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Fedenko, V.S.;  Landi, M.;  Shemet, S.A. Plant Phenolics as Ligands for Metal(loid)s. Encyclopedia. Available online: https://encyclopedia.pub/entry/40796 (accessed on 09 July 2025).
Fedenko VS,  Landi M,  Shemet SA. Plant Phenolics as Ligands for Metal(loid)s. Encyclopedia. Available at: https://encyclopedia.pub/entry/40796. Accessed July 09, 2025.
Fedenko, Volodymyr S., Marco Landi, Sergiy A. Shemet. "Plant Phenolics as Ligands for Metal(loid)s" Encyclopedia, https://encyclopedia.pub/entry/40796 (accessed July 09, 2025).
Fedenko, V.S.,  Landi, M., & Shemet, S.A. (2023, February 03). Plant Phenolics as Ligands for Metal(loid)s. In Encyclopedia. https://encyclopedia.pub/entry/40796
Fedenko, Volodymyr S., et al. "Plant Phenolics as Ligands for Metal(loid)s." Encyclopedia. Web. 03 February, 2023.
Plant Phenolics as Ligands for Metal(loid)s
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This entry is adapted from the peer-reviewed paper 10.3390/ijms231911370

Plant adaptive strategies have been shaped during evolutionary development in the constant interaction with a plethora of environmental factors, including the presence of metals/metalloids in the environment. Among adaptive reactions against either the excess of trace elements or toxic doses of non-essential elements, their complexation with molecular endogenous ligands, including phenolics, has received increasing attention.

anthocyanins binding sites complexation flavonoids

1. Introduction

The life processes of plants have evolved in coordination with environmental factors. In addition, intensified anthropogenic load on ecosystems has led to increasing levels of chemical contamination and resulted in the emergence of new pollutants, namely xenobiotics. To understand the peculiarities of plant–environment interactions, it is essential to take into account the environmental and ecological aspects of the problem [1]. Hazardous metals and metalloids are among the major and widespread pollutants due to their high toxicity to the biosphere and the amplitude of their contamination in the natural environment [2]. Mining and metal extraction, fossil fuel combustion, agricultural application of fertilizers, sewage sludge, metal-containing pesticides, wastewater irrigation, and atmospheric deposition are the main anthropic sources of metal(loid)s [3].
A special feature of plants regarding their relations to metal(loid)s is that a certain amount of trace metals is necessary for a number of biologically essential processes (metalloenzymes, mineral nutrition, photosynthesis, prooxidant/antioxidant systems, etc.), whilst both the overdoses of essential and toxic doses of non-essential elements negatively affect plant metabolism. Therefore, to optimize those processes, plants have evolved multiple regulatory and defence mechanisms to counteract metal(loid) toxicity [4][5][6].
Among others, plants detoxify metal(loid)s via their biotransformation into metabolically inactive compounds [7][8]. Among biotransformation reactions, the in vivo chelating of metal ions is pivotal, which is accomplished by endogenous chelators: glutathione (GSH), phytochelatins (PCs), metallothioneins (MTs), organic acids, nicotinamine, amino acids [6][9][10]. An important feature of those bioligands is their capacity to bind various metals [10]. Such a property of binding both essential and non-essential metal(loid)s has been confirmed in vivo for flavonoid pigments of anthocyanins (ACNs) [11][12][13]. Those results allow postulating a hypothesis about the participation of phenolic compounds (PCs) in metal(loid) detoxification in plants [12]. However, a comprehensive systemic analysis of the role of PCs as endogenous chelators in plant metal tolerance has not yet been performed.
It is noteworthy that the current level of metal(loid) tolerance in plants is characterized by the extensive use of the integrated “omics” approach [14][15][16][17][18]. The “omics” approach is aimed at the studying of the organism as a holistic system, based on the integrative analysis of and interrelations among major biological processes [19]. The “omics” research object related to the behaviour of metals in living organism is referred to as the “metallome” [20] and the corresponding research field as “metallomics” [21].
In recent years, the key significance of PCs was confirmed as pivotal and versatile plant defensive compounds against abiotic stresses, including metal(loid) tolerance [5][22][23][24]. Systematizing extensive experimental data resulted in the hypothesis of a universal dominant tendency to increased accumulation of PCs as components of the antioxidant defence system, which ensures the balance between the production and detoxification of reactive oxygen species (ROSs) under metal(loid) exposure [25]. Beside their ROS scavenging prerogative, other possible roles of those secondary metabolites in the metallomics context remain a poorly investigated issue.

2. Plant Phenolics as Ligands for Metal(loid)s

The systematization of the available information on the chelating capacity of plant PCs should be performed in two consecutive stages. In the first stage, it is necessary to analyse the binding of PCs with metal(loid)s’ ions in vitro to establish the structure of the metallocomplexes formed and the key criteria of such a binding. In the second stage, using the identified criteria of binding, it is possible to systematize the experimental results about the PC’s chelation with metal(loid) ions (Men+) in plants in vivo.

2.1. Complexing In Vitro

In the studies of PC–Men+ chelation, two main directions can be distinguished:
(1)
Evaluation of the complexation of individual PCs with Men+, based on the features of the ligands, which are modified due to chelation;
(2)
Assessment of metal chelating ability toward PCs and plant extracts based on the alterations in the absorption of metallochromic indicators.

2.1.1. Individual Phenolic Compounds

Various aspects related to the synthesis, identification of the structure, biological activity, and application of PC–Men+ complexes have been systematized in numerous reviews [26][27][28][29][30][31][32][33]. However, some aspects of this problem remain unclear due to the scarce attention given to the involvement of PCs in the modulation of the metallome
In this regard, researchers analysed the available data on the ability of natural phenolic metabolites to form metallocomplexes or identifying the binding of individual compounds to Men+ in vitro in order to answer the following questions:
(1)
How are metal binding properties manifested for the natural compounds from different PC subclasses, which are formed in the process of plant phenolic metabolism?
(2)
Which structural fragments of PCs are crucial for the complexation?
(3)
Can PCs be considered as universal ligands for multiple Men+?
Researchers systematized available experimental data, and the main results are presented in Table 1 for individual representatives of various plant PC subgroups. The structural formulas of the ligands are shown in Figure 1 with their division into separate subgroups (phenolic acids 1–12, coumarins 14–16, chalcones 17, dihydrochalcones 18, flavanones 19–23, flavanonols 24, 25, flavonols 26–37, flavan-3-ols 38–43, flavones 44–51, isoflavones 52–54, anthocyanidins 55–57, xanthonoids 58, stilbenes 59, curcuminoids 60, lignans 61, flavonolignans 62, lignins, tannins 63–65). For some flavonoids, the data on complexation are combined with their derivatives. The number of Men+ ions, for which the formation of metal complexes is confirmed, is presented regardless of the compounds with different stoichiometric ligand:Men+ ratios, or for one ligand with different Men+ ions (heterometallic complexes), or for one Men+ with different ligands (mixed complexes). For some ligands, radiolabelled complexes are included. Chemical elements, for which the complexation with PC ligands has been established, are provided in Figure 2. It should be noted that researchers analysed only the data on metal complexes with natural PCs; currently, however, numerous studies are being performed on synthetic ligands of a phenolic nature, obtained by structural modification of the binding sites of natural PCs in order to create new effective biologically active substances.
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Figure 1. Structure of individual representative ligands capable of binding metal/metalloid ions from various plant phenolic subgroups.
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Figure 2. The elements confirmed to form phenolic ligand–Men+ complexes (highlighted in red).
Table 1. Complexes of plant phenolic ligands with metal(loid) ions.
Phenolic Ligand Metal(loid) Ion Number of Metal Ions References
Phenolic acid
Hydroxybenzoic acids
Protocatechuic acid 1 Al(III), U(VI) 2 [34][35]
Vanillic acid 2 Zn(II), Y(III), La(III), Ce(III),
Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III), Np(V)		 17 [36][37][38]
Gallic acid 3 Fe(II), Zn(II), Fe(III), Eu(III) 4 [39][40][41][42]
Syringic acid 4 Li(I), Na(I), K(I), Rb(I), Cs(I),
Fe(II), Fe(III), Y(III), La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III)		 22 [43][44][45]
Hydroxycinnamic acids
Cinnamic acid 5 Li(I), Na(I), K(I), Rb(I), Cs(I), Hg(I), Ca(II), Co(II), Ni(II), Cu(II), Zn(II),
Ru(II), Cd(II), La(III), Eu(III), Tb(III), VO(IV), Th(IV)		 19 [46][47][48][49][50][51][52][53]
p-Coumaric acid 6 Li(I), Na(I), K(I), Rb(I), Cs(I), Co(II), Ni(II), Cu(II), Zn(II),Al(III) 10 [33][54][55]
Caffeic acid 7 Li(I), Na(I), K(I), Rb(I), Cs(I), Cu(II), Pb(II), Pt(II), Al(III),
Fe(III), Cr(III), Eu(III)		 12 [33][56][57][58][59]
Ferulic acid 8 Ca(II), Mn(II), Cu(II), Zn(II), Cd(II), Al(III), VO(IV), V(V) 8 [33][48][60][61]
Isoferulic acid 9 Na(I), Mg(II), Mn(II) 3 [62]
Sinapic acid 10 Cu(II), Pt(II), V(V) 3 [57][63]
Chlorogenic acid 11 Li(I), Na(I), K(I), Rb(I), Cs(I),
Ca(II), Zn (II), Fe(III), VO(IV)		 9 [64][65][66][67][68]
Rosmarinic acid 12 Li(I), Na(I), K(I), Rb(I), Cs(I), Ca(II), Cu(II) 7 [67][69][70]
Chicoric acid 13 Co(II), Ni(II), Cu(II), Zn(II) 4 [71]
Coumarins
Coumarin 14 La(III), Ce(III), Nd(III), Sm(III), Dy(III) 5 [59]
Umbellipherone 15 Ce(III) 1 [72]
Daphnetin 16 Cu(II), Zn(II), Ge(IV) 3 [73]
Chalcones
Butein 17 Cu(II), Zn(II) 2 [74]
Dihydrochalcones
Phloretin 18 Ru(III) 1 [75]
Flavanones
Naringenin 19,
naringin 20		 Fe(II), Cu(II), Ni(II), Zn(II), Pt(II), Fe(III), Cr(III), La(III), Y(III), Eu(III), Ce(IV), VO(IV), V(V) 12 [27][28][57][76][77][78]
Eriodictyol 21 Fe(II), Fe(III) 2 [79]
Hesperitin 22,
hesperidin 23		 Ni(II), Cu(II), Zn(II), Al(III), VO(IV), 5 [27]
Flavanonols
Taxifolin 24 Fe(II), Ni(II), Cu(II), Zn(II),
Fe(III)		 5 [28][76][80][81][82]
Dihydromyricetin 25 Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) 6 [83][84]
Flavonols
Kaempferol 26 Fe(II), Cu(II), Zn(II), Pb(II), Fe(III), VO(IV) 6 [27][28][32][85]
Quercetin 27,
rutin 28,
quercitrin 29, isoquercitrin 30		 Mg(II), Ca(II), Sc(II), Mn(II),
Fe(II), Co(II), Ni(II), Cu(II),
Zn(II), Mo(II), Pd(II), Cd(II),
Hg(II), Sn(II), Pb(II), Al(III),
Cr(III), Fe(III), Ga(III), Y(III), Rh(III), Sb(III), La(III), Pr(III), Nd(III), Eu(III), Gd(III), Tb(III), Dy(III), Tm(III), Au(III), Ge(IV), Zr(IV), Ru(IV), Sn(IV), Os(IV), Cr(VI), Mo(VI), W(VI), Tc(VII), Os(VIII), VO(IV), UO2(II),		 43 [27][28][29][32][86][87][88][89][90][91][92]
Isorhamnetin 31 Fe(II), Cu(II) 2 [93]
Tamarixetin 32 Fe(II), Cu(II) 2 [93]
Fisetin 33 Fe(II), Cu(II), Zn(II), Fe(III), VO(IV) 4 [28][94]
Morin 34 Mg(II), Ca(II), Mn(II), Co(II),
Ni(II), Cu(II), Zn(II), Sr(II), Pd(II),
Cd(II), Ba(II), Sn(II), Pt(II),
Al(III), Cr(III), Fe(III), Au(III), La(III), Eu(III), Gd(III), Lu(III), Zr(IV), VO(IV), Mo(VI), W(VI), Ti(COO)2 2+		 26 [27][28][29][95]
Myricetin 35,
myricitrin 36		 Cu(II), Zn(II), Al(III), Fe(III) 4 [27][29][88]
Galangin 37 Fe(II), Cu(II), Zn(II), Al(III) 4 [85]
Flavan-3-ols
(+)-Catechin 38,
(-)-epicatechin 39		 Fe(II), Cu(II), Zn(II), Hg(II),
Al(III), Fe(III), Cr(III), La(III), Yb(III), Gd(III)		 10 [28][96][97][98][99][100][101][102][103]
(+)-Epigallocatechin 40 Fe(II) 1 [100]
(-)-Epicatechin
3-gallate 41		 Fe(II), Cu(II), Zn(II), Al(III),
Fe(III)		 3 [31][98]
(-)-Epigallocatechin
3-gallate 42		 Fe(II), Mn(II), Cu(II), Zn(II),
Pt(II), Al(III), Fe(III)		 7 [31][98][100][104][105]
Theaflavin 43 Al(III), Fe(III) 2 [106][107]
Flavones
Primuletin 44 Zn(II), Cu(II); Pb(II), Al(III),
Fe(III)		 5 [28][85]
Chrysin 45 Cu(II), Pd(II), Al(III), Fe(III), La(III), Ho(III), Er(III), Yb(III), Ce(IV), VO(IV) 10 [27][28][85]
Apigenin 46 Cu(II), Pb(II), VO(IV) 3 [27][85][108]
Luteolin 47 Mn(II), Fe(II), Cu(II), Al(III), Fe(III), Y(III), Ho(III), Yb(III), Lu(III), VO(IV) 10 [27][28][32][108][109]
Tricetin 48 Fe(II), Fe(III) 2 [79]
Baicalein 49,
baicalin 50		 Fe(II), Cu(II), Fe(III), VO(IV) 4 [27][28][108]
Acacetin 51 Fe(III) 1 [110]
Isoflavones
Daidzein 52 Ce(IV) 1 [28]
Genistein 53 Cu(II), Fe(III) 2 [27][111]
Biochanin A 54 Cu(II), Fe(III) 2 [111]
Anthocyanidins
Cyanidin 55 and its glycosides Cs(I), Mg(II), Ca(II), Mn(II),
Fe(II), Co(II), Ni(II), Cu(II), Sr(II),
Zn(II), Cd(II), Sn(II), Ba(II),
Hg(II), Pb(II), B(III), Al(III),
V(III), Cr(III), Fe(III), Ga(III), As(III), Bi(III), Ge(IV), VO3−, MoO42−, WO42−		 27 [13][112][113][114][115]
Delphinidin 56 and its glycosides Mg(II), Zn(II), Sn(II), Al(III),
Cr(III), Fe(III), Ga(III)		 7 [13][115][116]
Petunidin 57 and its glycosides Mg(II), Sn(II), Al(III), Cr(III), Fe(III), Ga(III) 6 [116][117]
Xanthonoids
Mangiferin 58 Fe(II), Cu(II), Zn(II), Fe(III),
Se(IV), Ge(IV)		 6 [73][118][119]
Stilbenes
Resveratrol 59 Fe(II), Cu(II), Zn(II), Al(III),
Fe(III)		 5 [31][82][120][121]
Curcuminoids
Curcumin 60 Mg(II), Ca(II), Mn(II), Fe(II),
Co(II), Ni(II), Cu(II), Zn(II),
Se(II), Pd(II), Cd(II), Sn(II),
Hg(II), Pb(II), Al(III), Cr(III), Fe(III), Ga(III), Y(III), Ru(III), In(III), Re(III), Sm(III), Eu(III), Dy(III), Au(III), VO(IV), Nb(V)		 28 [82][122][123][124]
Lignans
Secoisolariciresinol diglucoside 61 Ag(I), Ca(II), Fe(II), Ni(II), Cu(II), Pb(II) 6 [125]
Flavonolignans
Silibinin(silybin) 62 Ni(II), Cu(II), Zn(II), Fe(III), Ga(III), VO(IV) 6 [108][126][127][128]
Lignins
Ligno-cellulosic substrate Mn(II), Cu(II), Fe(III) 3 [129]
Tannins
Condensed tannins Fe(II), Cu(II), Zn(II), Al(III) 4 [130][131]
Oenothein B 63 Al(III) 1 [132]
Ellagic acid 64 Mg(II), Ca(II), Mn(II), Fe(II),
Co(II), Cu(II), Fe(III)		 7 [133]
Tannic acid 65 Mg(II), Mn(II), Fe(II), Co(II),
Ni(II), Cu(II), Zn(II), Mo(II),
Cd(II), Al(III), V(III), Cr(III),
Fe(III), Ru(III), Rh(III), Ce(III), Eu(III), Gd(III), Tb(III), Ti(IV), Zr(IV)		 21 [105][134][135][136][137]
Phenolic acids, as the first structural subgroup of the plant PC metabolic pathway, could be divided into hydroxybenzoic and hydroxycinnamic acids depending on the direction of their biosynthesis [138]. Among natural hydroxybenzoic acids, the complexes with Men+ have been identified for protocatechuic acid 1, vanillic acid 2, gallic acid 3, and syringic acid 4. Protocatechuic acid 1, depending on the pH, coordinates with Al(III) and U(VI) ions via the carboxyl group or the ortho-dihydroxyl group [34][35]. For vanillic acid 2, the complexation with 17 Men+ ions has been identified (Table 1). For gallic acid 3, the coordination with Men+ can involve the carboxylate and neighbouring phenolic hydroxyl groups [42]. The largest number of complexes with Men+ (22 ions) among hydroxybenzoic acids was identified for syringic acid 4.
Transformation of cinnamic acid 5 in the shikimate pathway results in the formation of different hydroxycinnamic acids (p-coumaric acid 6, caffeic acid 7, ferulic acid 8, isoferulic acid 9, sinapic acid 10, chlorogenic acid 11, rosmarinic acid 12, chicoric acid 13) [138]. Cinnamic acid 5 forms complexes with 19 Men+ ions using its carboxyl group. In the binding of p-coumaric acid 6 with Men+, its hydroxyl group could additionally be involved. In caffeic acid 7, its o-dihydroxyl group as an additional chelating site increases the ability of this molecule for complexation. The ability to form metallocomplexes has been confirmed for their methoxy derivatives (ferulic acid 8, isoferulic acid 9, sinapic acid 10). The esters of caffeic acid with quinic acid (chlorogenic acid 11), dihydroxyphenyl-lactic acid (rosmarinic acid 12), and tartaric acid (chicoric acid 13) retain the capacity for complexation with Men+.
For the following subgroups, the complexation with Men+ has been exemplified by their representative compounds: coumarins—coumarin 14, umbellipherone 15, daphnetin 16; chalcones—butein 17; dihydrochalcones—phloretin 18 (Table 1, Figure 1).
Flavanones form metallocomplexes in the form of both aglycons (naringenin 19, eriodictyol 21, hesperitin 22) and glycosides (naringin 20, hesperidin 23). For flavanonols, the metal chelating capacity has been confirmed for their derivatives with catechol (taxifolin 24) and gallic (dihydromyricetin 25) fragments.
Among the most-studied PC bioligands are flavonols (kaempferol 26, quercetin 27, rutin 28, quercitrin 29, isoquercitrin 30, isorhamnetin 31, tamarixetin 32, fisetin 33, morin 34, myricetin 35, myricitrin 36, galangin 37) (Table 1, Figure 1). It is noteworthy that the greatest amount of coordinated metals (43 different Men+ ions) has been identified for quercetin 27 and its glycosides (rutin 28, quercitrin 29, isoquercitrin 30). This pronounced capacity of quercetin to chelate metals is associated with its structural features, which determine the possibility of different variants for the interaction with Men+. Thus, the quercetin molecule contains three potential binding sites (Figure 3): (1) between the 3-hydroxy and 4-carbonyl groups in the C ring; (2) between the 5-hydroxy (in A ring) and 4-carbonyl groups (in the C ring); (3) between the 3’- and 4’-hydroxy groups in the B ring [29].
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Figure 3. Possible binding sites of quercetin according to [28][29].
Complexation of flavan-3-ols with Men+ is carried out by catechol and gallic binding sites ((+)-catechin 38, its stereoisomer (-)-epicatechin 39, (+)-epigallocatechin 40, esters with gallic acid–(-)-epicatechin 3-gallate 41, (-)-epigallocatechin 3-gallate 42). In theaflavin 43, Men+ binding may also involve its tropolone moiety [107].
For flavones (primuletin 44, chrysin 45, apigenin 46, luteolin 47, tricetin 48, baicalein 49, baicalin 50, acacetin 51) without 3-hydroxy groups in the C ring, the complexation with Men+ may involve the binding sites between 5-hydroxy (in A ring) and 4-carbonyl (in the C ring) or the catechol and gallic moieties. In this subgroup, the greatest number of metallocomplexes was identified for chrysin 45 and luteolin 47 (each binds 10 various Men+ ions).
Isoflavone ligands are represented by daidzein 52, genistein 53, and its O-methylated derivative biochanin A 54.
Metal chelating capacity has been demonstrated for anthocyanins and their glycosides with two or three hydroxyl groups in the B ring: cyanidin 55, delphinidin 56, petunidin 57. In contrast to other flavonoids, a specific peculiarity of ACNs is a pH-dependent dynamic equilibrium of aqueous solutions between several structural forms, which are capable of Men+ binding [13]. Among these ligands, the greatest number of metal complexes was identified for cyanidin 55 and its glycosides (27 Men+, in cationic and anionic forms).
For xanthonoids, metallocomplex formation was exemplified by mangiferin 58 (glucosylxanthone) and for stilbenes by resveratrol 59.
Among curcuminoids, the most comprehensively studied ligand is curcumin 60, which may bind 28 various Men+ due to its capacity of keto-enol tautomerism.
The ability of lignans for complexation has been confirmed for secoisolariciresinol diglucoside 61 and of flavonolignans for silibinin 62 (10 Men+ ions each).
The metal binding capacity of lignin as a polymeric phenol was studied for a ligno-cellulosic substrate with Mn(II), Cu(II), and Fe(III) ions (Merdy et al., 2003).
The presence of a great number of hydroxy groups in the structure of tannins (oligomeric and polymeric phenols) determines their high capacity for complexation with Men+. This fact has been established for their different forms: condensed tannins (proanthocyanidins), oenothein B 63 (dimeric macrocyclic ellagitannin), ellagic acid 64, tannic acid 65. The latter is one of the most-studied PC ligands (21 Men+).
Thus, the attempt at systematizing the available experimental results revealed that metallocomplexes can be formed by numerous representative ligands from 18 subgroups of plant PCs, and they are capable of binding 69 different Men+ ions (63 chemical elements) in total (Figure 2).

2.1.2. Metal Chelating Ability

The metal chelating ability is recognized as a generally accepted integrated indicator of the complexing capacity of PCs; it is used as one of the indicators in antioxidant assays [139]. The main aspects of this approach were summarized in the reviews [82][140]. The approach is based on the ability of selected metallochromic indicators to form complexes with Men+, which absorb light in the visible wavelengths range. Upon the addition of the tested PC ligand, competitive binding with Men+ occurs, with a subsequent decrease in the absorption, which is expressed as equivalents of standard chelators or the percentage metal chelating [82]. The binding ability of ligands could also be evaluated by stability constants [140]. For example, in Fe chelation, ferrozine and 2,2′-bipyridine are used as metallochromic indicators and EDTA and deferoxamine as standard metal chelators [82][141]. This approach enables the evaluation of the dependence between the structure of the PC ligand and its metal chelating activity; thus, a comparative analysis of this indicator extracts of medicinal plants is possible [141][142].

2.2. Chelating Effects In Vivo

In the studies of in vivo binding between phenolic metabolites and Men+, two aspects should be highlighted: (1) production of blue anthocyanins (ACNs) in blue flowers and (2) the defensive role of chelation in plant tolerance to toxic metal exposure. It is noteworthy that the vast majority of the in vivo studies on this topic are devoted to ACNs as metal chelators. This is due to the fact of the availability of non-destructive methods for the binding identification based on the spectral characteristics of ACN-Men+ complexes in plant tissues [13]. Blue flower coloration is associated with copigmentation of ACNs and the formation of pigment–copigment–Men+ complexes (Yoshida et al., 2009). Copigmentation can be performed with and without the participation of metal ions [143]. Such studies could be systematized according to two directions, which differ in their levels of elucidation of the content and structural organization of the pigment complex. The first direction is the evaluation of various aspects of the formation of non-stoichiometric ACNs’ metallocomplexes, which are stabilized due to copigmentation with caffeoyl or coumaroyl derivatives of quinic acids or glycosylated flavonoids [144]. For Hydrangea macrophylla, Phacelia campanularia, and Tulipa gesneriana flowers, the major pigments of those complexes are the delphinidin glucosides, while the pigments of Meconopsis grandis flowers are primarily composed of cyanidin glucosides [144]. The binding with metal ions (Fe3+, Al3+, Mg2+) is considered as a necessary condition for the formation of those pigment complexes [144].
Another direction is systemic studies, which have resulted in the establishment of the unique structure of metalloanthocyanins. According to the term’s definition, metalloanthocyanin is a self-assembled, supramolecular complex metal-containing pigment, which comprises 6 ACN molecules, 6 flavone molecules, and 2 metal ions [144]. During blue flowers’ colour formation, three major mechanisms can be implemented, i.e., self-association, copigmentation, and metal complexation [144]. To date, the following metallochelates have been isolated and identified from blue flowers: protocyanin (Centaurea cyanus), commelinin (Commelina communis), protodelphin (Salvia paterns), cyanosalvianin (Salvia uliginosa), nemophilin (Nemophila menziesii) [144]. The constitutive components of those pigments are the ACNs having a chelating centre with two (cyanydin) or three (delphinidin) hydroxyls, flavonoid apigenin derivatives, and Mg2+ and Fe2+ ions [144]. In protocyanin, an additional coordination link of Ca2+ ions with flavone molecules has been established [144]. The advantages of the supramolecular structure for plants are the stability of the pigment complex at physiological pH and the increased tolerance to UV radiation, which play an important role in the implementation of the main function of ACNs during plant blooming under sun irradiation. The simultaneous presence of non-associated and chelated ACN molecules explains the phenomenon of purple coloration due to the mixing of the two colour stimuli, red and blue [13]. Different ratios between those ACN forms, when present in vivo, create various superpositions of their colour stimuli, thus resulting in colour variability with different hues of purple plant coloration, a feature that has an important evolutionary significance, as it allows a wide diversity of plant colours and better alignment with pollinators [145][146]. One peculiarity of metallo-anthocyanins is the ability to replace coordinated biogenic Men+ with abiogenic Me ions, while the spectral characteristics of the metallocomplexes are retained. Thus, commelinin-like pigments can be formed by replacing Mg2+ with Cd2+, Zn2+, Co2+, Ni2+, and Mn2+ [144].
The ACNs’ capability of binding various Men+ ions during the formation of pigment complexes in flowers allows hypothesizing that the chelating properties could be engaged for a different purpose—to decrease the toxicity of endogenous metals, thus increasing plant metal tolerance [147]. This hypothesis was confirmed using maize as a metal-excluder plant; the in vivo chelating effect of cyanidin-3-glucoside (Cya-3-glu) in maize root tissues was found for nine exogenous Men+ (Mg2+, Fe2+, Cd2+, Ni2+, Pb2+, Al3+, VO3−, MoO42−, Cr2O72−) [11][148]. The reversible nature of Cya-3-glu–Pb2+ binding was found in maize roots, which can be controlled by manipulating the pH in the root solution [13]. An increase in the Pb2+ concentration in the root nutrient solution resulted in the increased formation of Cya-3-glu–Pb2+ complexes in maize roots in a dose-dependent manner [148].
The formation of ACN–metal complexes in the hypocotyls of Brassica plants was found upon their treatment with MoO42− and WO42− ion solutions [149][150].
The capability of binding Men+ was also demonstrated for other PCs localized in various plant tissues. Thus, the study of ACNs’ distribution over the roots of Lotus pedunculatus Cav. confirmed the hypothesis about metal binding and detoxifying by proanthocyanins in plant vacuoles [151]. Al(III) metallocomplexes with epigallocatechin gallate and proanthocyanins were identified in the leaves, stems, and roots of Camellia sinensis [131] and an oenothein B (dimeric macrocycle ellagitannin) in the roots of Eucalyptus camaldulensis [132]. Cd2+ binding by polymerized polyphenols was demonstrated in the leaves of water plants [152]. According to Rocha et al. [153], the reduction of mercury toxicity in plants can be associated with the chelating activity of gallic acid.
The confirmation of the role of PC ligands in plant–metal homeostasis is the identification of the complexes of Cu(II) with quercetin, luteolin, and syringic acid in the berries of Euterpe oleraceae and Vaccinum myrtyllus [154].
Aluminium stimulates maize plants to secrete into the rhizosphere various endogenous PCs (catechin, catechol, quercetin) capable of complexing with Al3+, thus implementing one of the mechanisms of plant tolerance to the metal excess in the root nutrition medium [155]. The role of root-secreted coumarins was shown in iron-deficient plants by the acquisition of Fe through reduction and chelation [156][157]. It is noteworthy that the binding effects/capacity of the chelators with different structural groups (including PCs) by trace elements are considered as one of the mechanisms of the soil–plant interface [158]. In this relation, it should be highlighted that metal(loid)-induced accumulation of PCs by plants is associated with their protecting role in plant metal tolerance [25].

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Subjects: Plant Sciences
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Volodymyr S. Fedenko
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Marco Landi
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Sergiy A. Shemet
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Update Date: 03 Feb 2023
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