Redox Properties and Cytotoxicity of Synthetic Isomeric Mitochondriotropic Derivatives of the Natural Polyphenol Quercetin

The Theoretical and Experimental Insights into the Radical Scavenging Activity of Rubiadin

Rubiadin (RBD), an anthraquinone derivative, is obtained from Rubia cordifolia, a plant species classified under the Rubiaceae family. Rubiadin has proven beneficial properties, such as anticancer, neuroprotective, anti-inflammatory, and antidiabetic activity. The antioxidant activity of this molecule was suggested by some experimental results but has not been clearly established thus far. In this study, we employ DFT calculations to comprehensively assess the mechanism and kinetics of the HO•/HOO• radical scavenging activity of this compound in relation to solvents. RBD showed moderate HO• radical scavenging activity, with rate constants of 2.95 × 108 and 1.82 × 1010 M-1 s-1 in lipid and polar media, respectively. In the aqueous solution, the compound exhibited remarkable superoxide anion radical scavenging activity (k = 4.93 × 108 M-1 s-1) but modest HOO• antiradical activity. RBD also showed promising antiradical activity against a variety of radicals (CCl3O•, CCl3OO•, NO2, SO4•-, and N3•), while experimental and computational results confirmed that RBD has moderate activity in DPPH/ABTS•+ assays. Thus, RBD is predicted to be a good, albeit selective, radical scavenger.

Mechanistic and Kinetic Studies of the Radical Scavenging Activity of 5-O-Methylnorbergenin: Theoretical and Experimental Insights

5-O-Methylnorbergenin (5-OMB), a natural compound isolated from Rourea harmandiana, is a compound with potential antioxidant activity based on its chemical structure; however, this activity has not been investigated thus far. In this study, the antioxidant activity of 5-OMB was evaluated by experimental and computational methods. 5-OMB exhibited high activity in DPPH (IC₅₀ = 7.25 ± 0.94 μM) and ABTS^•+ (IC₅₀ = 4.23 ± 0.12 μM) assays, higher than the reference compound Trolox. The computational results consistently show that 5-OMB is an excellent HOO^• radical scavenger (k_overall = 8.14 × 10⁸ M^-1 s^-1) in water at physiological pH, however it only exerts weak activity in lipid medium (k_overall = 3.02 × 10² M^-1 s^-1). The reaction follows the formal hydrogen transfer mechanism in nonpolar solvents, whereas both the sequential proton loss electron transfer and the formal hydrogen transfer pathways contribute to the activity in aqueous solution. There is a good agreement between experimental and computational data, suggesting that 5-OMB is a promising natural radical scavenger in aqueous physiological environment.

Synthesis and Testing of Novel Isomeric Mitochondriotropic Derivatives of Resveratrol and Quercetin

Resveratrol and quercetin are among the most studied plant polyphenols, and have many health-promoting actions. Strategies to accumulate them into mitochondria may be of therapeutic relevance, since these compounds are redox active and are well known to impact mitochondria and mitochondrial proteins. We report here the procedures to synthesize mitochondria-targeted resveratrol and quercetin derivatives; the synthetic strategies reported are however expected to be adaptable to other polyphenols with similar reactivity at the phenolic hydroxyls. Mitochondrial targeting can be achieved by conjugation with triphenylphosphonium , a lipophilic cation; this was linked via a butyl spacer forming an ether bond with one of the phenolic oxygens. The first step toward the synthesis of all mitochondriotropic derivatives described in this work is the production of a regiospecific -(4-O-chlorobutyl) derivative. Triphenylphosphonium (P⁺Ph₃I^-) is then introduced through two consecutive nucleophilic substitution steps: -Cl → -I → -P⁺Ph₃I^-. Pure mono-substituted chlorobutyl regioisomers are obtained by purification from the reaction mixture in the case of resveratrol , while specific protection strategies are required for quercetin to favor alkylation of one specific hydroxyl.Functionalization of the remaining hydroxyls can be exploited to modulate the physicochemical properties of the derivatives (i.e., water solubility, affinity for cell membranes); we report here synthetic protocols to obtain acetylated and methylated analogs.A brief description of some methods to assess the accumulation of the derivatives in mitochondria is also given; the proposed techniques are the use of a TPP ⁺-selective electrode (with isolated rat liver mitochondria ) and fluorescence microscopy (with cultured cells).

Mitochondria-Targeted Triphenylphosphonium-Based Compounds: Syntheses, Mechanisms of Action, and Therapeutic and Diagnostic Applications

Mitochondria are recognized as one of the most important targets for new drug design in cancer, cardiovascular, and neurological diseases. Currently, the most effective way to deliver drugs specifically to mitochondria is by covalent linking a lipophilic cation such as an alkyltriphenylphosphonium moiety to a pharmacophore of interest. Other delocalized lipophilic cations, such as rhodamine, natural and synthetic mitochondria-targeting peptides, and nanoparticle vehicles, have also been used for mitochondrial delivery of small molecules. Depending on the approach used, and the cell and mitochondrial membrane potentials, more than 1000-fold higher mitochondrial concentration can be achieved. Mitochondrial targeting has been developed to study mitochondrial physiology and dysfunction and the interaction between mitochondria and other subcellular organelles and for treatment of a variety of diseases such as neurodegeneration and cancer. In this Review, we discuss efforts to target small-molecule compounds to mitochondria for probing mitochondria function, as diagnostic tools and potential therapeutics. We describe the physicochemical basis for mitochondrial accumulation of lipophilic cations, synthetic chemistry strategies to target compounds to mitochondria, mitochondrial probes, and sensors, and examples of mitochondrial targeting of bioactive compounds. Finally, we review published attempts to apply mitochondria-targeted agents for the treatment of cancer and neurodegenerative diseases.

Synthesis of methylated quercetin analogues for enhancement of radical-scavenging activity

Methylation of the catechol moiety of quercetin resulted in the enhancement of its radical-scavenging activity.

Synthesis and testing of novel isomeric mitochondriotropic derivatives of resveratrol and quercetin

We report here the synthetic procedures to obtain mitochondria-targeted resveratrol and quercetin derivatives. These two compounds were selected among plant polyphenols because both are well studied and have many health-promoting actions. The synthetic strategies reported here are however expected to be adaptable to other polyphenols with similar reactivity at the phenolic hydroxyls.Mitochondrial targeting can be achieved by incorporating in the molecule an "electrophoretic" membrane-permeant, triphenylphosphonium cation. We have generally chosen to link it via a butyl spacer forming an ether bond with one of the phenolic oxygens. The first step toward the synthesis of all mitochondriotropic derivatives described in this work is the production of a regiospecific -(4-O-chlorobutyl) derivative. Triphenylphosphonium (P(+)Ph3I(-)) is then introduced through two consecutive nucleophilic substitution steps: -Cl→-I→-P(+)Ph3I(-). Pure mono-substituted chlorobutyl regioisomers are obtained by purification from the reaction mixture in the case of resveratrol, while specific protection strategies are required for quercetin to favor alkylation of one specific hydroxyl.Physicochemical properties of the derivatives (i.e., water solubility, affinity for cell membranes) can be furthermore modulated by functionalization of the remaining hydroxyls; we report here synthetic protocols to obtain acetylated and methylated analogs.We also briefly describe how to assess mitochondrial accumulation of the derivatives; the proposed techniques are the use of a TPP(+)-selective electrode (with isolated rat liver mitochondria) and fluorescence microscopy (with cultured cells).