Insights into the Antioxidant Mechanism of Newly Synthesized Benzoxazinic Nitrones: In Vitro and In Silico Studies with DPPH Model Radical

Synthetic nitrone spin-traps are being explored as therapeutic agents for the treatment of a wide range of oxidative stress-related pathologies, including but not limited to stroke, cancer, cardiovascular, and neurodegenerative diseases. In this context, increasing efforts are currently being made to the design and synthesis of new nitrone-based compounds with enhanced efficacy. The most researched nitrones are surely the ones related to α-phenyl-tert-butylnitrone (PBN) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) derivatives, which have shown to possess potent biological activity in many experimental animal models. However, more recently, nitrones with a benzoxazinic structure (3-aryl-2H-benzo[1,4]oxazin-N-oxides) have been demonstrated to have superior antioxidant activity compared to PBN. In this study, two new benzoxazinic nitrones bearing an electron-withdrawing methoxycarbonyl group on the benzo moiety (in para and meta positions respect to the nitronyl function) were synthesized. Their in vitro antioxidant activity was evaluated by two cellular-based assays (inhibition of AAPH-induced human erythrocyte hemolysis and cell death in human retinal pigmented epithelium (ARPE-19) cells) and a chemical approach by means of the α,α-diphenyl-β-picrylhydrazyl (DPPH) scavenging assay, using both electron paramagnetic resonance (EPR) spectroscopy and UV spectrophotometry. A computational approach was also used to investigate their potential primary mechanism of antioxidant action, as well as to rationalize the effect of functionalization on the nitrones reactivity toward DPPH, chosen as model radical in this study. Further insights were also gathered by exploring the nitrone electrochemical properties via cyclic voltammetry and by studying their kinetic behavior by means of EPR spectroscopy. Results showed that the introduction of an electron-withdrawing group in the phenyl moiety in the para position significantly increased the antioxidant capacity of benzoxazinic nitrones both in cell and cell-free systems. From the mechanistic point of view, the calculated results closely matched the experimental findings, strongly suggesting that the H-atom transfer (HAT) is likely to be the primary mechanism in the DPPH quenching.

Amidinoquinoxaline-Based Nitrones as Lipophilic Antioxidants

The potential of nitrones (N-oxides) as therapeutic antioxidants is due to their ability to counteract oxidative stress, mainly attributed to their action as radical scavengers toward C- and O-centered radicals. Among them, nitrones from the amidinoquinoxaline series resulted in interesting derivatives, due to the ease with which it is possible to introduce proper substituents within their structure in order to modulate their lipophilicity. The goal is to obtain lipophilic antioxidants that are able to interact with cell membranes and, at the same time, enough hydrophilic to neutralize those radicals present in a water compartment. In this work, the antioxidant efficacy of a series of amidinoquinoxaline nitrones has been evaluated regarding the oxidation of 2-deoxyribose and lipid peroxidation. The results have been rationalized on the basis of the different possible mechanisms involved, depending on some of their properties, such as lipophilicity, the ability to scavenge free radicals, and to undergo single electron transfer (SET) reactions.

Monoalkylated Epigallocatechin-3-gallate (C18-EGCG) as Novel Lipophilic EGCG Derivative: Characterization and Antioxidant Evaluation

Epigallocatechin-3-gallate (EGCG) has the highest antioxidant activity compared to the others catechins of green tea. However, the beneficial effects are mainly limited by its poor membrane permeability. A derivatization strategy to increase the EGCG interaction with lipid membranes is considered as one feasible approach to expand its application in lipophilic media, in particular the cellular absorption. At this purpose the hydrophilic EGCG was modified by inserting an aliphatic C18 chain linked to the gallate ring by an ethereal bond, the structure determined by NMR (Nuclear Magnetic Resonance) and confirmed by Density Functional Theory (DFT) calculations. The in vitro antioxidant activity of the mono-alkylated EGCG (C18-EGCG) was studied by the DPPH and Thiobarbituric Acid Reactive Substances (TBARS) assays, and its ability to protect cells towards oxidative stress was evaluated in Adult Retinal Pigmented Epithelium (ARPE-19) cells. Molecular Dynamics (MD) simulation and liposomal/buffer partition were used to study the interaction of the modified and unmodified antioxidants with a cell membrane model: the combined experimental-in silico approach shed light on the higher affinity of C18-EGCG toward lipid bilayer. Although the DPPH assay stated that the functionalization decreases the EGCG activity against free radicals, from cellular experiments it resulted that the lipid moiety increases the antioxidant protection of the new lipophilic derivative.

Amidinoquinoxaline N-oxides: spin trapping of O- and C-centered radicals

Amidinoquinoxaline N-oxides represent a novel family of heterocyclic spin traps. In this work, their ability to trap O- and C-centered radicals was tested using selected derivatives with different structural modifications. All the studied nitrones were able to trap radicals forming persistent spin adducts, also in the case of OH and OOH radicals which are of wide biological interest as examples of ROS. The stability of the adducts was mainly attributed to the wide delocalization of the unpaired electron over the whole quinoxaline moiety. The nitroxide spectral parameters (hfccs and g-factors) were analyzed and the results were supported by DFT calculations. The N-19 hfccs and g-factors were characteristic of each aminoxyl and could aid in the identification of the trapped radical. The enhanced stability of the OH adducts under the employed reaction conditions could be ascribed to their possible stabilization by IHBs with two different acceptors: the N-O˙ moiety or the amidine functionality. DFT calculations indicate that the preferred IHB is strongly conditioned by the amidine ring size. While five membered homologues show a clear preference for the IHB with the N-O˙ group, in six membered derivatives this stabilizing interaction is preferentially established with the amidine nitrogen as an IHB acceptor.

A comprehensive spectroscopic investigation of α-(2-naphthyl)-N-methylnitrone: a computational study on photochemical nitrone–oxaziridine conversion and thermal E–Z isomerization processes

CASSCF and 2-layer hybrid ONIOM-based computational studies on α-(2-naphthyl)-N-methylnitrone have proposed its photochemical oxaziridine formation and thermal E–Z isomerization mechanisms.