efficiency of primary radicals converting to initiating radicals, f(M^•) , for each acrylate monomer was shown to be a good predictor of the amount of gel fraction formed during polymerization. Understanding radical formation and secondary radical reactions can help guide the structure/processing conditions/properties relationships that are currently underdeveloped for EB reactions.     

Rates of addition of tert-butyl and pivaloyl radicals to acrylonitrile measured by modulated ESR and μSR spectroscopy

For the rate constant of addition of tert-butyl radicals to acrylonitrile at T = 300 K in solution modulated ESR spectroscopy and muon spin rotation yield 10⁶ M^?1 s^?1 and 2.4 × 10⁶ M^?1 s^?1. The addition of pivaloyl radical to acrylonitrile proceeds with Arrhenius parameters log A/M^?1 s^?1 = 7.7 and E_a = 11.5 kJ/ mol. The results are discussed in terms of polar effects in radical addition reactions.     

Trapping of photogenerated group IV radicals by TEMPO: Potential new organometallic initiators for “living” free radical polymerization

A series of trialkyl and triaryl organometallic radicals from group IV generated by hydrogen abstraction by tert‐butoxyl radical from the parent hydrides have been examined using laser flash photolysis. The rate constants for the trapping of the metal‐centered radicals by the persistent radical TEMPO were measured and were found to be large and similar to those of the carbon‐centered radical systems, yet below the diffusion controlled limit. The metal‐centered radicals were found to be efficiently trapped by TEMPO and would appear to be candidates suitable for “living” free radical polymerization similar to carbon analogue stoichiometric initiators. The radical trapping rate constants for the trialkyl series (M = Si, Ge, Sn) were found to be 8.9 × 10⁸ M⁻¹ s⁻¹ (M = Si), 7.2 × 10⁸ M⁻¹ s⁻¹ (M = Ge), and 6.2 × 10⁸ M⁻¹ s⁻¹ (M = Sn), respectively. The triaryl (Ph₃M•) series gave slightly slower rates of 1.6 × 10⁸ M⁻¹ s⁻¹ (M = Si), 3.4 × 10⁸ M⁻¹ s⁻¹ (M = Ge), and 1.9 × 10⁷ M⁻¹ s⁻¹ (M = Sn), respectively. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 238–244, 2000     

Thioxanthone–anthracene‐9‐carboxylic acid as radical photoinitiator in the presence of atmospheric air

Thioxanthone–anthracene‐9‐carboxylic acid (TX‐ANCA) namely 14‐oxo‐14H‐naphthol [2,3‐b]thioxanten‐12‐carboxylic acid, is synthesized and characterized as part of our continuing interest for syntheses of polyaromatic initiators. Photoinitiator, TX‐ANCA have good absorption properties in the UV and visible region of the electromagnetic spectrum (ɛ₃₇₀: 9080 M⁻¹cm⁻¹, ɛ₄₃₀: 6151 M⁻¹ cm⁻¹). The fluorescence quantum yield is calculated as 0.1 which is slightly higher than of the parent thioxanthone compound (φ_f: 0.07). The phosphorescence lifetime is found to be 39 ms. The possible initiating mechanism of TX‐ANCA is based on photoexcitation of TX‐ANCA and quenching of triplet excited states of TX‐ANCA by molecular oxygen generates singlet oxygen. Singlet oxygen reacts with the anthracene moiety of TX‐ANCA possibly forms an endoperoxide. The endoperoxides undergoes photochemical or thermal decomposition to form radicals which are able to initiate free radical polymerization. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1878–1883     

The Mechanism of Photochemical Release of Nitric Oxide from S-Nitrosoglutathione

Abstract— Laser flash photolysis of S-nitroso complexes of glutathione (GSNO) and bovine serum albumin (BSANO) via excitation at 355 nm has been used to investigate the photogeneration of nitric oxide (NO) and subsequent radical reactions. In the case of GSNO, liberation of NO was confirmed by its oxidation of oxyhemoglobin to met hemoglobin. Initial NO release is via homolytic cleavage of the S-N bond to produce the glutathione thiyl radical, GS, which can subsequently react with (a) ground-state GSNO (k= 1.7 × 10⁹M^?1/i> s^?1) to yield additional NO and oxidized glutathione, GSSG; and (b) oxygen (k= 3.0 × 10⁹M^?1 s^?1) to give the glutathione peroxy radical, GSOO, which subsequently reacts with ground-state GSNO (k= 3.8 × 10⁸M^?1 s^?1), also producing additional NO and GSSG. The relative concentrations of oxygen and GSNO in the system determine the major pathway for removal of G'. These secondary reactions occur at such high rates that they preclude radical recombination under low-intensity irradiation conditions. The quantum yield of overall loss of GSNO thus varies with both GSNO and oxygen concentrations; a value of 0.66 was determined for an aerated solution of GSNO (0.86 mM). In the case of GSNO, therefore, generation of NO is not due solely to homolysis of the S-N bond; secondary reactions of the radicals formed lead to further NO liberation. In rationalizing the known phototoxicity of GSNO, possible contributions from thiyl and thiyl-derived radicals should be considered. In contrast to GSNO, direct excitation of BSANO (containing one bound NO group per molecule) led to photodecomposition with a quantum yield of 0.09 but no evidence was obtained for liberation of NO into the bulk medium.     

Kinetics of one-electron oxidation by the ClO radical

Pulse radiolysis studies were carried out to determine the rate constants for reactions of ClO radicals in aqueous solution. These radicals were produced by the reaction of OH with hypochlorite ions in N₂O saturated solutions. The rate constants for their reactions with several compounds were determined by following the build up of the product radical absorption and in several cases by competition kinetics. ClO was found to be a powerful oxidant which reacts very rapidly with phenoxide ions to form phenoxyl radicals and with dimethoxybenzenes to form the cation radicals (k = 7 × 10⁸ −2 × 10⁹ M^-1 s^-1). ClO also oxidizes ClO^-₂ and N^-₃ ions rapidly (9.4 × 10⁸ and 2.5 × 10⁸ M^-1 s^-1, respectively), but its reactions with formate and benzoate ions were too slow to measure. ClO does not oxidize carbonate but the CO^-₃ radical reacts with ClO^- slowly (k = 5.1 × 10⁵ M^-1 s^-1).     

Reactions of vinyl ethers with 2‐cyano‐2‐propyl and benzoyloxy radicals

tert‐Butyl, cyclohexyl, n‐propyl, and n‐dodecyl vinyl ethers have been used as comonomers with styrene and methyl methacrylate using ¹³C‐enriched samples of azobis(isobutyronitrile) and benzoyl peroxide as initiators at 60°C. Examination by ¹³C‐NMR spectroscopy of either (¹³CH₃)₂C(CN) or Ph¹³COO end‐groups in the products has shown that the vinyl ethers have low reactivities toward the 2‐cyano‐2‐propyl radical but high reactivities toward the benzoyloxy radical. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 771–777, 1999     

Reactive oxygen and nitrogen species (RONS) scavenging reactions of o-vanillin: Pulse radiolysis and stopped flow studies

Reactions of peroxyl radicals and peroxynitrite with o-vanillin (2-hydroxy 3-methoxy benzaldehyde), a positional isomer of the well-known dietary compound vanillin, were studied to understand the mechanisms of its free radical scavenging action. Trichloromethylperoxyl radicals (CCl₃O ₂ ^· ) were used as model peroxyl radicals and their reactions with o-vanillin were studied using nanosecond pulse radiolysis technique with absorption detection. The reaction produced a transient with a bimolecular rate constant of approx. 10⁵ M⁻¹s⁻¹, having absorption in the 400–500 nm region with a maximum at 450 nm. This spectrum looked significantly different from that of phenoxyl radicals of o-vanillin produced by the one-electron oxidation by azide radicals. The spectra and decay kinetics suggest that peroxyl radical reacts with o-vanillin mainly by forming a radical adduct. Peroxynitrite reactions with o-vanillin at pH 6.8 were studied using a stopped-flow spectrophotometer. o-Vanillin reacts with peroxynitrite with a bimolecular rate constant of 3 × 10³ M⁻¹s⁻¹. The reaction produced an intermediate having absorption in the wavelength region of 300–500 nm with a absorption maximum at 420 nm, that subsequently decayed in 20 s with a first-order decay constant of 0.09 s⁻¹. The studies indicate that o-vanillin is a very efficient scavenger of peroxynitrite, but not a very good scavenger of peroxyl radical. The reactions take place through the aldehyde and the phenolic OH group and are significantly different from other phenolic compounds.