Potentiometric, FTIR and NMR studies of the complexation of metals with theaflavin

Abstract

Aluminium(III) complexes of theaflavin (LH) were studied by potentiometric, UV-Vis, NMR and FTIR spectroscopic methods. The stoichiometries of the main species formed in the aluminium(III)–theaflavin systems are [AlL], [AlL₂], [AlL₃], [AlL₃H₋₁] and [AlL₃H₋₂]. AlL, AlL₂ and AlL₃ correspond to complexes formed between Al(III) and the benzotropolone ring of one, two and three theaflavins, respectively. The species [AlL₃H₋₁] and [AlL₃H₋₂] could be accounted for by deprotonation of free phenol groups in the complexed theaflavin. ¹H NMR and HMQC experiments provided structural information on the complexes and allowed the determination of the metal coordination site. FTIR analysis of model aluminium complexes together with the aluminium theaflavin complexation corroborated the coordination site and allowed further spectral analysis of the aluminium–theaflavin system. The following complex stability constants have been evaluated from potentiometric analysis; log β_AlL= 7.80 (± 0.07), log β_AlL2= 15.00 (± 0.12), log β_AlL3 = 22.46 (± 0.13), log β_{AlL3H−1 =} 16.98 (± 0.1), log β_AlLH−2 = 10.19 (± 0.1). UV-Vis spectroscopy confirmed in part the stoichiometric coordination of the aluminium theaflavin complex. The complexation of Mn(II) with theaflavin was also investigated potentiometrically. The stoichiometry of the major complex formed in the manganese(II)–theaflavin system is [MnLH₋₁] and the equilibrium constant has been evaluated as log β_MnL = 4.80 (± 0.03).