Review A Comprehensive Review of Phytochemistry and Biological Activities of Quercus Species 1, 2, 2, Ema Burlacu y, Adrian Nisca y and Corneliu Tanase * 1 Residency Department, “George Emil Palade” University of Medicine, Pharmacy, Sciences and Technology of Târgu Mures, , 38 Gheorghe Marinescu Street, Târgu Mures, , 540139 Mures, , Romania; [email protected] 2 Department of Pharmaceutical Botany, “George Emil Palade” University of Medicine, Pharmacy, Sciences and Technology of Târgu Mures, , 38 Gheorghe Marinescu Street, Târgu Mures, , 540139 Mures, , Romania; [email protected] * Correspondence: [email protected]; Tel.: +40-744-2155-43 These authors share the first authorship. y Received: 20 July 2020; Accepted: 17 August 2020; Published: 19 August 2020 Abstract: The Quercus genus provides a large amount of biomaterial with many applications in fields like pharmaceutics, cosmetics, and foodstuff 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. Keywords: Quercus; oak; phenolic compounds; bark; acorn; antioxidant; antibacterial; antitumoral 1. 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 [1–3]. 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 Forests 2020, 11, 0904; doi:10.3390/f11090904 www.mdpi.com/journal/forests Forests 2020, 11, 0904 2 of 24 Forests 2020, 11, x FOR PEER REVIEW 2 of 21 are able to tolerate some degree of flooding (Q. lyrata and Q. laurifolia). The most common deciduous broad-leavedbroad‐leaved trees trees are areQ. Q. ilex ilex, Q., Q. dentata dentata, Q., Q. acutisssima acutisssima, Q., Q. variabilis variabilis, ,Q. Q. acuta acuta,,Q. Q. glauca glauca,, Q.Q. serrataserrata and Q. salicinaQ. salicina. The. most The most studied studied oaks oaks are the are European the European oaks, oaks, principally principallyQ. robur Q. robu(englishr (english oak) oak) and Q.and petraea Q. (sessilepetraea oak (sessile or durmast oak or durmast oak) [4,5 oak)]. [4,5]. TheTheQuercus Quercusspecies species produce produce a a universally universally known fruit fruit (acorn) (acorn) and, and, together together with with bark bark and and leaves,leaves, has has been been used used in in traditional traditional medicine, medicine, applied as as antiseptic antiseptic or or in in gastrointestinal gastrointestinal disorders. disorders. DueDue to to their their nutritional nutritional role role acorns are are used used as as food food for for both both humans humans and and animals. animals. The oak The wood oak woodhas important role in wine maturation in oak barrels and in the wood industry for the wood color, has important role in wine maturation in oak barrels and in the wood industry for the wood color, durability, and protection against fungal decay [1,6–8]. durability, and protection against fungal decay [1,6–8]. Large areas of oak forests (Europe, Asia, North Africa, North America, Central and South Large areas of oak forests (Europe, Asia, North Africa, North America, Central and South America), America), large amounts of forest waste (oak bark and leaves) resulting from wood processing, high large amounts of forest waste (oak bark and leaves) resulting from wood processing, high availability availability and their drought resistance make Quercus species important sources of bioactive andcompounds. their drought In the resistance same time, make theQuercus diversityspecies of folk importantuses and the sources richness of of bioactive phytochemicals compounds. found In in the sameQuercus time, thespecies diversity make of this folk genus uses andinteresting the richness for assessing of phytochemicals biological found activities in Quercus and toxicologicalspecies make thiseffects. genus interesting for assessing biological activities and toxicological effects. TheThe aim aim of of this this review review is tois updateto update the the literature literature data data (2010–June (2010–June 2020) 2020) about about phytochemistry phytochemistry and biologicaland biological activities activities of Quercus of speciesQuercus and species to discuss and theirto discuss potential their as antioxidants,potential as antioxidants, antimicrobials, anti-inflammatories,antimicrobials, anti and‐inflammatories, other activities. and other activities. 2. Phytochemistry2. Phytochemistry TheTheQuercus Quercusspecies species have have been been reported reported toto containcontain several phytoconstituents phytoconstituents with with significant significant diffdifferenceserences between between species species due due to to the the high high variability.variability. Even so, so, there there are are some some classes classes of of compounds compounds thatthat are are ubiquitous ubiquitous in in all allQuercus Quercusspecies species (Figure (Figure1 1).). TheThe mainmain bioactive phytochemicals phytochemicals are are phenolic phenolic compounds,compounds, commonly commonly found found as glycosidesas glycosides [9]. [9]. Other Other compounds compounds that that can becan found be found in Quercus in Quercusspecies arespecies volatile are organic volatile compounds, organic compounds, vitamins vitamins (especially (especially vitamin vitamin E), sterols, E), aliphaticsterols, aliphatic alcohols alcohols and fatty acidsand [ 9fatty,10]. acids [9,10]. FigureFigure 1. 1.The The main main phytochemical phytochemical compoundscompounds and biological biological activities activities of of QuercusQuercus sp.sp. PhenolicPhenolic compunds compunds (Figure (Figure2 )2) are are one one of of the the largest largest groupsgroups of secondary metabolites metabolites in in the the plantsplants with with great great importance due due to to their their occurrence occurrence and the and pharmacological the pharmacological properties properties [11]. These [11 ]. Thesecompounds compounds show show a large a large diversity diversity of structures of structures from from simple simple molecules molecules (e.g., (e.g.,phenolic phenolic acids) acids) to topolyphenols (e.g., (e.g., stilbenes, stilbenes, flavonoids, flavonoids, and and derived derived polymers) polymers) [12]. [ 12These]. These compounds compounds protect protect the plants against some herbivores and affect positively mammals (humans included) due to the Forests 2020, 11, 0904 3 of 24 theForests plants 2020 against, 11, x FOR some PEER REVIEW herbivores and affect positively mammals (humans included) due3 of to 21 the antioxidant, antimicrobial, anti-inflammatory and anticarcinogenic activities [13]. Even though, 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 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 of the antioxidant activity, which has several proposed mechanisms of actions. The general idea is phenolicthat phenolic compounds compounds may provide may provide an electron an electron to be to donated be donated or the or the whole whole hydrogen hydrogen atom, atom, from from the O–Hthe bond, O–H tobond, be transferred to be transferred to free to radical free radical molecules, molecules,