Oxidative Damage in Aliphatic Amino Acids and Di- and Tripeptides by the Environmental Free Radical Oxidant NO3•: The Role of the Amide Bond Revealed by Kinetic and Computational Studies

Primary photophysical and photochemical processes for cerium ammonium nitrate (CAN) in acetonitrile

Cerium ammonium nitrate (CAN) is an important photolytic source of NO3• radicals in aqueous nitric acid solutions and in acetonitrile. In this work we performed the study of primary photochemical processes for CAN in acetonitrile by means of ultrafast TA spectroscopy and quantum chemical calculations. Photoexcitation of CAN is followed by ultrafast (< 100 fs) intersystem crossing; the vibrationally cooled triplet state decays to pentacoordinated Ce(III) intermediate and NO3• radical with the characteristic time of ca. 40 ps. Quantum chemical (QM) calculations satisfactorily describe the UV-vis spectrum of the triplet state. An important feature of CAN photochemistry in CH3CN is the partial stabilization of the radical complex (RC) [(NH4)2CeIII(NO3)5…NO3•], which lifetime is ca. 2 μs. The possibility of the RC stabilization is supported by the QM calculations.

Photochemistry of (n-Bu4N)2[Pt(NO3)6] in acetonitrile

Photochemistry of the (n-Bu4N)2[Pt(NO3)6] complex in acetonitrile was studied by means of stationary photolysis and nanosecond laser flash photolysis. The primary photochemical process was found to be an intramolecular electron transfer followed by an escape of an •NO3 radical to the solution bulk. The spectra of two successive Pt(III) intermediates were detected in the microsecond time domain, and their spectral and kinetic characteristics were determined. These intermediates were identified as PtIII(NO3)52- and PtIII(NO3)4- complexes. Disproportionation of Pt(III) species resulted in formation of final Pt(II) products.

Combined LC-MS-based metabolomics and GC-IMS analysis reveal changes in chemical components and aroma components of Jujube leaf tea during processing

Making tea from jujube leaves changed the chemical composition and aroma composition of jujube leaves. Here, Through LC-MS, GC-IMS, and GC-MS technology, we have revealed the effect of jujube leaf processing changes on metabolites. LC-MS identified 468 non-volatile metabolites, while GC-IMS and GC-MS detected 52 and 24 volatile metabolites, respectively. 109 non-volatile metabolites exhibiting more pronounced differences were screened. Most lipids and lipid-like molecules, organic acids, amino acids, and flavonoids increased significantly after processing. GC-IMS and GC-MS analysis revealed that the contents of aldehydes and ketones were significantly increased, while esters and partial alcohols were decreased after processing into jujube leaf tea. The main flavor substances of fresh jujube leaf and jujube leaf tea were eugenol and (E) - 2-Hexenal, respectively. Furthermore, amino acids and lipids were closely linked to the formation of volatile metabolites. Our study provided new insights into the changes in metabolites of jujube leaves processed into jujube leaf tea, and had great potential for industrial application. It laid a foundation for further research on fruit tree leaf tea.

Oxidative Damage of Aliphatic Amino Acid Residues by the Environmental Pollutant NO3·: Impact of Water on the Reactivity

The rate of oxidative damage of aliphatic amino acids and dipeptides by the environmental pollutant nitrate radical (NO₃^·) in an aqueous acidic environment was studied by laser flash photolysis. The reactivity dropped by a factor of about four for amino acid residues with secondary amide bonds and by a factor of up to nearly 20 for amino acid residues with tertiary amide bonds, compared with that in acetonitrile. According to density functional theory studies, the lower reactivity is due to protonation of the amide moiety, whereas in neutral water, hydrogen bonding with the amide should have little impact on the absolute reaction rate compared with that in acetonitrile. This finding can be rationalized by the high reactivity and broad reaction pattern of NO₃^·. Although hydrogen bonding involving the amide group raises the energies associated with some electron transfer processes, alternative low-energy pathways remain available so that the overall reaction rate is barely affected. The undiminished high reactivity of NO₃^· toward aliphatic amino acid residues in a neutral aqueous environment highlights the health-damaging potential of exposure to the combined air pollutants nitrogen dioxide (NO₂^·) and ozone (O₃).

Ligand Enabled Pd(II)-Catalyzed γ-C(sp3)-H Lactamization of Native Amides

γ-Lactams form important structural cores of a range of medicinally relevant natural products and clinical drugs, principal examples being the new generation of immunomodulatory imide drugs (IMiDs) and the brivaracetam family. Compared to conventional multistep synthesis, an intramolecular γ-C-H amination of aliphatic amides would allow for the direct construction of valuable γ-lactam motifs from abundant amino acid precursors. Herein we report a novel 2-pyridone ligand enabled Pd(II)-catalyzed γ-C(sp³)-H lactamization of amino acid derived native amides, providing the convenient synthesis of γ-lactams, isoindolinones, and 2-imidazolidinones. C6-Substitution of the 2-pyridone ligand is crucial for the lactam formation. This protocol features the use of N-acyl amino acids, which serve as both the directing group and cyclization partner, practical and environmentally benign tert-butyl hydrogen peroxide (TBHP) as the sole bystanding oxidant, and a broad substrate scope. The utility of this protocol was demonstrated through the two-step syntheses of a lenalidomide analog and brivaracetam from readily available carboxylic acids and amino acids.

Photochemical intermolecular dearomative cycloaddition of bicyclic azaarenes with alkenes

Dearomative cycloaddition reactions represent an ideal means of converting flat arenes into three-dimensional architectures of increasing interest in medicinal chemistry. Quinolines, isoquinolines, and quinazolines, despite containing latent diene and alkene subunits, are scarcely applied in cycloaddition reactions because of the inherent low reactivity of aromatic systems and selectivity challenges. Here, we disclose an energy transfer-mediated, highly regio- and diastereoselective intermolecular [4 + 2] dearomative cycloaddition reaction of these bicyclic azaarenes with a plethora of electronically diverse alkenes. This approach bypasses the general reactivity and selectivity issues, thereby providing various bridged polycycles that previously have been inaccessible or required elaborate synthetic efforts. Computational studies with density functional theory elucidate the mechanism and origins of the observed regio- and diastereoselectivities.

Oxidative damage of proline residues by nitrate radicals (NO3˙): a kinetic and product study

Tertiary amides, such as in N-acylated proline or N-methyl glycine residues, react rapidly with nitrate radicals (NO₃˙) with absolute rate coefficients in the range of 4-7 × 10⁸ M^-1 s^-1 in acetonitrile. The major pathway proceeds through oxidative electron transfer (ET) at nitrogen, whereas hydrogen abstraction is only a minor contributor under these conditions. However, steric hindrance at the amide, for example by alkyl side chains at the α-carbon, lowers the rate coefficient by up to 75%, indicating that NO₃˙-induced oxidation of amide bonds proceeds through initial formation of a charge transfer complex. Furthermore, the rate of oxidative damage of proline and N-methyl glycine is significantly influenced by its position in a peptide. Thus, neighbouring peptide bonds, particularly in the N-direction, reduce the electron density at the tertiary amide, which slows down the rate of ET by up to one order of magnitude. The results from these model studies suggest that the susceptibility of proline residues in peptides to radical-induced oxidative damage should be considerably reduced, compared with the single amino acid.

Oxidative Repair of Pyrimidine Cyclobutane Dimers by Nitrate Radicals (NO3•): A Kinetic and Computational Study

Pyrimidine cyclobutane dimers are hazardous DNA lesions formed upon exposure of DNA to UV light, which can be repaired through oxidative electron transfer (ET). Laser flash photolysis and computational studies were performed to explore the role of configuration and constitution at the cyclobutane ring on the oxidative repair process, using the nitrate radical (NO3•) as oxidant. The rate coefficients of 8–280 × 107 M−1 s−1 in acetonitrile revealed a very high reactivity of the cyclobutane dimers of N,N’-dimethylated uracil (DMU), thymine (DMT), and 6-methyluracil (DMU6-Me) towards NO3•, which likely proceeds via ET at N(1) as a major pathway. The overall rate of NO3• consumption was determined by (i) the redox potential, which was lower for the syn- than for the anti-configured dimers, and (ii) the accessibility of the reaction site for NO3•. In the trans dimers, both N(1) atoms could be approached from above and below the molecular plane, whereas in the cis dimers, only the convex side was readily accessible for NO3•. The higher reactivity of the DMT dimers compared with isomeric DMU dimers was due to the electron-donating methyl groups on the cyclobutane ring, which increased their susceptibility to oxidation. On the other hand, the approach of NO3• to the dimers of DMU6-Me was hindered by the methyl substituents adjacent to N(1), making these dimers the least reactive in this series.