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Quantitative Estimation of the Hydrogen-Atom-Donating Ability of 4-Substituted Hantzsch Ester Radical Cations.

ABSTRACT: The purpose of this study is to investigate thermodynamic and kinetic properties on the hydrogen-atom-donating ability of 4-substituted Hantzsch ester radical cations (XRH^•+), which are excellent NADH coenzyme models. Gibbs free energy changes and activation free energies of 17 XRH^•+ releasing H^• [denoted as ΔG _HD ^o(XRH^•+) and ΔG _HD ^≠(XRH^•+)] were calculated using density functional theory (DFT) and compared with that of Hantzsch ester (HEH₂) and NADH. ΔG _HD ^o(XRH^•+) range from 19.35 to 31.25 kcal/mol, significantly lower than that of common antioxidants (such as ascorbic acid, BHT, the NADH coenzyme, and so forth). ΔG _HD ^≠(XRH^•+) range from 29.81 to 39.00 kcal/mol, indicating that XRH^•+ spontaneously releasing H^• are extremely slow unless catalysts or active intermediate radicals exist. According to the computed data, it can be inferred that the Gibbs free energies and activation free energies of the core 1,4-dihydropyridine radical cation structure (DPH^•+) releasing H^• [ΔG _HD ^o(DPH^•+) and ΔG _HD ^≠(DPH^•+)] should be 19-32 kcal/mol and 29-39 kcal/mol in acetonitrile, respectively. The correlations between the thermodynamic driving force [ΔG _HD ^o(XRH^•+)] and the activation free energy [ΔG _HD ^≠(XRH^•+)] are also explored. Gibbs free energy is the important and decisive parameter, and ΔG _HD ^≠(XRH^•+) increases in company with the increase of ΔG _HD ^o(XRH^•+), but no simple linear correlations are found. Even though all XRH^•+ are judged as excellent antioxidants from the thermodynamic view, the computed data indicate that whether XRH^•+ is an excellent antioxidant in reaction is decided by the R substituents in 4-position. XRH^•+ with nonaromatic substituents tend to release R^• instead of H^• to quench radicals. XRH^•+ with aromatic substituents tend to release H^• and be used as antioxidants, but not all aromatic substituted Hantzsch esters are excellent antioxidants.

SUBMITTER: Shen GB

PROVIDER: S-EPMC8444320 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO(*) (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl ((t)Bu(2)NO(*)), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and (t)Bu(2)NOH. The reactions have been monitored by UV-vis stopped-flow methods, using the different optical spectra of the nitroxyl radicals. The HAT reactions all have |DeltaG (o)| < or = 1.4 kcal mol(-1) and therefore are close to self-exchange reactions. The reaction of 4-oxo-TEMPO(*) + TEMPO-H --> 4-oxo-TEMPO-H + TEMPO(*) occurs with k(2H,MeCN) = 10 +/- 1 M(-1) s(-1) in MeCN at 298 K (K(2H,MeCN) = 4.5 +/- 1.8). Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k(2D,MeCN) = 0.44 +/- 0.05 M(-1) s(-1) with k(2H,MeCN)/k(2D,MeCN) = 23 +/- 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH(2)Cl(2), 23 +/- 4, suggesting that the large KIE is not caused by solvent dynamics or hydrogen bonding to solvent. The related reaction of 4-oxo-TEMPO(*) with 4-MeO-TEMPO-H(D) also has a large KIE, k(3H)/k(3D) = 21 +/- 3 in MeCN. For these three reactions, the E(aD) - E(aH) values, between 0.3 +/- 0.6 and 1.3 +/- 0.6 kcal mol(-1), and the log(A(H)/A(D)) values, between 0.5 +/- 0.7 and 1.1 +/- 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of (t)Bu(2)NO(*) + TEMPO-H(D) in MeCN has a large KIE, 16 +/- 3 in MeCN, and very unusual isotopic activation parameters, E(aD) - E(aH) = -2.6 +/- 0.4 and log(A(H)/A(D)) = 3.1 +/- 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH(3))(2)NO(*) + (CH(3))(2)NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO((*)/H), (t)Bu(2)NO((*)/H), and Ph(2)NO((*)/H) self-exchange reactions reveal why the phenyl groups make the last of these reactions several orders of magnitude faster than the first two. By inference, the calculations also suggest why tunneling appears to be more important in the self-exchange reactions of dialkylhydroxylamines than of arylhydroxylamines.

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Quantitative Estimation of the Hydrogen-Atom-Donating Ability of 4-Substituted Hantzsch Ester Radical Cations.

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