The search for new natural antioxidants is a growing area of research due to the broad
spectrum of their biological properties, which are associated with the prevention
of chronic diseases that originate in oxidative stress. In this context, the fact
that they promote a health capacity is noteworthy and has determined a marked trend
towards the supplementation of food products with natural antioxidants. This Special
Issue of Molecules contains a collection of 28 research works and 4 reviews, overall
covering distinct aspects related to natural antioxidants.
Several authors highlighted the relevance of specific structural features of natural
compounds in their antioxidant capacities. In this context, the comparison made by
Ouyang et al. [1] between galangin and 3,5,7-trihydroxychromone (characterized by
the presence or absence of a null B-ring) allowed the authors to elucidate that, regardless
of it not affecting the antioxidant pathways, the presence of the null B-ring in flavonols
improves the antioxidant levels, since the π–π conjugation can provide more resonance
forms and bonding sites. Moreover, Li and coworkers [2] showed that the presence of
3,8″-dimerization on flavonoids could enhance the antioxidant capacity through the
electron-transfer pathway, possibly by allowing a partial π–π conjugation. Relevance
was also given to the influence of substituents on the modulation of the antioxidant
activity. In particular, when comparing the antioxidant potential of scutellarein
and scutellarin (scutellarein-7-O-glucuronide), Liu and coworkers [3] revealed that
the glucuronidation of pyrogallol-type phytophenol antioxidants caused a dual effect
by decreasing the antiradical potential towards several radicals such as 1,1-diphenyl-2-picrylhydrazyl,
2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl
3-oxide radicals and the ability for radical adduct formation, and by in turn enhancing
the Fe2+-chelating potentials. Also worth noting, the work performed by Liang et al.
[4] with the two chalcones echinatin and licochalcone A allowed them to conclude that
the 1,1-dimethyl-2-propenyl substituent increased its antioxidant potential, which
in aqueous solutions may occur through an electron transfer and proton transfer mechanism
and, in addition, in an alcoholic solution through hydrogen atom transfer preferentially
at 4-OH.
Another research direction in focus in this special edition aims to elucidate the
mechanisms of protection of natural compounds and/or to explore new sources of health-promoting
compounds. Among the published works, phenolic compounds or derivatives were spotlit
because of their antioxidant abilities, albeit other potential bioactivities such
as cytoprotective, antiproliferative and cytotoxic ones were also investigated. Notably,
Das and coworkers [5] elucidated multiple protective mechanisms involving the phenolic
(catecholic) diterpene carnosic acid against cadmium-provoked nephrotoxicity. As for
the natural sources rich in phenolic compounds, these included distinct parts of plants
(the root, xylem, phloem, petiole, leaves and bud of Boehmeria nivea L. [6]), with
emphasis on medicinal plants (aerial parts of Salvia africana, Salvia officinalis
‘Icterina’ and Salvia mexicana [7], leaves of Sorbus domestica [8], leaves of Cotoneaster
zabelii, Cotoneaster bullatus and Cotoneaster integerrimus [9], leaves of Mahonia
bealei (Fort.) Carr [10], leaves of Vaccinium vitis-idaea L. [11], flowers of Astragalus
membranaceus var. mongholicus [12], rhizomes of Reynoutria japonica, Reynoutria sachalinensis
and Reynoutria x bohemica [13], fruits of Lycium barbarum L. [14]), the pericarp of
Dimocarpus longan Lour. [15], the resin of the tree species Croton lechleri (Müll.
Arg) [16] and a herbal standardized product containing an extract of persimmon leaves
[17]. In parallel, nonphenolic compounds, including monoterpes and their derivatives
from three mentha species [18], alkaloids from Uncaria tomentosa [19], polysaccharides
from white ginseng [20] and protein hydrolysates from walnut meal [21], were also
demonstrated to hold the potential to be used as natural antioxidants and/or to fight
oxidative stress related disorders.
Attention was also given to the impact of distinct factors on the levels of relevant
natural compounds, including bioactive components of plants/vegetables. For example,
the work of Liu et al. [22] demonstrated that sunny hours and temperature were the
main drivers affecting the accumulation of Cyclocarya paliurus phenolics and their
antioxidant properties. In addition, Guan and coworkers [23] concluded that the combination
of methyl jasmonate treatment with wounding stress could stimulate phenolic accumulation
in broccoli. Moreover, Pereira et al. [24] described the variable effects of commercial
biostimulants and irrigation regimes on the chemical composition and bioactive properties
of two spinach genotypes, alerting one to the need for further research in order to
make solid conclusions on the effects of the use of biostimulants under water stress
conditions.
The application of natural compounds as ingredients in novel products to emphasize
specific characteristics or new strategies to improve the quality and antioxidant
properties of health-promoting natural products/bioactive compounds were also addressed
by some contributing authors. This included the formulation of a novel fermented glutinous
rice product with the supplementation of Fu brick tea (i.e., a beverage processed
through the postfermentation of Camellia sinensis L.) to increase its sensorial features,
the levels of phenolics, and its antioxidant and DNA protective activities [25]; the
supplementation of pasta flour with salmon fish powder to manipulate the glycaemic
index, protein digestibility, release of phenolic compounds and antioxidant capacity
of the digested pasta [26]; and the use of cold saponification on commercial natural
soaps manufactured from plant oils and additives in order to retain unsaponified fatty
acids, phenolic compounds and antioxidant activities in the final products [27]. In
addition, Wu et al. [28] explored a new strategy for ameliorating the stability and
enhancing the stability, solubility and safety of resveratrol through the preparation
of novel resveratrol transfersomes.
Campos et al. [29] provided an overview on the valorization of fruit byproducts from
food-processing industries in order to overcome a global problem, highlighting the
application of sustainable and green methodologies for the conversion of fruit waste
into high-value products with a significant biological activity. Santos and Silva
[30] summarized different prenylation patterns of natural and synthetic flavonoids
that have been focused on in the past two decades, aiming at the elucidation of structure-antioxidant
activity relationships and the development of efficient routes for the synthesis of
natural derivatives. All of the studies published over the last decade on the relationship
between moderate alcohol consumption and coronary heart disease were revised by Castaldo
et al. [31], who summarized the various red wine components and the putative mechanisms
that influence their activity. Cione et al. [32] described four polyphenols used as
nutritional supplements: quercetin, epigallocatechin gallate, curcumin and resveratrol,
summarizing the current knowledge about them, ranging from dietary sources to human
microRNA modulation.