Potentiometric and (1)H NMR studies of complexation of Al(3+) with (-)-epigallocatechin gallate, a major active constituent of green tea.
The acid dissociation of (-)-epigallocatechin gallate (abbreviated as egcg) and its complexation with Al(3+) were studied by potentiometric titrations, and were compared with those of (-)-epicatechin (ec) and (-)-epigallocatechin (egc). In Al(3+)-ec and Al(3+)-egc reaction systems, [Al(LH(-2))](+), [Al(LH(-2))(OH)](0), and [Al(LH(-2))(2)](-) are formed, as reported for Al(3+)-catechin (c). Reactions between Al(3+) and egcg at pH <4.1 yield AlLH(-2) and AlLH(-3) species. The 1H NMR studies have shown that two hydroxyl groups of the gallate (D) ring are deprotonated and coordinated to an Al(3+) ion in [Al(egcgH(-2))](+). The AlLH(-3) species of egcg is supposed to be formulated as [Al(egcgH(-3))](0) in which one hydroxyl group of the pyrogallol (B) ring and two hydroxyl groups of the D ring are deprotonated; an Al(3+) ion is coordinated to two oxygen atoms of the D ring and one oxygen atom from the B ring of the neighboring chelate molecule, resulting in the formation of a polymeric structure. In the Al(3+) complex of egcg, the gallate group forms major coordinate bonds and results in solution properties that are different from those of ec, egc and c which have no gallate group.
Inoue
Michiko B
MB
Department of Chemistry, University of Arizona, Tucson, AZ 85721-0041, USA. minoue@u.arizona.edu
Inoue
Motomichi
M
Fernando
Quintus
Q
Valcic
Susanne
S
Timmermann
Barbara N
BN
D000535:Aluminum / Q000737:chemistry*; D000970:Antineoplastic Agents / Q000737:chemistry; D002392:Catechin / Q000031:analogs & derivatives / Q000737:chemistry*; D004563:Electrochemistry; D008024:Ligands; D019906:Nuclear Magnetic Resonance, Biomolecular; D009942:Organometallic Compounds / Q000737:chemistry; D013662:Tea / Q000737:chemistry*; D017186:Titrimetry
D000970:Antineoplastic Agents; D008024:Ligands; D009942:Organometallic Compounds; D013662:Tea; D002392:Catechin; C045651:epigallocatechin gallate; D000535:Aluminum