A Comprehensive Review of Phytochemistry and Biological Activities of Quercus Species

: The Quercus genus provides a large amount of biomaterial with many applications in ﬁelds like pharmaceutics, cosmetics, and foodstu ﬀ areas. Due to the worldwide dissemination of the genus, many species were used for centuries in traditional healing methods or in the wine maturing process. This review aims to bring together the results about phytoconstituents from oak extracts and their biological applicability as antioxidants, antimicrobial, anticancer, etc. The literature data used in this paper were collected via PubMed, Scopus, and Science Direct (2010–June 2020). The inclusion criteria were papers published in English, with information about phytoconstituents from Quercus species (leaves, bark and seeds / acorns) and biological activities such as antioxidant, antibacterial, antiobesity, anti-acne vulgaris, antifungal, anticancer, antiviral, antileishmanial, antidiabetic, anti-inﬂammatory. The exclusion criteria were the research of other parts of the Quercus species (e.g., galls, wood, and twigs); lack of information about phytochemistry and biological activities; non-existent Quercus species reported by the authors. The most studied Quercus species, in terms of identiﬁed biomolecules and biological activity, are Q. brantii , Q. infectoria and Q. robur. The Quercus species have been reported to contain several phytoconstituents. The main bioactive phytochemicals are phenolic compounds, volatile organic compounds, sterols, aliphatic alcohols and fatty acids. The, Quercus species are intensely studied due to their antioxidant, anti-inﬂammatory, antimicrobial, and anticancer activities, provided by their phytochemical composition. The general conclusion is that oak extracts can be exploited for their biological activity and can be used in research ﬁelds, such as pharmaceutical, nutraceutical and medical.


Introduction
The Quercus genus is an evergreen or deciduous tree, belonging to the Fagaceae family. These genus contain about 450 species and represent an important tree group widespread in Europe, Asia, North Africa, North, Central and South America. The white oaks (section Quercus), live oaks (series Virentes) golden cup or intermediate oaks (section Protobalanus) and red oaks (section Lobatae) are present in America. The cycle cup oaks (subgenus Cyclobalanopsis) are found in Asia and the white oaks along with black oaks (section Cerris) are extended into Eurasia [1][2][3].
Quercus species are widespread in Northern Hemisphere, in temperate seasonally dry forests, and tend to be distributed in well-drained upland areas and often in montane areas and some species Phenolic compunds (Figure 2) are one of the largest groups of secondary metabolites in the plants with great importance due to their occurrence and the pharmacological properties [11]. These compounds show a large diversity of structures from simple molecules (e.g., phenolic acids) to polyphenols (e.g., stilbenes, flavonoids, and derived polymers) [12]. These compounds protect the plants against some herbivores and affect positively mammals (humans included) due to the Phenolic compunds (Figure 2) are one of the largest groups of secondary metabolites in the plants with great importance due to their occurrence and the pharmacological properties [11]. These compounds show a large diversity of structures from simple molecules (e.g., phenolic acids) to polyphenols (e.g., stilbenes, flavonoids, and derived polymers) [12]. These compounds protect the plants against some herbivores and affect positively mammals (humans included) due to the antioxidant, antimicrobial, anti-inflammatory and anticarcinogenic activities [13]. Even though, phenolic compounds display a large variety of biological activities, most of them are secondary effects of the antioxidant activity, which has several proposed mechanisms of actions. The general idea is that phenolic compounds may provide an electron to be donated or the whole hydrogen atom, from the O-H bond, to be transferred to free radical molecules, thus transforming the free radicals into harmless species. This process also transforms the phenolic into a radical, with an odd electron, but because of the aromatic structure the odd electron has the possibility to be spread over the entire molecule, resulting in a radical stabilization [14].
Forests 2020, 11, x FOR PEER REVIEW  3 of 21 antioxidant, antimicrobial, anti-inflammatory and anticarcinogenic activities [13]. Even though, phenolic compounds display a large variety of biological activities, most of them are secondary effects of the antioxidant activity, which has several proposed mechanisms of actions. The general idea is that phenolic compounds may provide an electron to be donated or the whole hydrogen atom, from the O-H bond, to be transferred to free radical molecules, thus transforming the free radicals into harmless species. This process also transforms the phenolic into a radical, with an odd electron, but because of the aromatic structure the odd electron has the possibility to be spread over the entire molecule, resulting in a radical stabilization [14]. The investigation of phenolic compounds Quercus species shows their presence in all organs (leaves, bark, and acorn). There is a complex variation in the production of individual phenolic compounds across Quercus species. Significant differences exist between absolute and relative concentration of polyphenols. For example, the ellagitannin production in white Quercus leaves may be characterized as products of castalagin and vescalagin [15], whereas in the black Quercus species these compounds are in relatively lower amounts without derivatives. These differences between oak phenolic compounds explain ecological differences between species [15].
The phenolic compounds identified in the Quercus species are mostly flavonoid and nonflavonoid phenolic constituents that intermediate the phenylpropanoid metabolism by shikimate pathway [16]. The resulting hydroxycinnamic acids and esters participate in reductase, oxygenase and transferase processes, resulting in a characteristic pattern of secondary metabolites for each plant species. For example, flavan-3-ols, flavonols and acylated kaempferol glucosides were identified and quantified in Q. ilex extracts [17]. Below, the main bioactive compounds present in the Quercus genus, at the level of bark, leaves and acorns, will be presented.
The Quercus bark can be considered a possible renewable source of bioactive compounds due to the large quantities of polyphenols. The main phenolic compounds identified in Quercus bark extracts (Table 1) are phenolic acids (e.g., caffeic acid, ellagic acid, gallic acid, and protocatechuic acid), tannins (ellagitannins, roburins) and flavonoids. The Quercus species in which the phenolic profile of the bark extracts was analyzed are Q. acutissima, Q. alba, Q. macrocarpa, Q. petraea, Q. robur and Q. The investigation of phenolic compounds Quercus species shows their presence in all organs (leaves, bark, and acorn). There is a complex variation in the production of individual phenolic compounds across Quercus species. Significant differences exist between absolute and relative concentration of polyphenols. For example, the ellagitannin production in white Quercus leaves may be characterized as products of castalagin and vescalagin [15], whereas in the black Quercus species these compounds are in relatively lower amounts without derivatives. These differences between oak phenolic compounds explain ecological differences between species [15].
The phenolic compounds identified in the Quercus species are mostly flavonoid and non-flavonoid phenolic constituents that intermediate the phenylpropanoid metabolism by shikimate pathway [16]. The resulting hydroxycinnamic acids and esters participate in reductase, oxygenase and transferase processes, resulting in a characteristic pattern of secondary metabolites for each plant species. For example, flavan-3-ols, flavonols and acylated kaempferol glucosides were identified and quantified in Q. ilex extracts [17]. Below, the main bioactive compounds present in the Quercus genus, at the level of bark, leaves and acorns, will be presented.
The Quercus bark can be considered a possible renewable source of bioactive compounds due to the large quantities of polyphenols. The main phenolic compounds identified in Quercus bark extracts (Table 1) are phenolic acids (e.g., caffeic acid, ellagic acid, gallic acid, and protocatechuic acid), tannins (ellagitannins, roburins) and flavonoids. The Quercus species in which the phenolic profile of the bark extracts was analyzed are Q. acutissima, Q. alba, Q. macrocarpa, Q. petraea, Q. robur and Q. sideroxyla (Table 1). Hosam et al. [22] showed the presence in bark extracts of catechin, caffeic acid, ellagic acid, epicatechin, epigallocatechin, epigallocatechin gallate, gallic acid, and in Q. acutissima, the presence of caffeic acid, ellagic acid, epicatechin, epigallocatechin, gallic acid and protocatechuic acid in Q. macrocarpa and the presence of catechin, ellagic acid, gallic acid, protocatechuic acid and vanillic acid in Q. robur [22].
The Quercus genus may also contain other compounds, like fatty acids, for example, palmitic, oleic and linoleic acids being the most abundant fatty acids in the acorns ( Table 1). The kernel oil obtained from Q. robur and Q. cerris also rich in these fatty acids, stearic, arachidic and α-linolenic acids being also present, but in smaller quantities [30].

Biological Activity of Quercus Extracts
The ethnobotanical data show that Quercus spp. can be valuable plants, especially for the treatment of gastrointestinal disorders, skin and urinary tract infections (e.g., Q. ilex, Quercus oblongata D. Don) [32,33]. The Quercus acorns are edible (animal feed or human diet), astringent and diuretic, used in diarrhea, indigestion and asthma [23]. The wood can be used for timber, fuel or in agricultural tools (handles of plough, axes, gun buts, and walking sticks) [34,35]. Starting from ethnobotanical results, the researchers performed numerous experiments to demonstrate the biological activity of phytochemicals found in Quercus species.

Antioxidant Activity
Due to the biomolecules variety present in different parts of oaks, their power as an antioxidant and their beneficial aspects in multiple applications has been reported [36].
Some studies showed a highly and significant correlation between phenolic contents and antioxidant activity and that may be the reason for stronger radical scavenging activities [37]. Nedamani et al. [38] concluded that antioxidant activity showed by Q. brantii extract's reducing power is related to their total phenolic content, and Tuyen et al. [39] showed a strong correlation between phenolic content in Q. mongolica ssp. crispula leaves and bark extract and its antioxidant power. Many other studies showed the antioxidant effect of oak tannins in somatic cells beside other effects [40,41]. Kim et al. [42] reported that methanolic extract of Q. acuta showed the highest radical scavenging activity and total phenolic content, while the reducing power was the highest in the water extract. Along with the type of extract, it was showed that the radical scavenging activity was increased by increasing concentrations of Q. brantii leaf extract [43].
As to applicability, Ferreira et al. [44] revealed the protective effect of acorn extract of Q. ilex against oxidative degradation of lipids and proteins carbonylation. These effects are probably related to the intense antioxidant activity of polyphenols from acorns and so they may be used as preservatives in the alimentary industry, in nutraceutical and pharmacology activities. Another applicability of antioxidant power is noted by Horvathova et al. [45]. The Q. robur extract showed stimulation of antioxidant enzymes, decreasing damage to proteins and lipids, and a moderate increase in the total antioxidant capacity of plasma on human subjects. The authors concluded that the extract can be used as a natural supplement for improving the life quality in humans.
The most common antioxidant compounds in Quercus species are gallic and ellagic acid. Gallic acid (3,4,5-trihydroxybenzoic acidis) is a most popular phenolic compound, a natural antioxidant that is basically a secondary metabolite [46]. The ellagic acid is a polyphenol that occurs largely in woody eudicotyledons plants [47]. This phenolic acid is one of the highly investigated phytochemicals, with antioxidant, antimutagenic, and anticancer properties [48]. Thus, the antioxidant activity of Quercus extract can be attributed and to gallic and ellagic acid. Other common phenolic compounds from Quercus species are ellagitannins, including castalagin, vescalagin and roburin. It has been previously demonstrated that these compounds have potent antioxidant activity [49]. Methanol extract Alkaloids, flavonoids, steroids, tannins -Analgesic activity [80,81] Water extract --Anti-inflammatory activity and non-toxic activity on acute and chronic administration in rats [82] Bark Acetone extract Phenolic compounds -Anti-quorum sensing activity-reduced the pyocyanin, protease, elastase production and biofilm formation in Pseudomonas aeruginosa [83] Acorn Hidroalcoholic axtract --Antiangiogenic activity-the extract prevented the formation of endothelial tubular structures by endotelial cells, inhibited the ability of cellular migration and decreased the vascular endothelial growth factor secretion [84] Acetone and methanol extract --Antibacterial activity of acorn extract in combination with Vancomycin against two strains of MRSA. The time-kill curves showed a faster killing rate than Vancomycin alone and a synergic interaction [85] Ethanol extract --Antifungal activity against Candida albicans compared with fluconazole [86] Aqueous extract --Antibiofilm activity against Pseudomonas aeruginosa [87] Ethanol extract Phenolic acids (tannic acid, gallic acid, ellagic acid), flavonoids -Anti-inflammatory activity-extract treatment ameliorated the inflammatory phenotype of bone marrow derived macrophages induced by prediabetic or diabetic environments [88] Water extract Gallic acid, syringic acid -Antiosteoporothic-Q. infectoria semipurified fractions combined with osteoporotic drug pamidronate induce bone formation in human fetal osteoblast cell model, and increase the efficiency of pamidronate acting on osteoblast cell.

Antibacterial and Antifungal Activity
Since ancient times, the traditional use of oak bark in medicine field was well known and applied topically to burns and wounds to prevent infection, or applied orally for gastrointestinal diseases. The Quercus extract (especially bark extract) contains important antimicrobial compounds such as gallic acid, ellagic acid, vescalagin or castalagin [119,120]. Due to its history of antibacterial uses, Deryabin and Tolmacheva [101] used Q. robur cortex against Chromobacterium violaceum. The results showed an anti-quorum sensing effect determined by the extract's bioactivity.
The antimicrobial activity is sustained by diverse results of experiments reported against Streptococcus aureus, Acinetobacter baumannii, Klebsiella pneumoniae, Pseudomonas aeruginosa. Thus, Q. alba bark extracts inhibited growth in the strains of Acinetobacter baumannii and growth of Staphylocccus epidermidis and showed antibiofilm activity against Staphylococcus aureus [54]. Not only the oak bark but the leaves of Q. brantii have antibacterial and antifungal activity, as shown by Tahmouzi [43]. Acorn extract of Q. brantii showed antifungal activity against vaginal candidiasis [60] and Q. floribunda acorn extract showed mildly antibacterial activity against Bacillus subtilis, Escherichia coli, Klebsiella pneumonie and Staphylococcus aureus.
The antibacterial and antifungal activity can be the result of extract type, harvesting time, oak tree species, and the instrument quality and measurement methods. Tayel et al. [86] showed the antifungal activity against Candida albicans of the Q. infectoria acorn ethanolic extract. The acorns are concentrated in tannins (ellagitannins) more than other parts of oak trees and the antibacterial and antifungal activity can be attributed to these compounds [76]. Tannins have been shown to inhibit growth in a wide range of bacteria, fungi and viruses, inactivating the microbial enzymes, sequestering essential metal ions in complexes and inhibition of membrane transport [121]. Zhou et al. [118] showed antibacterial activity of Q, variabilis acorn extract, destroying the bacteria's wall cell and inhibits their normal growth and cellular metabolism. Khairon et al. [79] showed the antibacterial activity against MRSA of Q, infectoria acorn water extract through the interference of various metabolisms and functions of the bacterial proteome. Tannins also act as biofilm inhibitors by binding to matrix proteins [122]. Chusri et al. [76] highlighted the anti-biofilm activity on MRSA and MSSA due to Q. infectoria extract effect on Staphilococcus aureus cell wall and cell surface hydrophobicity.
Another use of oak extracts is in the cosmetic and pharmaceuticals industries [90], or in alimentary industry. The Quercus extract can be applied as a natural disinfectant and decontaminant for chicken eggs or for helping farmers to avoid fungicides due to their human and environmental hazards [97].

Anti-Inflammatory and Anticancer Activity
Along the time, more anticancer treatment were developed, but recently the evolving of cancer resistancy and side effects of the chemoterapic therapy, demands new chemicals with high potency, low side effects, and high selectivity at molecular level [75].
The Quercus genus is intensely studied due to its high potency against inflammation and proliferation of cancer cells. The main anticancer compounds identified in Quercus species are ellagic acid, kaempferol and its glycosides, quercetin, myricetin [123][124][125][126]. Farhad et al. [57] describe antiproliferative activity of Q. brantii crude extract that suppresses the proliferation of cancer cells through induction of early apoptosis as mechanism. The betulin compound found in Q. incana leaves extract by Hasan et al. [75] has completely eradicated large cells' lung cancer even after 15 days of incubation in culture media during colony formation assay compared to the control. Different cancer cells as MCF-7 (breast cancer cell line), HeLa (cervical cancer cells), Jurkat (human T lymphocyte cells), HT-29 (human colon cancer cell line), HEK 293 (human embryonic kidney cells) and T24 (human urinary bladder cancer cells) were tested by Elansary et al. [22] with great regression done by Q. macrocarpa and Q. robur bark extracts.
There are several studies showing anti-inflammatory activity. Castejón Martínez et al. [72] describe that phenolic extracts from Q. ilex leaves decreased neutrophil infiltration and reduced the inflammatory citokines in TNBS-induced ulcerative colitis rats. Chokpaisarn et al. [88] have described the Q. infectoria acorn extract amelioration of inflammatory phenotype of bone marrow-derived macrophages induced by prediabetic or diabetic environments, potentially by inhibiting the Set-7/NF-κB pathway.
The phenolic compounds (e.g., ellagic acid) improves anti-inflammation through isolated compounds from Q. mongolica bark extract which showed inhibitory activities towards inflammatory cytokines and chemokines induced in ultraviolet B (UVB)-irradiated keratinocites by increasing the cell migration ability of cells and enhancing their regeneration when exposed to UVB, and these compounds can be further developed for treating the chronic inflammatory skin diseases, like atopic dermatitis and psoriasis [127].

Antidiabetic Activity
Diabetes mellitus is a metabolic disorder distinguished by a failure of glucose homeostasis with disturbances of carbohydrate, fat and protein metabolism as a result of defects in insulin secretion [128]. Phloridzin is a dihydrochalcone glycoside detected at higher concentrations in Quercus leaves, (e.g., Q. resino) [129]. This compound is recognized for its astringent properties and antidiabetic effects [129]. For example, an effect of Q. coccifera bark extracts is inhibition of α-glucosidase. This effect could be important in the treatment of diabetus mellitus, as Sari et al. [64] and Muccilli et al. [111] concluded. The other antidiabetic phenolic compounds from Quercus species can be ellagic acid, rutin or vescalagin [127,130,131]. The Q. floribunda acorn extract has mild to moderate α-amylase inhibition and this strategy is an essential target to manage blood glucose level in noninsulin dependent diabetes mellitus, as found by Soto et al. [115]. Another way to treat diabetes is observed after 15 days of oral administration of Q. mongolica acorn extract in alloxan-induced diabetic rats by Yin et al. [95]. They reported a hypoglycemic activity, but also the fasting blood glucose, cholesterol and triglyceride levels were significantly decreased.
Thus, the significant antidiabetic effect of the Quercus phenolic compounds could be due to the presence of the phenolic compounds, which could act synergistically or independently to enhance the activity of glycolytic enzymes.

Other Activities
Wine taste is clearly defined by the infusion of phytochemical components found in Quercus species such as Q. petraea, by Sindt et al. [132], who determined that the taste of three compounds extracted from oak bark, that were described for their bitterness and their influence in wine and brandy, increased in bitterness during oak aging. Jiménez-Moreno et al. [106] exhibit more clearly that the wine flavor which was in contact with oak bark displayed a more intense wood and spicy aroma, and more body and persistence in the mouth than the control wine, obtained after applying the toasted oak extract of Q. robur.
Antileishmanial activity against Leishmania tropica by Q. floribunda was studied with interest by Kheirandish et al. [77] who found that promastigotes treated with oak extracts were able to infect only 33.2% of the peritoneal macrophages and their infectiveness reduction was 51.3%. After 4 weeks of Q. infectoria acorn extract treatment, a decrease in the number of parasites compared to the control group was observed. Daryani et al. [61] showed anti-toxoplasma activity of Q. castaneifolia acorn methanolic extract by increasing the survival rate of the mice compared to the mice in the untreated infected control.
Antiadipogenesis is crucial for the prevention of obesity because adipogenesis occurs progressively throughout human life [133]. The antiadipogenic activity was revealed by Q. acutissima acorn extract, which decreased the intracellular lipid droplets in fat tissue and hepatic lipid accumulation and downregulates the diglyceride acyltransferase 2 gene expression as found by Youn et al. [51] and Hwang et al. [52]. The Youn et al. [51] speculated that inhibitory activities of aqueous/methanolic Q. acutissima acorn extracts and particularly the gallic acid, is mediated by regulation of the cell cycle and insulin signaling in the early stage of adipogenesis. Therefore, further study regarding the precise mechanism for the anti-adipogenic activity of acorn shell extracts is required. Antiacne activity was found by Koseki et al. [53], which explained that Q. acutissima bark extract inhibited androgen-related pathogenesis of acne through inhibition of α-reductase activity in testosterone-induced sebum synthesis in rats.
Q. brantii acorn ethanol extract showed antiviral activity against HSV-1 (Herpes Simplex Virus type 1) by modulating the replication mechanism as suggested by Karimi et al. [56].
One of the interesting effects is shown by Q. salicina leaves and bark extract as the vasodilatator effect on porcine endothelium coronary artery, experienced by Park et al. [112]. Q. gilva leaves and bark extract inhibits the development of urolithiasis in the animal models, as described by Youn et al. [69], and Q. infectoria can be used as analgesic for mild pain relief [79] or for its antiosteoporothic activity, shown by Abdullah et al. [89].
The nutritional role is reminded by Paswan and Sahoo [92] for sustaining the minimal level of production (milk, meat) in wintertime by using Quercus oblongata leaves' extract.
Finally we have some reports regarding the toxicity of obtained extracts. The Q. infectoria acorn water extract showed non-toxic activity on acute and chronic administration in rats and no significant adverse effects [82]. One the other hand, the Q. crassifolia bark water extract appears to have toxic activity on kidney and was associated with short-term repeated administration in rats, as Valencia et al. [65] discovered.

Conclusions and Future Directions
This review unified results about biological activities of Quercus extracts and its isolated compounds. Most bioactivity studies were focused on antioxidant, antimicrobial and anticancer effects. Thus, the Quercus extracts are a great source of phytoconstituents, especially polyphenols. The general conclusion of scientists is that these extracts can be exploited for their antioxidants, antimicrobial and anticancer potential activities and can be used in diverse research fields, such as pharmaceutical, nutraceutical, medical, and for improving the wine sensory quality. Most of the pharmacological effects of Quercus genus can be explained by the high amount of phenolic compounds' content, especially tannins and their antioxidant potential. The pharmacological studies have mostly been performed in vitro and in vivo, and clinical studies are very limited. Thus, clinical studies are needed to confirm in vitro and in vivo results for a rational use in phytotherapy. These studies should be continuously developing newer techniques for treating multidrug resistance and quorum sensing activity as a bacterial biofilm formation. New anticancer therapies should be continuously developed because of the tendency of resistance to classical treatments. That is why more studies should concentrate on in vivo experiments. More studies are needed to show the link between the chemical compound and bioactivity and to discuss their action mechanism. Even if Quercus products are generally safe, more toxicological data are needed.
Large areas of oak forests, large amounts of forest waste (oak bark and leaves) resulting from wood processing, high availability and their drought resistance make Quercus species important sources of bioactive compounds.