Sn(II) binds to kaempferol (HKaem, 3,4 ,5,7-tetrahydroxy-2-(4-hydroxyphenyl)-4H-1benzopyran-4-one) at the 3,4-site forming [Sn(II)(Kaem) 2 ] complex in ethanol. DPPH • scavenging efficiency of HKaem is dramatically decreased by SnCl 2 coordination due to formation of acid inhibiting deprotonation of HKaem as ligands and thus reduces the radical scavenging activity of the complex via a sequential proton-loss electron transfer (SPLET) mechanism. Moderate decreases in the radical scavenging of HKaem are observed by Sn(CH 3 COO) 2 coordination and by contact between Sn and HKaem, in agreement with the increase in the oxidation potential of the complex compared to HKaem, leading to a decrease in antioxidant efficiency for fruits and vegetables with Sn as package materials. be increased by the presence of copper (II), iron (III), and zinc (II) by other authors and our recent studies [7,8,[23][24][25][26]. In the present study, HKaem was selected as a typical flavonoid with two possible chelation sites (3,4 and 4,5) for metal ions, and complex formation by Sn(II) coordination to HKaem was investigated together with the complex as a radical scavenger. The effect on radical scavenging efficiency of HKaem by Sn(II) coordination is of relevance as Sn is used for packaging of food, and accordingly deserves more attention.Molecules 2020, 25, x FOR PEER REVIEW
Six structurally related flavonoids including two flavones apigenin and luteolin, one isoflavone genistein, and three flavonols, galangin, kaempferol and quercetin, are affected differently as radical scavengers by presence of Ca(II). Except for apigenin showing no reaction with Ca(II), other flavonoids all coordinate with Ca(II) to form 1 : 2 Ca(II)‐flavonoid complexes confirmed by UV‐vis and mass spectra. Coordination with Ca(II) for luteolin with the 3′, 4′‐catechol group and for flavonols with 3‐hydroxyl and 4‐carbonyl significantly enhance the DPPH• scavenging efficiencies shown by stopped‐flow spectra. Addition of Ca(II) shows no effect on radical scavenging rate of Genistein, as an isomer of apigenin, although both containing the same 4‐carbonyl and 5‐hydroxyl as a potential chelating group as confirmed by mass spectra and cyclic voltammetry. The two structure elements, 3′, 4′‐catechol and the combination of 3‐hydroxyl and 4‐carbonyl, are identified as important factors for Ca(II) enhancement of radical scavenging of flavonoids. Coordination of Ca(II) to kaempferol forming a 1 : 2 Ca(II)‐kaempferol complex at 3,4 site shows the most significant enhancement in radical scavenging efficiency by a factor of 4.4 compared to other investigated flavonoids.
The interactions
of luteolin (Lut) with bovine serum albumin (BSA)
mediated by Cu(II) were investigated by spectroscopic, calorimetric,
and molecular dynamic (MD) methods. Fluorescence studies showed that
the binding of Lut to BSA was significantly enhanced by Cu(II) coordination
with the number of binding sites and binding constant increasing from
n
= 1 and
K
a
= 3.2 × 10
5
L·mol
–1
for Lut to
n
= 2 and
K
a
= 7.1 × 10
5
L·mol
–1
for a 1:1 Cu(II)–luteolin
complex, in agreement with the results from isothermal titration calorimetry
(ITC). Site-specific experiments with warfarin and ibuprofen and MD
confirmed that two binding sites of BSA were sequentially occupied
by two Cu(II)–luteolin complexes. Cu(II) coordination increased
the antioxidant activity of luteolin by 60% in the inhibition of carbonyl
formation from the oxidation of amino groups in the side chain of
BSA induced by the peroxyl radical ROO
•
; however,
it counteracted the antioxidant effects of luteolin and played pro-oxidative
roles in BSA aggregation induced by
•
OH.
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