Substituent Inductive Effects on the Electrochemical Oxidation of Flavonoids Studied by Square Wave Voltammetry and Ab Initio Calculations

Flavonoid Oxidation Potentials and Antioxidant Activities-Theoretical Models Based on Oxidation Mechanisms and Related Changes in Electronic Structure

Herein, I will review our efforts to develop a comprehensive and robust model for the estimation of the first oxidation potential, Ep1, and antioxidant activity, AA, of flavonoids that would, besides enabling fast and cheap prediction of Ep1 and AA for a flavonoid of interest, help us explain the relationship between Ep1, AA and electronic structure. The model development went forward with enlarging the set of flavonoids and, that way, we had to learn how to deal with the structural peculiarities of some of the 35 flavonoids from the final calibration set, for which the Ep1 measurements were all made in our laboratory. The developed models were simple quadratic models based either on atomic spin densities or differences in the atomic charges of the species involved in any of the three main oxidation mechanisms. The best model takes into account all three mechanisms of oxidation, single electron transfer-proton transfer (SET-PT), sequential proton loss electron transfer (SPLET) and hydrogen atom transfer (HAT), yielding excellent statistics (R2 = 0.970, S.E. = 0.043).

Electrochemistry of lyophilized blueberry and raspberry samples: ROS activation of the antioxidant ability of anthocyanins

An electrochemical study of lyophilized blueberry (Vaccinium corymbosum L.) and raspberry (Rubus idaeus L.) fruits was carried out using microparticulate deposits from ethanolic extracts of fruits in contact with different aqueous electrolytes. This voltammetry is dominated by the oxidation of anthocyanins. It was hypothesized that under conditions of electrochemical generation of reactive oxygen species (ROS), new anthocyanin-derived species with high antioxidant capacity are formed over a wide pH range. In the case of ROS-activated anthocyanins, electrochemical oxidation is associated with loss of sugar moieties. Experimental voltammetric and scanning electrochemical microscopy (SECM) imaging data consistent with this hypothesis are presented.

Electrochemical Sensors for Detection of Phytomolecules: A Mechanistic Approach

High demand and ongoing technological advancements have created a market for sensors that is both varied and rapidly evolving. Bioactive compounds are separated systematically to conduct an in-depth investigation, allowing for the profiling or fingerprinting of different Plantae kingdoms. The profiling field is significant in elucidating the complex interplay of plant traits, attributes, and environmental factors. Flexible technology advancements have enabled the creation of highly sensitive sensors for the non-destructive detection of molecules. Additionally, very specialized integrated systems that will allow multiplexed detection by integrating many hybrid approaches have been developed, but these systems are highly laborious and expensive. Electrochemical sensors, on the other hand, are a viable option because of their ability to accomplish exact compound detection via efficient signal transduction. However, this has not been investigated because of some obstacles to learning minimum metabolites' fundamentals and nonredox properties. This article reviews the electrochemical basis of plants, contrasting it with more conventional techniques and offering both positive and negative perspectives on the topic. Because few studies have been devoted to the concept of merging the domains, we've expanded the scope of this work by including pertinent non-phytochemical reports for better report comparison.

Active Learning Guided Computational Discovery of Plant-Based Redoxmers for Organic Nonaqueous Redox Flow Batteries

Organic nonaqueous redox flow batteries (O-NRFBs) are promising energy storage devices due to their scalability and reliance on sourceable materials. However, finding suitable redox-active organic molecules (redoxmers) for these batteries remains a challenge. Using plant-based compounds as precursors for these redoxmers can decrease their costs and environmental toxicity. In this computational study, flavonoid molecules have been examined as potential redoxmers for O-NRFBs. Flavone and isoflavone derivatives were selected as catholyte (positive charge carrier) and anolyte (negative charge carrier) molecules, respectively. To drive their redox potentials to the opposite extremes, in silico derivatization was performed using a novel algorithm to generate a library of > 40000 candidate molecules that penalizes overly complex structures. A multiobjective Bayesian optimization based active learning algorithm was then used to identify best redoxmer candidates in these search spaces. Our study provides methodologies for molecular design and optimization of natural scaffolds and highlights the need of incorporating expert chemistry awareness of the natural products and the basic rules of synthetic chemistry in machine learning.

Electrochemistry of Flavonoids

This review presents a description of the available data from the literature on the electrochemical properties of flavonoids. The emphasis has been placed on the mechanism of oxidation processes and an attempt was made to find a general relation between the observed reaction paths and the structure of flavonoids. Regardless of the solvent used, three potential regions related to flavonoid structures are characteristic of the occurrence of their electrochemical oxidation. The potential values depend on the solvent used. In the less positive potential region, flavonoids, which have an ortho dihydroxy moiety, are reversibly oxidized to corresponding o-quinones. The o-quinones, if they possess a C3 hydroxyl group, react with water to form a benzofuranone derivative (II). In the second potential region, (II) is irreversibly oxidized. In this potential region, some flavonoids without an ortho dihydroxy moiety can also be oxidized to the corresponding p-quinone methides. The oxidation of the hydroxyl groups located in ring A, which are not in the ortho position, occurs in the third potential region at the most positive values. Some discrepancies in the reported reaction mechanisms have been indicated, and this is a good starting point for further investigations.

Antioxidant Properties of Selected Flavonoids in Binary Mixtures-Considerations on Myricetin, Kaempferol and Quercetin

Flavonoids, secondary plant metabolites with many health-promoting properties, including antioxidant, are a valuable component of food products, especially functional foods. In the latter, plant extracts are commonly used, the properties of which are attributed to the characteristic main ingredients. However, in a mixture the antioxidant properties of the individual ingredients do not always show an additive effect. This paper presents and discusses the antioxidant properties of naturally occurring flavonoid aglycones and their binary mixtures. In the experiments, model systems were used that differed in the volume of the alcoholic antioxidant solution in the measuring system and its concentration in the range in which it occurs in nature. Antioxidant properties were determined by ABTS and DPPH methods. The presented data proved that the dominant resultant effect in the mixtures is antioxidant antagonism. The magnitude of the observed antagonism depends on the mutual relations of individual components, their concentrations and the method used to assess antioxidant properties. It was shown that the observed non-additive antioxidant effect of the mixture results from the formation of intramolecular hydrogen bonds between phenolic groups of the antioxidant molecule. The presented results may be useful in the context of proper design of functional foods.

Electrochemical Profiling of Plants

The profiling, or fingerprinting, of distinct varieties of the Plantae kingdom is based on the bioactive ingredients, which are systematically segregated to perform their detailed analysis. The secondary products portray a pivotal role in defining the ecophysiology of distinct plant species. There is a crucial role of the profiling domain in understanding the various features, characteristics, and conditions related to plants. Advancements in variable technologies have contributed to the development of highly specific sensors for the non-invasive detection of molecules. Furthermore, many hyphenated techniques have led to the development of highly specific integrated systems that allow multiplexed detection, such as high-performance liquid chromatography, gas chromatography, etc., which are quite cumbersome and un-economical. In contrast, electrochemical sensors are a promising alternative which are capable of performing the precise recognition of compounds due to efficient signal transduction. However, due to a few bottlenecks in understanding the principles and non-redox features of minimal metabolites, the area has not been explored. This review article provides an insight to the electrochemical basis of plants in comparison with other traditional approaches and with necessary positive and negative outlooks. Studies consisting of the idea of merging the fields are limited; hence, relevant non-phytochemical reports are included for a better comparison of reports to broaden the scope of this work.

Origin and Function of Structural Diversity in the Plant Specialized Metabolome

The plant specialized metabolome consists of a multitude of structurally and functionally diverse metabolites, variable from species to species. The specialized metabolites play roles in the response to environmental changes and abiotic or biotic stresses, as well as in plant growth and development. At its basis, the specialized metabolism is built of four major pathways, each starting from a few distinct primary metabolism precursors, and leading to distinct basic carbon skeleton core structures: polyketides and fatty acid derivatives, terpenoids, alkaloids, and phenolics. Structural diversity in specialized metabolism, however, expands exponentially with each subsequent modification. We review here the major sources of structural variety and question if a specific role can be attributed to each distinct structure. We focus on the influences that various core structures and modifications have on flavonoid antioxidant activity and on the diversity generated by oxidative coupling reactions. We suggest that many oxidative coupling products, triggered by initial radical scavenging, may not have a function in se, but could potentially be enzymatically recycled to effective antioxidants. We further discuss the wide structural variety created by multiple decorations (glycosylations, acylations, prenylations), the formation of high-molecular weight conjugates and polyesters, and the plasticity of the specialized metabolism. We draw attention to the need for untargeted methods to identify the complex, multiply decorated and conjugated compounds, in order to study the functioning of the plant specialized metabolome.

Mitigating Heat Wave and Exposure Damage to "Cabernet Sauvignon" Wine Grape With Partial Shading Under Two Irrigation Amounts

Rising temperatures in most agricultural regions of the world are associated with a higher incidence of extreme weather events such as heat waves. We performed an experiment to mitigate the impact of heat waves and exposure of berries in grapevine (Vitis vinifera cv. "Cabernet Sauvignon") with untreated vines (Exposed) or with fruit-zone partial shading (Shaded) under 40 and 80% replacement of crop evapotranspiration (ET _c ) with sustained deficit irrigation in a factorially arranged experiment. The trial was performed in a vineyard with vertically shoot positioned trellis with a row orientation that concentrated solar radiation exposure on the southwest aspect of the fruit zone. Leaf stomatal conductance (g _s ) and net carbon assimilation (A _N ) were significantly lower in shaded leaves under partial fruit-zone shading that resulted in lower pruning mass for Shaded treatments. Stem water potential (Ψ _stem ) responded to a large extent to increased irrigation. However, grapevines with partial fruit-zone shading had transiently better water status under 40% ET _c . Cluster maximum temperatures were 3.9°C greater in Exposed grapevines. Exposed clusters had transiently lower acidity and higher pH. However, Exposed clusters on 40% ET _c had higher total soluble solids (TSS). The experimental vineyard suffered a 4-day heat wave 21 days before harvest, resulting in 25% of the clusters being damaged in Exposed treatment, regardless of irrigation amount. Furthermore, berries in Exposed treatments suffered a great loss of anthocyanins and flavonols even if they were not damaged by direct solar exposure. The pre-planting decision of using a vertically shoot positioned trellis that concentrated solar radiation on the Southwest aspect offered mild protection in a hot climate region with a sunny growing season with extreme heat events during the execution of study. The extreme conditions under which this study was conducted are not unusual, and have become more expected. Our work provided evidence of the vulnerability of grape berry to heat waves and exposure during heat wave events and possible protection methods to mitigate these effects in situ in context of climate change.

Discovery and Development of Inflammatory Inhibitors from 2-Phenylchromonone (Flavone) Scaffolds

Flavonoids are compounds based on a 2-phenylchromonone scaffold. Flavonoids can be divided into flavonoids, flavonols, dihydroflavones, anthocyanins, chalcones and diflavones according to the oxidation degree of the central tricarbonyl chain, the connection position of B-ring (2-or 3-position), and whether the tricarbonyl chain forms a ring or not. There are a variety of biological activities about flavonoids, such as anti-inflammatory activity, anti-oxidation and anti-tumor activity, and the antiinflammatory activity is apparent. This paper reviews the anti-inflammatory activities and mechanisms of flavonoids and their derivatives reported in China and abroad from 2011 till date (2011-2020), in order to find a good drug scaffold for the study of anti-inflammatory activities.

Control of Reactive Oxygen Species for the Prevention of Parkinson's Disease: The Possible Application of Flavonoids

Oxidative stress reflects an imbalance between the production of reactive oxygen species (ROS) and antioxidant defense systems, and it can be associated with the pathogenesis and progression of neurodegenerative diseases such as multiple sclerosis, stroke, and Parkinson's disease (PD). The application of antioxidants, which can defend against oxidative stress, is able to detoxify the reactive intermediates and prevent neurodegeneration resulting from excessive ROS production. There are many reports showing that numerous flavonoids, a large group of natural phenolic compounds, can act as antioxidants and the application of flavonoids has beneficial effects in the adult brain. For instance, it is well known that the long-term consumption of the green tea-derived flavonoids catechin and epigallocatechin gallate (EGCG) can attenuate the onset of PD. Also, flavonoids such as ampelopsin and pinocembrin can inhibit mitochondrial dysfunction and neuronal death through the regulation of gene expression of the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway. Additionally, it is well established that many flavonoids exhibit anti-apoptosis and anti-inflammatory effects through cellular signaling pathways, such as those involving (ERK), glycogen synthase kinase-3β (GSK-3β), and (Akt), resulting in neuroprotection. In this review article, we have described the oxidative stress involved in PD and explained the therapeutic potential of flavonoids to protect the nigrostriatal DA system, which may be useful to prevent PD.

An electrochemical method for plant species determination and classification based on fingerprinting petal tissue

The identification of plant species not only is a hobby but also has important application value in plant resources science. Traditional plant identification often relies on the experience of botanists. The infrageneric identification of plants is easily mistaken due to similarities in organ features. In this work, we propose an electrochemical method to obtain fingerprints of plant petal tissue. Fourteen species of Lycoris were used as a model for validating this methodology. Pattern and color recognition were established for visualization of electrochemical fingerprints recorded after various solvent extractions. In addition, the infrageneric relationships of these Lycoris species were deduced from the electrochemical fingerprints since the type and content of electroactive compounds in plants are controlled by genes. The results indicate that the electrochemical fingerprints of Lycoris petals are correlated with the infrageneric relationships of native Lycoris species.

Theoretical Studies on the Electronic Structure Parameters and Reactive Activity of Neu5Gc and Neu5Ac under Food Processing Solvent Environment

The animal product hazard factor N-glycolylneuraminic (Neu5Gc) and brain nutrient substance N-acetylneuraminic acid (Neu5Ac) were studied at the M062X/6-311 + G(d,p) geometry optimization level. We considered the electronic structure parameters with different solvents: (benzene ε = 2.27, acetic acid ε = 6.25, ethanol ε = 24.85, lactic acid ε = 22.00, formic acid ε = 51.1, water ε = 78.35). The maximum molecular surface electrostatic potentials, which were 62.77 for Neu5Gc and 60.90 kcal/mol for Neu5Ac, are both located on the carboxyl group hydrogen. The orbital analysis showed that the amide group and carboxyl group confer the sites with susceptibility to nucleophilic and electrophilic attack, respectively. The solvent effect showed that polar solvents, such as formic acid and water, can enhance the two molecules’ nucleophilic activity. To better understand the roles of the hydroxyl group in the two molecules, the independent gradient model theory confirmed the four intramolecular hydrogen bonds of Neu5Gc at gas phase, whereas Neu5Ac only has two. The lowest bond dissociation energy in solvent occurs at O7-H, which is 104.03 kcal/mol in water for Neu5Gc and 104.57 kcal/mol in lactic acid for Neu5Ac. The lowest proton affinity value for Neu5Gc (20.34 kcal/mol) and Neu5Ac (20.76 kcal/mol) was both occur at the carboxyl group O6-H under ethanol. The antioxidant mechanisms of the two sialic acid are prone to sequential proton-loss electron transfer under polar or non-polar solvents.

Partial Solar Radiation Exclusion with Color Shade Nets Reduces the Degradation of Organic Acids and Flavonoids of Grape Berry (Vitis vinifera L.)

The incidence of solar radiation on red-skinned grapes can promote the synthesis of flavonoids desirable for wine production, but elevated temperature may impair their accumulation. We performed a shade cloth trial covering the fruit zone (from pepper-corn size to maturity) with four polyethylene 1 m curtains with different optical properties (20% shading factor Pearl colored and 40% shading factor Aluminet, Blue, and Black colored) and a Control with no cover. Cluster temperature was 3.7 °C lower on the Southwest side in Black-40% clusters during the warmest part of the day compared to Control. Results indicated a lower berry weight under the Aluminet-40%. Berries under the nets often had significantly lower pH and higher TA than Control, but only the Black-40% were significant at harvest. Black-40% had higher values of anthocyanins than Control toward the last weeks of development. Berry skin flavonol and anthocyanin composition and concentration were measured by C18 reversed-phased HPLC; and proanthocyanidin isolates were characterized by acid catalysis in the presence of excess phloroglucinol followed by reversed-phase HPLC. Proanthocyanidins and flavonol contents were lower in Black-40% before veraison and the first part of ripening, respectively. However, their contents in Control decreased toward the end of ripening to a point where any net was different from Control. Anthocyanin and flavonol profiles were richer in 3', 4', 5' hydroxylated forms. Proanthocyanidin chain length was not affected while small changes were observed in the proportion of terminal catechin/epicatechin and in seed galloylation in response to treatments. Results show that shade cloths may efficiently palliate temperature spikes, especially the last weeks before harvest, while transmitting enough radiation into the fruit zone to achieve a better grape composition compared to uncovered grapes.