On the Inhibition of Hydroxyl Radical Formation by Hydroxycinnamic Acids: The Case of Caffeic Acid as a Promising Chelating Ligand of a Ferrous Ion

Revisiting the HO●-initiated oxidation of L-proline amino acid in the aqueous phase: influence of transition metal ions

The oxidation of L-proline (Pro) by HO^● radical in water and the influence of transition metal ions on this process has been revisited by using the density functional theory (DFT) method at the M05-2X/6-311 + + G(3df,3pd)//M05-2X/6-311 + + G(d,p) level of theory at the temperature of 298.15 K. The main reactive sites of the HO^●-initiated oxidation of Pro via hydrogen atom transfer (HAT) reactions are at the β- and γ-carbon, with the branching ratios being 44.6% and 39.5%, respectively. The overall rate constant at 298.15 K is 6.04 × 10⁸ M^-1 s^-1. In addition, Pro tends to form stable complexes with both Fe and Cu ions via the -COO functional group of dipole-salt form. The most stable Cu(II)-Pro complexes have high oxidant risks in enhancing the HO^● formation in the presence of reducing agents. Besides this, the high oxidation state metal complexes, i.e. Fe(III)-Pro and Cu(II)-Pro, may be oxidized by HO^● radical via HAT reactions but with a lower rate constant than that of free-Pro. By contrast, the low oxidation state metal complexes (i.e. Fe(II)-Pro and Cu(I)-Pro) have higher oxidation risks than the free ligands, and thus, the complexation enhances the oxidation of Pro amino acid.

Neighboring Carboxylic Acid Boosts Peroxidase-Like Property of Metal-Phenolic Nano-Networks in Eradicating Streptococcus mutans Biofilms

Developing nature-inspired nanomaterials with enzymatic activity is essential in combating bacterial biofilms. Here, it is reported that incorporating the carboxylic acid in phenolic/Fe nano-networks can efficiently manipulate their peroxidase-like activity via the acidic microenvironment and neighboring effect of the carboxyl group. The optimal gallic acid/Fe (GA/Fe) nano-networks demonstrate highly enzymatic activity in catalyzing H₂ O₂ into oxidative radicals, damaging the cell membrane and extracellular DNA in Streptococcus mutans biofilms. Theoretical calculation suggests that the neighboring carboxyl group can aid the H₂ O₂ adsorption, free radical generation, and catalyst reactivation, resulting in superb catalytic efficiency. Further all-atom simulation suggests the peroxidation of lipids can increase the cell membrane fluidity and permeability. Also, GA/Fe nano-networks show great potential in inhibiting tooth decay and treating other biofilm-associated diseases without affecting the commensal oral flora. This strategy provides a facile and scale-up way to prepare the enzyme-like materials and manipulate their enzymatic activity for biomedical applications.

Oxidation of l-leucine amino acid initiated by hydroxyl radical: are transition metal ions an enhancement factor?

Hydroxyl radical (HO·) formation initiated by the Fenton-type reactions of Fe and Cu complexes of l-leucine (Leu) amino acid as well as its oxidation reaction by HO· was computationally investigated by using the density functional theory method at the M05-2X/6-311++G(3df,2pd)//M05-2X/6-311++G(d,p) level of theory in the aqueous phase. The results showed that dipole-salt is the main form of Leu in the physiological condition. Leu exhibits high chelating potential towards both Fe(III)/Fe(II) and Cu(II)/Cu(I) ions with the most favourable coordinating positions at two oxygen atoms of the -COO functional group. Furthermore, the Leu-ions complexes show a high risk of HO· formation via Fenton-like reactions, especially when ascorbate anion exists in the environment as a reducing agent. Finally, the oxidation reaction of l-leucine by HO· demonstrated a relatively high overall apparent reaction rate, k _overall, being 1.18 × 10⁹ M^-1 s^-1, in which formal hydrogen transfer reactions of the dipole-salt form occur as the primary mechanism. Consequently, the Leu oxidation by HO· radical can be promoted by the Fenton reaction enhancement of its transition metal complexes.

On the free radical scavenging and metallic ion chelating activities of pyridoxal - Could the pro-oxidant risk be competitive?

Primary and secondary antioxidant activities of pyridoxal have been investigated by using density functional theory (DFT) at the M05-2X level combined with 6-311++G(d,p) basis set for non-metallic atoms and LanL2DZ for metallic ions. The former has been examined by its free radical scavenging activity towards HOO^●, HO^●, and NO₂^●via different mechanisms including formal hydrogen transfer (FHT), proton transfer (PT), single electron transfer (SET), and radical adduct formation (RAF). The latter has been accomplished through its transition metal-chelating ability with Fe(III)/Fe(II) and Cu(II)/Cu(I) ions. The results show that pyridoxal illustrates as an efficient radical scavenger, especially, for HO^● and NO₂^● in water. The overall rate constants (k_overall) for the reactions with HOO^●, HO^●, and NO₂^● radicals are 1.30 × 10⁴, 5.76 × 10⁹, and 1.43 × 10⁹ M^-1s^-1, respectively. The SET from the anionic state is the most dominant for the HOO^● and NO₂^● scavenging reactions, while both RAF and SET contribute largely to the reaction with highly reactive HO^● radicals. Moreover, the anionic form of pyridoxal demonstrates a better role as a metal chelator than the neutral. However, the pro-oxidant risks of the formed complexes could be observed if there are superoxide radical anion (O₂^●-) and ascorbate (Asc^-) in aqueous media.

Efficient decontamination of ciprofloxacin at neutral pH via visible light assisted Fenton-like process mediated by Fe(III)-GLDA complexation

This work demonstrated a novel N,N-bis(carboxymethyl)glutamic acid (GLDA) modified visible (vis) light assisted Fenton-like system which could effectively removal ciprofloxacin (CIP) at neutral pH. The removal rate of CIP in the GLDA/Fe(III)/H₂O₂/vis system was 97.9% within 120 min and was approximately twice as high as that in the Fe(III)/H₂O₂/vis system. GLDA could significantly accelerate the Fe(III)/Fe(II) cycle under visible light irradiation. Radical scavenging experiments demonstrated that 74.2% of the Fe(II) in the GLDA/Fe(III)/H₂O₂/vis system originated from the ligand-to-metal charge transfer reaction between Fe(III) and GLDA. The hydroxyl radical was the dominant species for CIP degradation. H₂O₂ utilization kinetic modeling exhibited that 90.1% of H₂O₂ was used for CIP mineralization. The effects of experimental parameters and coexisting substances on the removal of CIP in the Fe(III)/GLDA/H₂O₂/vis system were investigated in detail. The intermediate products of CIP were explored via the high-performance liquid chromatography-mass spectrometry.

New insights into the competition between antioxidant activities and pro-oxidant risks of rosmarinic acid

Direct and indirect antioxidant activities of rosmarinic acid (RA) based on HOO˙/CH₃OO˙ radical scavenging and Fe(iii)/Fe(ii) ion chelation were theoretically studied using density functional theory at the M05-2X/6-311++G(2df,2p) level of theory. First, four antioxidant mechanisms including hydrogen atom transfer (HAT), radical adduct formation (RAF), proton loss (PL) and single electron transfer (SET) were investigated in water and pentyl ethanoate (PEA) phases. Regarding the free radical scavenging mechanism, HAT plays a decisive role with overall rate coefficients of 1.84 × 10³ M⁻¹ s⁻¹ (HOO˙) and 4.49 × 10³ M⁻¹ s⁻¹ (CH₃OO˙) in water. In contrast to PL, RAF and especially SET processes, the HAT reaction in PEA is slightly more favorable than that in water. Second, the [Fe(iii)(H₂O)₆]³⁺ and [Fe(ii)(H₂O)₆]²⁺ ion chelating processes in an aqueous phase are both favorable and spontaneous especially at the O5, site-1, and site-2 positions with large negative Δ_rG⁰ values and great formation constant K_f. Finally, the pro-oxidant risk of RA⁻ was also considered via the Fe(iii)-to-Fe(ii) complex reduction process, which may initiate Fenton-like reactions forming reactive HO˙ radicals. As a result, RA⁻ does not enhance the reduction process when ascorbate anions are present as reducing agents, whereas the pro-oxidant risk becomes remarkable when superoxide anions are found. The results encourage further attempts to verify the speculation using more powerful research implementations of the antioxidant activities of rosmarinic acid in relationship with its possible pro-oxidant risks.

Direct and indirect antioxidant activities of rosmarinic acid (RA) based on HOO˙/CH₃OO˙ radical scavenging and Fe(iii)/Fe(ii) ion chelation were theoretically studied using density functional theory at the M05-2X/6-311++G(2df,2p) level of theory.

Enhancing Fenton-like process at neutral pH by Fe(III)-GLDA complexation for the oxidation removal of organic pollutants

N,N-bis(carboxymethyl)glutamic acid (GLDA) was utilized in this study to significantly enhance the Fe(III) mediated Fenton-like oxidation removal of organic pollutants at neutral pH, in which ciprofloxacin (CIP) was used as the model pollutant. The CIP degradation rate in the GLDA/Fe(III)/H₂O₂ system reached 96.5% within 180 min and was nearly 14 times higher than that in the Fe(III)/H₂O₂ system. This enhancement was contributed to the acceleration of the cycle of Fe(III)/Fe(II) caused by GLDA, which was verified by UV-vis spectroscopy, cyclic voltammetry, and radical quenching experiments. The results proved that the GLDA could complex with Fe(III) and greatly modify the redox potential of Fe(III)/Fe(II). Moreover, radical quenching experiments confirmed that •OH and O₂^·- were the mainly species for CIP degradation, and O₂^·- was responsible for 81.9% •OH generation. In addition, H₂O₂ utilization kinetic modeling was also investigated. The optimum parameters of the 100 μM Fe(III)-GLDA complex and 15 mM H₂O₂ were attained by lot-size optimization experiments. Two possible CIP degradation pathways were proposed on the basis of the intermediates identified by MS/MS. The GLDA/Fe(III)/H₂O₂ system performed better than common chelating agents at the same condition, manifesting good potential for environmental concerns.

Antioxidant and UV-radiation absorption activity of aaptamine derivatives - potential application for natural organic sunscreens

Antioxidant and UV absorption activities of three aaptamine derivatives including piperidine[3,2-b]demethyl(oxy)aaptamine (C1), 9-amino-2-ethoxy-8-methoxy-3H-benzo[de][1,6]naphthyridine-3-one (C2), and 2-(sec-butyl)-7,8-dimethoxybenzo[de]imidazo[4,5,1-ij][1,6]-naphthyridin-10(9H)-one (C3) were theoretically studied by density functional theory (DFT). Direct antioxidant activities of C1–C3 were firstly evaluated via their intrinsic thermochemical properties and the radical scavenging activity of the potential antioxidants with the HOO˙/HO˙ radicals via four mechanisms, including: hydrogen atom transfer (HAT), single electron transfer (SET), proton loss (PL) and radical adduct formation (RAF). Kinetic calculation reveals that HOO˙ scavenging in water occurs via HAT mechanism with C1 (k_app, 7.13 × 10⁶ M⁻¹ s⁻¹) while RAF is more dominant with C2 (k_app, 1.40 × 10⁵ M⁻¹ s⁻¹) and C3 (k_app, 2.90 × 10⁵ M⁻¹ s⁻¹). Antioxidant activity of aaptamine derivatives can be classified as C1 > C3 > C2. Indirect antioxidant properties based on Cu(i) and Cu(ii) ions chelating activity were also investigated in aqueous phase. All three studied compounds show spontaneous and favorable Cu(i) ion chelating activity with ΔG⁰ being −15.4, −13.7, and −15.7 kcal mol⁻¹, whereas ΔG⁰ for Cu(ii) chelation are −10.4, −10.8, and −2.2 kcal mol⁻¹ for C1, C2 and C3, respectively. In addition, all compounds show UVA and UVB absorption; in which the excitations are determined mostly as π–π* transition. Overall, the results suggest the potential applications of the aaptamines in pharmaceutics and cosmetics, i.e. as a sunscreen and antioxidant ingredient.

Antioxidant and UV absorption activities of three aaptamine derivatives were theoretically studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT).

A Review on Coordination Properties of Al(III) and Fe(III) toward Natural Antioxidant Molecules: Experimental and Theoretical Insights

This review focuses on the ability of some natural antioxidant molecules (i.e., hydroxycinnamic acids, coumarin-3-carboxylic acid, quercetin, luteolin and curcumin) to form Al(III)- and Fe(III)-complexes with the aim of evaluating the coordination properties from a combined experimental and theoretical point of view. Despite the contributions of previous studies on the chemical properties and biological activity of these metal complexes involving such natural antioxidants, further detailed relationships between the structure and properties are still required. In this context, the investigation on the coordination properties of Al(III) and Fe(III) toward these natural antioxidant molecules might deserve high interest to design water soluble molecule-based metal carriers that can improve the metal’s intake and/or its removal in living organisms.

Pseudomonas aeruginosa Presents Multiple Vital Changes in Its Proteome in the Presence of 3-Hydroxyphenylacetic Acid, a Promising Antimicrobial Agent

Pseudomonas aeruginosa, a widely distributed opportunistic pathogen, is an important threat to human health for causing serious infections worldwide. Due to its antibiotic resistance and virulence factors, it is so difficult to combat this bacterium; thus, new antimicrobial agents are in search. 3-Hydroxyphenylacetic acid (3-HPAA), which is a phenolic acid mostly found in olive oil wastewater, can be a promising candidate with its dose-dependent antimicrobial properties. Elucidating the molecular mechanism of action is crucial for future examinations and the presentation of 3-HPAA as a new agent. In this study, the antimicrobial activity of 3-HPAA on P. aeruginosa and its action mechanism was investigated via shot-gun proteomics. The data, which are available via ProteomeXchange with identifier PXD016243, were examined by STRING analysis to determine the interaction networks of proteins. KEGG Pathway enrichment analysis via the DAVID bioinformatics tool was also performed to investigate the metabolic pathways that undetected and newly detected groups of the proteins. The results displayed remarkable changes after 3-HPAA exposure in the protein profile of P. aeruginosa related to DNA replication and repair, RNA modifications, ribosomes and proteins, cell envelope, oxidative stress, as well as nutrient availability. 3-HPAA showed its antimicrobial action on P. aeruginosa by affecting multiple bacterial processes; hence, it could be categorized as a multitarget antimicrobial agent.

Layered nanocomposite of zinc sulfide covered reduced graphene oxide and their implications for electrocatalytic applications

Herein, we have synthesized zinc sulfide nanospheres (ZnS NPs) encapsulated on reduced graphene oxide (RGO) hybrid by an ultrasonic bath (50 kHz/60 W). The physical and structural properties of ZnS NPs@RGO hybrid were analyzed by TEM, XRD, EIS and EDS. As-prepared ZnS NPs@RGO hybrid was applied towards the electrochemical determination of caffeic acid (CA) in various food samples. The ZnS NPs@RGO hybrid modified electrode (GCE) exhibited an excellent electrocatalytic performance towards caffeic acid detection and determination, when compared to other modified electrodes. Therefore, the electrochemical sensing performance of the fabricated and nanocomposite modified electrode was significantly improved owing to the synergistic effect of ZnS NPs and RGO catalyst. Furthermore, the hybrid materials provide highly active electro-sites as well as rapid electron transport pathways. The proposed electrochemical caffeic acid sensor produces a wide linear range of 0.015-671.7 µM with a nanomolar level detection limit (3.29 nM). In addition, the real sample analysis of the proposed sensor has applied to the determination of caffeic acid in various food samples.